has the green revolution been sustained? the quantitative impact of the seed-fertilizer revolution...

Pergamon

World Development, Vol. 22, No. 9, pp. 1345-l 361, 1994 Elsevier Science Ltd

Printed in Great Britain

0305-750X(94)00052-2

Has the Green Revolution Been Sustained? The Quantitative Impact of the Seed-Fertilizer

Revolution in Pakistan Revisited

DEREK BYERLEE” CIMMYT Mexico, D.E

and

AKMAL SIDDIQ” University of Illinois, Urbana-Champaign

Summary.-Quantitative impacts of the Green Revolution on food production in the Punjab of Pakistan are reviewed and the effects of different technologies on wheat yields over the past two decades decomposed. New quantitative evidence of sustainability problems in irrigated systems is presented. The yield increases expected in the post-Green Revolution period from the further spread of modem wheat var- eties, a tripling of fertilizer dosage, and the release of newer higher yielding varieties have been cancelled by problems resulting from increased cropping intensity, use of poor quality groundwater, low fertilizer efficiency, and increased weed and disease losses. New directions in institutional policies and research and extension strategies are outlined to improve efficiency and sustainability in wheat production and prevent Pakistan from becoming a major food grain importer in the coming decades.

I. INTRODUCTION

Pakistan was one of the first countries that widely adopted the new seed-fertilizer technology, based on the diffusion of semidwarf wheat and rice varieties, in what came to be known as the Green Revolution. With the largest contiguous irrigation system in the world providing a favorable and relatively homoge- neous environment, the country was ideally situated to take advantage of the vastly increased potential offered by the new technology when it became available in the mid-1960s. In fact, soon after it became clear that the technology was being adopted rapidly, the Government of Pakistan commissioned a perspective plan to the year 1985, to prepare for the possibility of a food grain surplus and explore the potential for exports. For this plan, Cownie, Johnston, and Duff (1970) developed detailed projections of the supply of wheat and rice to 1985, based on assumptions about the spread of the new varieties and the adoption of associated inputs. Their projections seemed to confirm the growing optimism about Pakistan’s potential as a major food producer and exporter.

It is now clear that Pakistan has been unable to live up to these expectations. In almost every year since 1970, when Cownie, Johnston, and Duff’s landmark study (hereafter referred to as CJD) was published, Pakistan has been a significant importer

of wheat, the major food staple. Since 1970, deficits in domestic wheat production have required the import of at least one million tons nearly one year in two, and on average, imports accounted for over 10% of total supply in the early 1990s.

Of course, all projections are subject to a wide margin of error and we should not be surprised that the reality is different from what was projected over two decades ago. CJD felt reasonably confident, however, that the range of assumptions they employed (from the least to the most optimistic) would cover the feasible set of outcomes. Moreover, their projections were based on results of an exten- sive program of on-farm demonstrations and experiments with the new technology, and the example provided by its rapid early adoption. CJD observed that

enough is known about the yield potential of the new varieties, the ability of these varieties to respond to fertilizer, and the receptivity of Pakistani farmers to these technical innovations to provide a basis for alternative sets of assumptions that represent the range of production possibilities.

As it turns out, the error in their projections was large. CJD had forecast that wheat production in 1985 would range between 16 and 20 million tons,

*Final revision accepted: March 16, 1994.

1345

1346 WORLD DEVELOPMENT

depending on the scenario. The actual level of wheat production for Pakistan in 1984-86 was 12 million tons. The projections of CJD were particularly optimistic for changes in yield, which were only half of what CJD predicted in their most pessimisric scenario.

Our interest in the CJD study, however, is not merely historical. There are good reasons for revisit- ing the scenario envisaged in the early years of the Green Revolution. First, as we shall show, the expansion of modern input use in food grain production in Pakistan has closely followed expectations; the problem lies rather in the translation of those inputs into outputs, as well as possible negative influences on yields resulting from a decline in the quality of the resource base occurring as cropping systems have intensified. Second, these issues are important because current development strategies in Pakistan and some other post-Green Revolution countries have not adjusted to the reality of very low input-output coefficients in the agricultural sector (that is, low efficiency in input use), in part because the problem is not yet well recognized or under- stood. For example, the National Commission on Agriculture (Government of Pakistan, 1988) in Pakistan projected food grain production using strategies of input intensification that are a continuation of the scenarios employed by CJD and which have clearly not lived up to expectations. An analysis of the expectations and reality of food grain production therefore has potentially important implications for current food policies and strategies. Finally, these issues are broadly important in the intensively cropped irrigated systems of Asia, given that productivity growth appears to be stagnating in some of these systems, and concern about their sustainability is increasing (Pingali and Rosegrant, 1993; Pingali, Moya and Velasco 1990; Byerlee, 1992).

This paper revisits the growth of food grain production in Pakistan during the last two decades to identify key policy issues for the future. We employ a methodology similar to the one used by CJD in tracing out changes in inputs and outputs over the past two decades. The original CJD paper provides a very comprehensive description of the basic parameters used, so that these parameters can be compared with more recent estimates. To simplify the presenta- tion and to focus on key issues for the future, we analyze trends in the yield of the dominant food grain, wheat, and the effects of biochemical technology on wheat yields. This is done for the major province, the Punjab, because it accounts for more than 70% of all wheat produced, and because data are generally more available for the Punjab. Because the Punjab dominates food grain production. our results for wheat in the Punjab can be safely extrapo- lated to the whole country.

What emerges from this analysis is a serious con-

cern about the sustainability of Pakistan’s agricultural and irrigation systems. Unless these issues are resolved quickly, the scenario for food production in Pakistan could be bleaker in the next two decades than it has been up to now. Indeed the scenario suggests that over the next two decades, Pakistan may become a major importer of food grains.

2. AN OVERVIEW OF WHEAT PRODUCTION TRENDS

Trends in wheat production in Pakistan since Independence can be divided into three distinct periods: 1947-65, prior to the release of semidwarf wheats; 1966-76, the so-called Green Revolution period when semidwarf or modern varieties (MVs) were adopted rapidly on over two-thirds of total wheat area; and 1976-90, a post-Green Revolution period, when MVs continued to spread slowly to cover almost all the irrigated areas and were taken up rapidly over much of the rainfed area.’

Yield changes for each of these periods were estimated by a spline function of the form:

100 X IQ,) = h,, + b, w, + bzwz + b,uj,.

where:

y, = yield in year f, M?, = 1, w2=OiftS 1966,

=r-1966ift> 1966, w,=Oift11976

= t-1976 if f> 1976.

and the coefficients bz and b, test for significant dif- ferences in trends in wheat yields between succeed- ing periods. The fitted function for the Punjab is:

100 x In (y) = 678 - O.O3w, + 5.06~~ - 3.68w,, (.08) (5.68)*** (3.68)***

R’ = 0.89, n = 44. where r-values are given in parentheses and *** denotes significance at the I % level (see also Figure 1).

Prior to 1966, yield growth was negligible and nonsignificant. The highly significant positive coefficient for w2 indicates that yield growth accelerated to 5.1% per year in the Green Revolution period.? In the post-Green Revolution period, the growth of yields has slowed markedly relative to the previous period, as indicated by the significant negative coefficient for wl.

The yield increases observed in the past two decades can conceptually be disaggregated into three major components: increased yields due to conver-

HAS THE GREEN REVOLUTION BEEN SUSTAINED? 1347

1.5

0.5

g=ennual growth rate

0.0 , I I I I I I I I I-

1945 1950 1955 1960 1965 1970 1975 1960

Figure 1. Average wheat yields in the Punjab, Pakistan, 1948-90.

1965 1990

sion of rainfed land to irrigated area; increased yields due to the switch from traditional tall varieties (TVs) to modern or high-yielding varieties (MVs); and increased yields in the areas already sown to MVs due to release of newer higher yielding varieties, increased fertilizer use, increased water supplies in irrigated areas, and improvements in other cultural practices.

Official statistics available from 1966 allow some disaggregation of trends in area, yields, and production in the Punjab by these different components. Over 196690, irrigated area expanded by more than 50%, and its share of total wheat area jumped from 68% to 87%.3 A comparison of irrigated areas and rainfed areas (Table 1) indicates that a significant part of the expansion in irrigated area resulted from conversion of rainfed wheat land to irrigated land. Increased cropping intensity at a rate of about 1% annually also added to the expansion in irrigated wheat area. Finally, wheat area as a proportion of total cropped area has grown from 35.4% to 37.6%.

The most striking trend is the slow rate of progress in yields in areas already sown to MVs (Table 1) (see also Hamid er al., 1987). Over 1966- 90, average yields of MVs at the farm level showed no significant trend around an average yield of about 1.8 t/ha. At first glance, then, the growth rate in wheat yields in the Punjab since the beginning of the Green Revolution is almost entirely a combination of conversion of rainfed to irrigated land and the switch

from TVs to MVs (also associated with increased fertilizer use). We will examine these trends later in more detail after reviewing the situation with respect to input use.

3. TRENDS IN USE OF MAJOR INPUTS

Three major inputs were central to the Green Revolution technology for wheat production: irrigation water, semidwarf wheat varieties, and fertilizer.

(a) Irrigation water

In addition to the growth of irrigated area, increased supplies of irrigation water per unit of area can increase yields on land that is already at least partially irrigated. Growth in supplies of irrigation water in the Punjab differed markedly between the decade 1967-76 and the decade that followed (Table 2). During 1964-76, total rabi (winter season) water supply doubled. Canal water supply increased by more than 50% in this period after completion of the Mangla and Tarbela Dams. As installation of private tubewells expanded, groundwater supply increased even more rapidly, tripling in 1967-1976. By 1976 groundwater provided nearly half of the rabi water supply. The overall annual growth in water supply of 6.7% during this period was used to convert rainfed


Table 1. Rates of growth (%/yr) of wheat area, yield, and production by irrigation status and varietal class, Punjab,

Pakistan*

Total Punjab 1967-76 1977-90

Irrigated areas 1967-76 1977-90

Rainfed areas 1967-76 1977-90

Modem varieties 1961-76 1977-90

Tall varieties 1967-76 1977-90

Area Yield Production

0.5 3.9t 4.5t 1.51 1.8t 3.3t

2.3t 2xt 5.01 2. I i I .4$ 3.5t

4.7t 3.9t -0.8 -1.6? 2.55 0.9

27.0-F 0.3 27.3t 3.7t 0.8 4.51

-10.6t -1.9 -12.5t -15.0t -0.2 -15.0t

Source: Calculated from Agricultural Statistics of Pakistan (various issues). *Estimated from log linear time-trend regression. tsignificant at 1% level. $Significant at 5% level. PSignificant at 10% level.

land to irrigated land (on 15% of the wheat area

sown) and also to increase water supplies to irrigated

wheat land from an average of 47 cm/ha to 67 cm/ha (under the reasonable assumption that 80% of rabi water is applied to wheat).

In the next decade, 1976-86, growth in supplies of irrigation water slowed substantially to 1.9% annually, slightly below the growth of total irrigated wheat area; hence water supply per irrigated hectare remained steady at about 65 cm/ha, and the proportion of irrigated wheat area increased by only 4% of total wheat area. Total canal water supplies remained

unchanged in this period, and all increases in water supply were provided by tubewells. By 1986, tubewell water accounted for 59% of total rabi water supply, compared to only 36% two decades earlier. Nonetheless, since 1980, investment in tubewells has slowed sharply, and the current rate of increase in water supply per hectare is negligible.

These trends are also consistent with the projections of CJD, who correctly anticipated the significant increase in irrigation water supplies as a result of the completion of new dams in the 1970s and the rapid expansion of tubewells. In fact the CJD projections were made on the assumption that irrigation water supplies w&d not be a major constraint on expanding cropping intensity and wheat production.

Table 2. Growth in source.s of water supply to wheat, Punjab, Pakistan, 1967-86

Groundwater Surface Total water water

Total supply* (m’ x 10mx) 1967 45 78 1986 173 121

Supply per ha (cm/ha) 1967 17 30 1986 38 27

Water supply by source (%) 1967 36 64 1986 59 41

Growth rate of water supply per ha (%lyr) 1967-76 7.9 1.8 1977-86 2.2 -3.4

123 294

47 65

100 100

4.4 xl.3

*Allocates 80% of total rubi water supply to wheat in proportion to irrigated cropped area. Surface water supply at farm gate assumed to be 60% of total supply.

(b) Adoption of MVs

In irrigated areas, the rate of adoption of MVs was most rapid in 1966-76, when the proportion of area sown to new varieties increased from less than 1% to more than 80%. Since 1976, the area under MVs in irrigated areas has continued to expand slowly on the remaining area, reaching 99% by 1988. In addition, the original MVs that initiated the Green Revolution, such as Mexipak, have been replaced by a succession of newer varieties, which have not only increased yield potential but have provided new sources of resistance against evolving disease pathogens (Byerlee and Moya, 1993). Adoption of MVs in rainfed areas was negligible up to 1976 but has expanded rapidly since, to reach over 80% of the rainfed area of the Punjab by 1988.

(c) Fertilizer

Estimated fertilizer use on wheat increased from less than 10 kg nutrient/ha in 1966 to 130 kg nutrient/ha in 1986 in irrigated areas, and 45 kg nutrient/ha in rainfed areas - a growth rate surpass- ing 10% annually. Measured by total nutrients applied, the increase in fertilizer use has been somewhat higher in the second decade, 1977-86, compared to the first decade, 1966-76. During the first decade, almost 90% of the fertilizer applied was nitrogenous fertilizer. Since then, use of phosphatic fertilizer has expanded rapidly to account for about 40% of all fertilizer applied to wheat in 1986. The expansion of fertilizer use is very close to what was

100

90

ii 8o .9 70 c 160

50

40

30

160


Year

60

20 ’ ’ I I I I I I I I I J

1970 1972 1974 W76 1978 1980 1982 1984 1986

Figure 2. Projected and actual input use in the irrigated Punjab, Pakistan. Note: projections based on Cownie et al., 1970.


projected by CJD, who forecast total nutrient use on irrigated wheat of 125 kg/ha and 20 kg/ha on MVs in rainfed areas by 1985.

(d) Input use: Expectation verstu reality

A comparison of the growth of input use projected by CJD and actual input use shows surprising congruence of expectations and reality. Figure 2, for example, compares the growth of the area covered by MVs and the use of fertilizer in irrigated areas. In both cases, input use has met or exceeded expectations, and in both cases the projections were remark- ably accurate.’ For the other major input, water, the actual expansion of irrigated area and water supply per irrigated hectare were also above the projections. Thus slow growth in input use does not appear to be the reason that yields of modern varieties have remained stagnant.

4. CHANGES IN INPUTS AND OUTPUTS IN WHEAT: THE MICRO-LEVEL PICTURE

Farm survey data provide further evidence to support these general trends in the use of inputs on irrigated wheat observed in secondary data. Data from the farm level also offer some insight into changes in other practices. Two farm surveys conducted around 1970 give quite detailed information on irrigated wheat production practices in the Punjab

soon after the introduction of MVs and fertilizer, and results of these surveys can be compared with survey data on wheat production practices from the mid- 1980s (Byerlee et al., 1984; Akhter et al., 1986). In the case of Multan District, comparable data exist for 1970 and 1985. In addition, the Water and Power Development Authority (WAPDA) has conducted large sample surveys over the whole province in 1977 and 1988.

The farm-level survey data on area sown to MVs and fertilizer use (Tables 3 and 4) are generally consistent with the trends observed in secondary data. Trends in other practices are also evident. For example, largely through the development of rental markets for machinery services, tractor use has become much more widespread since 1970. By the 1980s. tractors were the dominant power source for land preparation.5

It is also evident that there has been a major shift in the date of wheat planting over the period of analysis. In the early 1970s almost all wheat was

planted at the optimal time (the middle of

November) in both the rice-wheat and cotton-wheat

systems, two of the major cropping systems of the

irrigated Punjab. By the mid-late 1980s most wheat

was planted late (i.e., after November 30). Together

these surveys suggest a steady progression toward late planting of wheat in both cropping systems

(Tables 3 and 4).h The delay in wheat planting is caused by an increase in cropping intensity (Table 4). This trend is especially apparent in the cotton-wheat area, where wheat is often planted

Table 3. Comparative duta ott wheat production pructicrs in the irrigated Punjab oj’ Pakistan from f&n-level surveys, 1969-M

Survey location and year

Rotation

Sahiwal Multan Gujranwala and Multan and District, District, Sheikhupura, Bahawalpur,

1969 1970 1984 1985

Cotton-wheat Cotton-wheat Rice-wheat Cotton-wheat

Farm size (ha) Percent use tractor Seed rate (kg/ha) Percentage plant on time* Percentage use MVs Percentage apply nitrogen Percentage apply phosphorus Average dose (kgiha)

Nitrogen Phosphorus

Percentage apply animal manure Yield MV (t/ha)

9.8

:; ‘)4t 65 94 na

6.8 8.4 8.2 24 81 51 86 I25 I IO na 40 30 12 98 99 89 93 95

I 83 83

55$ 50: 61 95

5$ 5f 44 46 na 85 9 13

2.1 2.1 1.8 2.3

Sources: Sahiwal - Eckert (1970); MUkin - Lowdermilk, 1972; Gujranwala and Sheikhupura - Byerlee et nl., 1984; Multan and Bahawalpur - Akhter et cd., 1986. *Before December I. tEstimated from mean and standard deviation. $High-yielding varieties only.


Table 4. Comparative data on wheat production practices in the irrigated Punjab of Pakistan from large-scale

wheat yield, y*, for the Punjab is the weighted average of rainfed and irrigated yields, as follows:

WAPDA surveys, 1977 and 1988

Number of observations Farm size (ha) Cropping intensity (%) Seed rate (kg/ha) Percentage plant on time* Percentage use MVs Percentage apply nitrogen Percentage apply phosphorus Average dose (kg/ha)

Nitrogen Phosphorus Total

Percentage applying animal manure

Average yield (t/ha)

1976-77 1987-88

1,159 293 7.2 8.6

127 144 89 100 68 43 94 80 ;; 39 79

48 93

19 s 67 143

46 36 I .55 1.80

y* =py’+ (1 -p)y”

where:

(1)

y’ = average yield in irrigated areas, y” = average yield in rainfed areas, and p = proportion of wheat area irrigated.

Using subscripts to denote year, the change in yields from a base period t = 0 to year t can be readily disaggregated into three components:

Source: S. Bashiruddin, Enterprise and Development Consulting, Islamabad, Pakistan (personal communication). *Based on time wheat planting is finished.

p&v, - y,‘), the component due to yield changes in irrigated areas, (1 - p,)(y,” - y,“), the component due to yield changes in rainfed areas, and (p, - p,)Cy; - y,“), the component due to conversion of rainfed to irrigated land.

Likewise, average yields in irrigated areas, yl, can be expressed as:

after cotton in a double cropping pattern, unlike the traditional fallow-wheat and cotton-fallow systems (Byerlee, Akhtar and Hobbs, 1987).

y’ = qy’” + ( 1 - q)y’Z

where:

(2)

Seed rates have also increased by about one-third in an effort to improve plant stand and adjust to late planting. Finally, use of farm yard manure has apparently dropped sharply, probably in response to increased use of chemical fertilizer, reduced num- bers of draft animals, and growing use of animal manure for cooking fuel.7

The survey data on yields are also consistent with official statistics in suggesting that yields are stagnating at a little less than two t/ha in areas already sown to MVs. Because the surveys were undertaken in relatively advanced and productive districts, it is evident that this yield stagnation is not just a result of MVs diffusing to more marginal areas, which would tend to reduce overall average yields for Punjab Province. For example, the average surveyed yield of MVs in Multan District was 2.1 t/ha in 1970 compared to 2.3 t/ha in 1985 (Table 3), even though the use of chemical fertilizer on MVs in Multan District almost tripled over this period.

Y’“, Y” = yield of MVs (s) and local varieties (z) in irrigated areas, respectively, and

9 = proportion of irrigated area sown to MVs.

Finally, the increase in yields of MVs in irrigated areas over time can be represented by the following expression:

yr’” = yol\e@ + h(F, - F,) + K, (3)

where:

yOls,Y,‘” = yield of MVs in the base period and in year t,

5. QUANTITATIVE DECOMPOSITION OF CHANGES IN WHEAT YIELDS, 1966-86

F,, F, = fertilizer nutrients applied in the base period and in year t,

; = exponential rate of genetic gains in yields, = marginal grain-nutrient ratio for fertilizer

application, and

K = the combined effects of changes in other cultural practices and in the quality of the resource base.

(a) Basic framework of analysis

The next step is to decompose the growth in wheat yields into various sources. First, the average

Equation 3 simplifies the relationship between yields and inputs in a number of ways. First, the expression assumes no interaction between variety and fertilizer response within the group 0fMVs - an assumption consistent with recent findings (Traxler and Byerlee,

I352 WORLD DEVELOPMENT

1993). Second, the coefficient h implies a linear response to fertilizer, although this can be adjusted easily to incorporate nonlinear responses. Finally, the variable K, is essentially a residual to measure the effects of changes in other cultural practices and in the quality of the resource base, and it may be positive (e.g., due to improvements in cultural practices) or negative (e.g., due to degradation of the resource base). No algebraic expression is developed for the effect of these other factors, but they will be considered individually below.

(b) Yield gains due to conversion of rrrinfed land to irrignted land

The parameters for Equation 3 are readily available from secondary statistics. Over 1964-66 to 1984-86, the proportion of wheat area that was irrigated increased from 68% to 84%, so that p, - p0 = 0.16. The increase in yields in rainfed areas (J” - y,,“) was about 500 kg/ha. while the increase in yields in irrigated areas (y,’ - v,‘) was about 825 kg/ha. Conversion of rainfed land to irrigated land increased yields (i.e.. (J,’ - ?;“) by some 900 kg/ha. Substituting these values into Equation I, we estimate that the overall increase in average yields of 870 kg/ha in the Punjab in 196441986 can be disaggregated as in Table 5.

By far the largest contribution to average yields has been made by increasing yields in irrigated areas. This is not surprising, because irrigated wheat area is much greater than rainfed wheat area and because MVs spread first and rapidly in irrigated areas. The conversion of rainfed to irrigated area has contributed a little less than 20% of the overall increase in yields in the province.

(c) Yield gains in irrigated areus

By substituting Equation 3 into Equation 2 (see appendix A), the yield increase in irrigated areas can be decomposed into four components as follows:

- (Yt - &Xv,” - v,“) is the component due to changing from TVs to MVs on 100 (ql - q,) per-

Table 5. Punjab, 1964-86: Components #yield increasr

Effect on average yield

Increase in yields in irrigated areas 560 kg/ha Increase in yields in rainfed areas 160 kg/ha Conversion of rainfed to irrigated land 150 kg/ha

Total 870 kg/ha

cent of the area, with an absolute yield increase of v,” - v,,“. - y,,“q,(ee’ - I) is the component due to genetic improvements in yields of newer MVs at a rate of lOOg% per year. - q,h(F, - F,,) is the component due to increased fertilizer use on MVs, and - q,K, is a residual effect of all other factors influencing yields of MVs. Because the initial adoption of MVs. strongly

interacts with fertilizer use and the two inputs were in fact adopted almost simultaneously, it is impossi- ble to separate out their individual effects. Hence we assume that initial adoption of MVs is accompanied by adoption of a modest dose of fertilizer, empiri- cally estimated at about 40 kg/ha in irrigated areas, based on survey information available in the early years of the Green Revolution. Thus the yield difference between TVs and MVs, (y,,” - y,“), includes the combined effects of adoption of variety and the initial fertilizer dose.

To analyze changes in yields in irrigated areas, it is convenient to consider two periods. First, the period from 1964-66 to 197 l-73 was the period of rapid adoption of MVs. Average wheat yields in irrigated areas are estimated to have increased from 1,050 kg/ha in 1964-66 to 1,450 kg/ha in l97l-73.8 In the more recent period, 1971-73 to 1984-86, yields increased more slowly from 1,450 kg/ha to 1,825 kg/ha. Table 6 summarizes the estimated inputs of MVs, fertilizer, and water in each of these periods.

The increase in irrigated wheat yields during the first period can be explained almost entirely by adoption of MVs together with a modest dose of nitrogenous fertilizer. Data from various studies (Eckert, 1970; Lowdermilk, 1972; Nagy, 1984; Narvaez and Borlaug, 1966; Mirza Qari and Khan, 1980). can be used to summarize the expected increment in yields under farmers’ conditions from using MVs and nitrogenous fertilizer (Figure 3). With a modest dose of fertilizer, MVs provided an average increment in yields of about 680 kg/ha over unfertil- ized TVs; this is our estimate of y,” - rOIZ above.

Using these data as estimates of the parameters for the first component of changes in irrigated yields,

(4, - 4”)(‘i’ - J,“), the increase in average yields in irrigated areas associated with the adoption of MVs from 1964-66 to 1971-73 on 61% of the irrigated wheat area is estimated to be 4 I5 kg/ha (0.6 I x 680). This is just over the actual increase in irrigated wheat yields of 400 kg/ha during this period. Since varieties grown in this period were the original Green Revolution varieties, the parameter g, measur- ing genetic gains in nrbver varieties, has no effect. Likewise, two-thirds of the increased fertilizer use in this period can be explained by adoption of MVs


Table 6. Key inputs and yields in irrigated wheat production for three periods, Punjab, Pakistan

1964-66 1971-73 1984-86

Average wheat yield (kg/ha) 1,050 1,450 1,825 Area under modem varieties (%) <l 61 93 Fertilizer applied to wheat (kg/ha) 10 40 114 Irrigation water supply (cm/ha) 49 57 65

with a dose of 40 kg/ha, and this effect is already accounted for by y,ls - yoiZ above. Nonetheless, the increase in irrigation water supplies per hectare during this period (Table 6) and the increase in fertilizer use not accounted for by initial adoption of MVs would be expected to have some positive yield effect, and suggest that there may be some negative factors influencing yields.

In the more recent period, 1971-73 to 1984-86,

the adoption of MVs increased from 61% to 93% of the area (Table 6). Since MVs were first adopted over the best irrigated land with the best infrastruc- ture, it is reasonable to assume that much of the first adoption of MVs in the later period occured on more marginal irrigated land that has problems of water scarcity, waterlogging, or salinity. Hence, for this second period we have assumed that the yield advantage of MVs over TVs at a fertilizer level of 40

2160

1680

1200

1100

1000

0

/ E e P

___________.__.__-_-.

Fertilizer (kg nut/ha)

Figure 3. Estimated average fertilizer response at the farm level for local varieties and modem varieties under irrigation. Punjab, Pakistan.


kg/ha, is only half that in the first period, which gives a total yield advantage from adopting MVs, y,”

- y,“, of 440 kg/ha. In the 1970s and 198Os, newer higher yielding

varieties became available in areas where farmers had already adopted modern varieties. These varieties eventually replaced the original Green Revolution varieties. The estimated contribution to yield potential of these newer varieties under experimental conditions was about 1 .O% per year over this period (Byerlee, 1993). Gains realized in farmers’ fields were probably less, especially because of the effects of late planting. We have therefore reduced the value of g to 0.75% per year.

Fertilizer applied to irrigated wheat in the second period increased by 73 kg nutrient/ha. The switch from tall to MVs on an additional 32% of the area, assuming the simultaneous adoption of 40 kg/ha of fertilizer, accounts for 13 kg nutrient/ha of the overall increase in fertilizer use (.32 x 40), and this effect has already been included in y,” - ?,,I above. On- farm fertilizer experiments (Aslam, 1989; NFDC, 1989) suggest an average grain-nutrient ratio, h, of 8: 1 associated with increasing fertilizer use from 40 kg/ha to 100 kg/ha on MVs.

Substituting these various parameters into the yield decomposition model (above and appendix) gives the estimates of the various components of yield increases in irrigated wheat from 1971-73 to 1984-86 shown in Table 7.

This projected yield increase can be compared in two ways: with the actual increase in irrigated yields in the same period and with the increase projected by CJD. The actual yield increase in irrigated areas during this period was 375 kg/ha (Table 6). Hence K,

can be calculated as the residual, 375-725 = - 350 kg/ha. The surprising result is not that K, is negative, but that it is so large - about 20% of irrigated yields in 1984-86.

On the other hand, CJD projected an average irr- gated wheat yield in 1985 of 3,900 kg/ha, double the actual yields for MVs in 1984-86 (Figure 4). This difference is due entirely to the fact that CJD projected that yields of MVs would increase by 64% because of increased fertilizer doses and improved fertilizer efficiency. CJD expected that fertilizer efficiency would increase from a marginal grain-nutrient ratio of 12 to a ratio of I6 as farmers gained more experience with this relatively new input and improved other cultural practices, such as plant stand management and weed control. In fact, the actual grain-nutrient ratio over this period fell sharply to less than 5:1 (Figure 4). This difference between expectations based on on-farm experimental data available in 1970 and the reality is indicative of the unexpected negative trends in yields over this period.

from 1971-73 to 1984-86

Eff&ct on average yield

Further switching from tall varieties to MVs

Genetic gains in yield potential of new varieties

Increased fertilizer use on MVa

Total

141 kg/ha

I38 kg/ha 446 kg/ha

725 kg/ha

6. SUSTAINABILITY 1SSUES: NEGATIVE INFLUENCES ON YIELDS

In the irrigated Punjab, the unexpected stagnation of yields of MVs of wheat over the past two decades, in light of substantial increases in inputs, especially fertilizer, raises serious questions about the sustainability of the system. Resolving these questions should be a major research and policy issue for the future. The increase in wheat yields expected since 1970 from the further spread of MVs. a tripling of fertilizer dosage, and the release of newer even higher-yielding varieties appears to have been cancelled by various negative influences on yields. These factors may include the following:

(a) h-reused cropping intensity

This has been particularly important in delaying wheat planting and has probably had other negative effects on soil structure, soil health, and nutrient availability. In the rice-wheat and cotton-wheat systems, two of the dominant cropping systems of the Punjab, an average of 50% of the wheat may be sown late. Conservatively, the average date of planting of MVs in the Punjab is estimated to be at least seven days later now than it was in 1970. Given an approximate yield decline per day for every day’s delay in planting of 1% (25 kg/ha”) under farmers’ conditions (Hobbs, 1985), an average delay in planting of seven days would decrease wheat yields by about 175 kg/ha over the period. Although increased cropping intensity and delayed planting of wheat have undoubtedly had negative influences on wheat yields, they may have increased overall productivity of the system as farmers trade off wheat yields, for higher yields of more profitable cash crops, and increased cropping intensity. A recent estimate, however, of total factor productivity for the Punjab suggests that, in fact, productivity for the whole system also stagnated during 1970-85 (Evenson and Bloom,

HAS THE GREEN REVOLUTION BEEN SUSTAINED? 135.5

1Y70 1972 1974 1976 1978 1980 1982 1984 1986

3.5

g 2.5

?I i-

2.0

. ...-@

Projected ,@Jor

. ..w”M

&” sfl

I I I I I I I I I I

lY70 1972 1974 1976 1978 1980 1982 1984 1986

Ycpr

Figure 4. Projected and actual grain-to-nutrient ratios and yieldsfor modem varieties, Punjab, Pakistan, 197&S. Note: projections based on Cownie et al., 1970.

1990) and that the problems experienced in wheat productivity have not been compensated by increased productivity of the system as a whole.

(b) Poor quality groundwater

The data presented above show that groundwater from tubewells has increased dramatically as a share of all irrigation water. In some major wheatgrowing areas, tubewell water provides 75% of the irrigation

water applied to the wheat crop (Tetlay, Byerlee, and Ahmed, 199O).‘O In widespread testing of water, however, from more than 1,000 tubewells in the 198Os, the Punjab Soil Fertility Institute has classified only 25% of tubewells as providing usable water, 21% as marginally usable, and 54% as “haz- ardous.” More than a decade ago, Choudhri, Mian and Rafiq (1978) arrived at a similar conclusion, that sodicity caused by unsuitable tubewell water was a major problem affecting half of the cultivated area in


the Punjab. Sodicity, which is a cumulative process over time, causes hardening of the topsoil, reduced plant stand through poor seed emergence and seedling survival (especially in wheat), and reduced water infiltration. Data from the Soil Fertility Institute from thousands of on-farm fertilizer experiments on wheat during 1975-1983 also suggest declining wheat yields of the order of 1.5% per year at the recommended fertilizer level in areas that depend primarily on tubewell water.

Sodicity from poor quality tubewell water can be arrested at least partly by soil amendments such as gypsum (Choudhri Mian and Rafiq 1978), although at the moment the use of gypsum in the Punjab is negligible.” Effects of secondary salinity on seedling emergence may also be reduced by increasing the seed rate. Data presented in Table 3 indicate that farmers have in fact increased seed rates by about one-third over the past decade, but wheat plant stand still remains low (Aslam et al.. 1989; WAPDA, 1979). Finally, use of organic manure can slow the effects of sodicity. The evidence presented above suggests, however, that use of organic manure has declined sharply and probably aggravated the problem of sodicity.

(c) Low fet-tilizer t$icienq

A number of factors suggest that the efficiency of fertilizer applied to irrigated wheat in the Punjab is low. The balance of nutrients applied may be inap- propriate. In Pakistan the conventional wisdom is that nitrogen and phosphorus should be applied in a 2:l ratio. The current ratio for fertilizer applied to wheat is about 3: 1. On the other hand, in 42 fertilizer experiments sown in the rice-wheat zone of the Punjab in 1984-88, no overall phosphorus response was observed (Aslam et al., 1989). Hence farmers in this area, who apply an average of about 50 kg/ha of phosphorus, appear to be using above the optimum dosage of this nutrient. Clearly phosphorus use must be tailored to the specific agroclimatic, soil, and economic conditions of each location. In addition, potassium and micronutrients may be limiting nutrients in some locations, reducing overall efficiency of applied nitrogen.

There is also evidence that the efficiency of applied nitrogen is low - that is, the efficiency with which applied nutrients are converted into nitrogen removed through grain and straw. Given average farm yields of two t/ha with the application of 90 kg/ha of nitrogen (N), the estimated efficiency of N recovery is at most 28%.” Even in on-farm fertilizer trials in the Punjab, N-use efficiency for application of 150 kg/ha is estimated at a low 33%. Reasons for low N-efficiency may include application methods

(uneven fertilizer distribution in the field and volatilization of urea applied without sufficient moisture) as well as reduced use of organic manure and increased problems of sodicity, weeds, and other pests.

(d) Increased breed and diseuse 1osse.s

The weed Phalaris minor has spread very rapidly in the rice-wheat system and in the central areas of the Punjab in the past two decades. This weed causes average losses of about 500 kg/ha in about 30% of the fields that were classified as seriously infested in the rice-wheat system (Byerlee et ul., 1984). Likewise, Malik (1986) observed a yield loss to weeds of 230 kg/ha in 65 on-farm “constraints” experiments in the rice-wheat zone. Assuming that the production systems in which this weed is a problem cover one-third of the Punjab’s wheat area, the average loss over the whole Punjab may be equiva- lent to at least 50 kg/ha (0.3 x 500 x 0.33).

Increased losses to disease are also a problem. During the past decade, much of the wheat area has been sown to disease-susceptible varieties due to a slow rate of varietal replacement in farmers’ fields. Although an epidemic has occurred in only one year, 1978, annual losses to rust diseases may be of the order of 5-10% in many years (PARC, 1987).

Other factors may be leading to yield declines as well, such as increasing waterlogging in some areas, soil health problems caused by continuous planting of wheat without rotation in the winter season, and soil compaction due to the widespread use of tractors for land preparation. At present only fragmentary data are available to quantify the magnitude and extent of these problems. For example, in the rice- wheat area, rotation appears to be a key determinant of yields; fields planted continuously to wheat for three or more years showed a significant negative tendency in yields (Byerlee ef crl., 1984).

Overall, the negative effects listed above could account for much of the apparent yield gap observed between actual yields and yields predicted by Equation 3 on the basis of increased use of MVs and fertilizer. Assigning average losses (weighted by the area affected) of 175 kg/ha to late planting, 300 kg/ha to increased secondary salinity/sodicity from use of poor quality groundwater (i.e., 1.5% per year as suggested by Soil Fertility Institute data), and 100 kg/ha to increased weed and disease losses accounts for the discrepancy of 350 kg/ha between actual yields and predicted yields.

7. A SCENARIO FOR THE FUTURE

In light of the above analysis, it is instructive to


look forward to the next decade. Based on reasonable assumptions about population and income growth rates, wheat consumption in Pakistan is projected to grow at 3.3% annually over the next decade or more.13 What are the chances that production increases over the next decade will prevent the import gap from widening?

(a) Increasing wheat area

Growth in wheat area is determined by bringing new land under cultivation, especially through increased irrigation water supplies; by increased cropping intensity; and by increasing wheat area as a percentage of total cropped area. There is little reason to expect any significant increase in total cultivated area over the next decade or more (Government of Pakistan, 1988). Increases in canal water supplies will be marginal (no new dams are under construction), and, as discussed earlier, increases in tubewell water supplies slowed signifi- cantly in the 1980s. There is substantial scope, however, to improve water-use efficiency and to use sav- ings in water to expand cropped area through higher cropping intensity. Newer varieties also facilitate more double cropping, and plant breeders are giving greater emphasis to developing varieties that tit more intensive cropping patterns (Tetlay, Byerlee, and Ahmed, 1990). Earlier maturing varieties of cotton and Basmati rice varieties have been released in recent years and their rapid adoption has not only facilitated increased cropping intensity but also tend- ed to reduce the problem of late planting of wheat (Byerlee, Akhter and Hobbs, 1987; Sharif er al., 1992). Hence cropping intensity can be expected to increase at about the same rate as in the past (about 0.8% annually). It is unlikely, however, that wheat will expand as a proportion of total cropped area,‘j but with a small margin for new land being brought under cultivation - say 0.2% annually - it may be possible to increase wheat area at 1 .O% annually.

(b) Increasing wheat yields

With a somewhat optimistic projection that wheat area will increase at 1.0% per year, an increase in yields of about 2.3% per year will be needed to match the growth in wheat demand.15 To attain the required rate of yield gain in the future, however, will demand a strategy different to the past, which emphasized intensification of input use and cropping systems, and neglected maintenance of the quality of the resource base. The process of switching from TVs to MVs is now complete in irrigated areas. Newer varieties with steadily increasing yield poten-

tial are continually released. Release and adoption of these newer varieties, especially varieties that yield better at late planting, have the potential to con- tribute about 0.75% per year to increased yields.

Given fertilizer responses estimated by the National Fertilizer Development Centre ( 1989) and Aslam et al. (1989), and a price ratio of nitrogen to wheat of about 3:1, the optimum nitrogen dosage that Punjab farmers should apply would appear to be no more than 130 kg/ha. i6 To reach this optimum, total fertilizer use (nitrogen, phosphorus, and potassium) on wheat would need to increase by only 2.9% per annum to the year 2000 (from 120 kg/ha to 170 kg/ha), compared to the overall average increase in fertilizer use of 5% annually projected by the National Commission on Agriculture. The marginal grain-nutrient ratio observed from on-farm experiments (Malik, 1986; Aslam et al., 1989; NFDC, 1989) for increasing fertilizer use from the current 120 kg nutrient/ha to 170 kg nutrient/ha (mostly through increasing nitrogen) is about 6: 1. Hence fertilizer use may add only about a 1 .O% annual growth rate to average yields over the next decade.

In the past, government policies, such as fertilizer subsidies and extension campaigns, emphasized higher doses of fertilizer - that is, movement along the response curve. Given the apparently low technical efficiency of fertilizer use, future policies should emphasize moving the response curve upward through improved management practices (e.g., weed control, improved plant stand, correction of secondary salinity/sodicity) to increase fertilizer efficiency and raise the marginal returns to using current levels of fertilizer.

A number of other technological components can increase productivity and help improve the @ciency of water and fertilizer use. Weed losses are a major problem in many irrigated areas; herbicides are now beginning to be adopted in some areas, even by small farmers, and should spread rapidly over the next decade (Ahmed et al., 1991). Reduced and zero tillage promise not only to lower costs, but to allow more timely planting to increase yields; adoption could be rapid in the next few years.

The combination of genetic gains, increased fertilizer use, and improvements in other management practices should allow an increase in yields of 2.0% annually, without considering for the moment the negative injluences on yields discussed above. There, however, are a number of obstacles to achieving even these modest yield gains:

(i) Realizing genetic gains from the release of newer varieties requires that new varieties spread much more rapidly to farmers’ fields (Heisey, 1990). At present rates of varietal diffusion, a new variety requires more than 10 years from the time it is released until it reaches its maximum


adoption (assuming the variety is accepted by farmers). Major changes in seed distribution and extension are needed to ensure that new varieties are adopted more rapidly in the future (Heisey, 1990). (ii) Improvements in other cultural practices, such as weed control, balanced fertilizer doses, and irrigation scheduling will require a well- developed adaptive research and extension system to formulate location-specific recommenda- tions and provide farmers improved technical information to address the management needs of an increasingly complex agriculture (Byerlee, 1987).

(c) Overall assessment

The above projections suggest that even under optimistic assumptions, wheat production will increase at only 3.0% annually versus a demand growth of at least 3.3% annually. Two major factors will influence the final outcome. First, Pakistan has generally taxed producers and subsidized consumers, especially for wheat (for a review, see Byerlee and Morris, 1993). Thus one scenario would be to allow real prices to rise to manage the import gap (Renkow, 1991). This would reverse the downward trend in real prices to producers and consumers over the past two decades and would promote greater allocative efficiency in the agricultural sector. Poor consumers, however, including many food-deficit households, would be adversely affected by this scenario (Renkow, 1991). Second, the magnitude of the supply-demand imbalance will depend critically on the success in reversing the apparent negative tendency in yields brought about by intensification and degradation of the resource base. The evidence presented in this paper suggests that these negative influences could cancel most of the expected yield gain from adoption of improved technology. Some short-term solutions are possible. For example, the pressure of increased cropping intensity could be partly alleviated through the greater research emphasis on varieties to fit intensive cropping systems. Likewise, success with improving supplies of seed of new varieties and herbicides will reduce the problem of increasing disease and weed losses. Further gains are also possible through application of soil amendments to ameliorate the effects of sodic tubewell water, and through use of microelements.

The most urgent need at present, however, is a concentrated research program to quantify the magnitude of the yield decline (for given input levels) and to identify its causes more precisely. This will require a truly interdisciplinary longer term research approach that departs sharply from current research

strategies. The emerging sustainability problems demand the skills of soil scientists, agronomists, pathologists, irrigation specialists, and social scientists to integrate information across disciplines and commodity research programs for a specific cropping system in order to understand critical soil- water-pest-rotation interactions. Long-term experiments and surveys must be undertaken to monitor trends in productivity and key management practices (e.g., rotation, use of unsuitable tubewell water, use of organic manures, and phosphorus levels).

There is no tradition in the research system of this type of long-term integrated research. Substantial institutional reform is therefore required to meet future challenges (for example, establishing multidisciplinary research institutes for major cropping systems).

8. CONCLUSIONS

This paper has revisited the quantitative impacts of the Green Revolution on food production in Pakistan. The earlier optimistic view of Pakistan becoming a major food exporter remains unfulfilled. A disaggregation of actual changes in wheat production in the Punjab over the past two decades indicates that there have been three major sources of production increases: area increases, especially in 1976-86, through increased cropping intensity; yield increases resulting from the conversion of rainfed to irrigated land; and yield increases arising from the switch from TVs to MVs, together with the adoption of fertilizer.

The major source of the projected yield increase, however, has still not materialized - that is, the increase in the yields of MVs expected from rapid growth in fertilizer use, the release of newer, higher yielding MVs, and the adoption of better cultural practices. This source of yield gain has apparently been cancelled by a number of negative factors influencing yield trends. These factors are not well under- stood and a vigorous long-term research effort needs to be mounted to quantify and correct the gap between current yields and yields that could be achieved with current levels of inputs.

In contrast to the optimism of two decades ago, we believe that the evidence presented in this paper raises serious concerns about Pakistan’s ability to sustain even present levels of food self-sufficiency to 2000 and beyond. Even under an optimistic scenario that ignores negative influences in yields due to intensification and resource degradation, it is likely that demand for the main food staple, wheat, will outstrip growth in supply. The major sources of production increases in the past two decades will play a much smaller role in the future; only increased area from higher cropping intensity will be significant.

Fertilizer doses will rise but the marginal payoff is now relatively low under current production practices and low levels of fertilizer efficiency. The emphasis in Pakistan on boosting yields through higher levels of inputs must change to one of pro- moting greater efficiency of input use at current levels, and in maintaining and improving the quality of the resource base.

Clearly the major source of growth over the next decade will have to be in irrigated areas that have already adopted MVs and are already using moder- ate to high doses of fertilizer (more then 100 kg/ha). Average yields in these areas are still less than 2 t/ha; hence there is substantial potential to expand production further through yield increases. It will also be important, however, to reduce the effects of factors that are apparently leading to a deterioration in the quality of the resource base and causing a long-term tendency for yields to decline. To achieve improved efficiency and sustainability in irrigated

crop production in Pakistan will require a reorienta- tion of institutional policies and strategies, especially those of research and extension.

We do not want to speculate on the extent that the disappointing performance of post-Green Revolution agriculture in Pakistan is representative of other areas. In the Indian Punjab, where extension is much stronger and linkages with research are well developed, the situation is clearly different. Yields of MVs have increased rapidly in the past two decades, and there is little evidence of negative influences on yields (Sidhu and Byerlee, 1992). In other areas of Asia, however, there is concern about the long-term sustainability of yields in intensive irrigated systems, in light of an apparent degradation of the resource base (Pingali Moya and Velasco, 1990). This paper provides new quantitative evidence that reinforces the urgency of identifying and addressing sustainability problems in Asia’s post-Green Revolution agriculture.


NOTES

1. While the initiation of the Green Revolution was clearly marked by the release and rapid adoption of modem wheat varieties, there is no clear demarcation of the post- Green Revolution period, defined here as the period when most wheat area was sown to modem varieties. Here we use 1976 as the dividing point, since about 80% of the irn’- gored area was sown to MVs by that year. The choice of year is arbitrary and does not affect the conclusions of this analysis. The reader should not interpret this demarcation to mean that there there was a sharp break in the rate of change in yields in that year.

2. The growth rate of yields (in percentage per year) for the first period is given by w,, for the second period by w, + w2, and for the third period, w, + w? + w2.

3. This increase is considerably higher than what was projected by CJD, who assumed an increase in irrigated area of only 20% for West Pakistan as a whole and no change in rainfed area.

4. ‘Ihe actual use of MVs in the early years is somewhat higher than projected since the projections refer to Pakistan as a whole, while the actual adoption is for the Punjab, which led the Green Revolution.

5. CJD projected increased tractorization under certain scenarios, especially the continuation of an overvalued exchange rate. They failed, however, to predict the rapid development of a tractor rental market that would foster small-scale farmers’ wide participation in the trend toward increased merchanization.

6. These data are not strictly comparable, because the WAPDA data are for the date a farmer finished planting whereas the other surveys provided data on planting dates for specific fields.

7. A similar phenomenon is observed for other intensively cropped systems of South Asia (Fujisaka, Harrington and Hobbs, forthcoming).

8. We use averages of three years in these calculations to reduce the effects of random weather phenomena that often dominate yield data from only one year.

9. Assumes a base yield of 2.5 t/ha and that on average farmers plant 20 days beyond the optimum date.

10. Over the whole of the irrigated Punjab, 47% of the wheat area is supplied by a combination of canals and tubewells, 19% by tubewells only, and 34% by canals only.

11. Gypsum use is, however, quite widespread in the Indian Punjab.

12. Based on a harvest index of 35%, a base yield with out nitrogen of 1,000 kg/ha, and grain and straw N content of 1.8% and 0.4%, respectively (M. Bell, CIMMYT, personal communication).

13. Assumes annual growth rates of population and per capita income of 2.7% and 2.9%, respectively, and an income elasticity of demand for wheat of 0.2.

14. In fact there will be pressure to reduce wheat as a percentage of cropped area in order to satisfy the increasing demand for other crops, especially oilseeds.

15. This is a more modest target than the 3% yield growth specified by the Government of Pakistan’s National Commission on Agriculture (1988).

16. Recall that farmers already appeal to be using close to the optimum level of phosphorus.


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APPENDIX: DECOMPOSITION OF YIELD CHANGES

This appendix provides the algebraic expressions for decomposing changes in yields into components that reflect

y:’ - ?jd* = q-v,” - q-v,” + ( 1 - q,)y,” - (I - q,)y,“. (3)

the contribution of various inputs to changing yields. Let yields in irrigated areas at time t, y,‘*. be represented

Assuming y,” = y,‘” and substituting Equation (2) into

by the following two equations: Equation (3), we get:

Y,” = qyt” + ( l-q)y,” (1) 4’1” - y,,‘,” = q,v,‘\e@ + q,h(F,-F,) + q,K, - y,“(q,-q,),

?;” = y 0 “e%’ + h( F,F,) + K, (2) and rearranging:

Equation (1) simply states that irrigated yields are the average of yields of MVs, yt’“. and TVs, y?. weighted by the proportion of area sown to semidwarfs, q. Equation (2) rep- resents the yield of MVs as the sum of three effects:

(a) The effect of genetic gains at an annual rate of 100% per year. (b) The effect of increasing fertilizer from F, in the base period to F, with a grain-nutrient ratio of h. (c) The effect K, of all other factors, such as weed control, changing residual fertility, etc. K, is arbitrarily assigned a value of zero.

Yield changes from the base period (t = 0) to time t can be represented by:

y,'*-y," = Cy,” -ynv,“)(q, -4,) +y,“q,(e@- I) + q,h (F, - F,,)

+ q&

where

cv,” - Y,") (4, - 4”) = the effect due to the switch from TVs to MVs,

4, (e”‘- 1) = the effect due to genetic gains in yields of MVs,

q&F,-F,) = the effect due to increasing fertilizer doses, and

q,K, = a residual of all other effects, given

)11’ * - Y,,".

has the green revolution been sustained? the quantitative impact of the seed-fertilizer revolution...

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