FEEDING DAIRY COWS:

1.FIBRE IMPORTANCE ON ANIMAL WELFARE, MILK PRODUCTION AND COMPOSITION

A.B. Rodríguez1-2, P. Llorente3, S. Andrés1, F.J. Giráldez1

1 Instituto de Ganadería de Montaña, CSIC-ULE 2 Pania Animal S.L.

3 INATEGA S.L.

Emails:

A.B. Rodríguez (rganabel@gmail.com)

P. Llorente (pablollorente@inatega.com)

S. Andrés (sonia.andres@eae.csic.es)

F.J. Giráldez (j.giraldez@eae.csic.es)

INTRODUCTION

Forage is an essential ingredient in dairy cattle feed. Its inclusion in the diet is necessary to

ensure an adequate supply of fibre, proper rumen function, animal welfare and productive

performance, i.e. milk production and composition.

To understand the importance of forage in the dairy cow's diet, it is necessary to know what

fibre is, the role it plays in ruminant nutrition and the relationship between its effect and chopped

forage size. This first article will address the following questions:

What is fibre? Are there different types of fibre?

What role does insoluble fibre play in the dairy cow's diet?

What is rumen acidosis?

Does rumen acidosis affect milk yield and composition?

What percentage of fibre should be included in the dairy cow's ration?

WHAT IS FIBRE? ARE THERE DIFFERENT TYPES OF FIBRE?

The term forage refers to fibrous or voluminous foods, i.e. feeds that are high in fibre and

low in energy.

Fibre is only found in feeds of plant origin, and in general terms it consists of the plant cell

wall components. The intracellular content basically includes non-structural polysaccharides such

as starch and fructans.

The importance of the content and type of fibre included in rations renders it necessary to

conduct a detailed analysis of the fibre fractions necessary to achieve a correct formulation; these

are crude fibre (CF), neutral detergent fibre (NDF) and acid detergent fibre (ADF). These various

fractions must be included in the calculation because there is no single laboratory method for

determining the total fibrous fraction, and not everyone possesses the technical capacity to

routinely determine the different types.

From an analytical perspective, determination of crude fibre, the most basic type of fibre, is

the first method to separate cell wall components from the remaining plant cell components.

However, this procedure does not achieve complete separation, and underestimates the content

of structural polysaccharides, i.e. those that comprise the cell wall and are less digestible than

polysaccharides found within the cell, as shown in Figure 1.

Hence, a new method has been developed that optimises separation of structural cell wall

polysaccharides and polyphenols from the other components (Figure 2). This new method makes

it possible to determine neutral detergent fibre (NDF), also known as insoluble fibre, which

includes all the cellulose, hemicellulose and lignin present in feed. Acid detergent fibre (ADF) is the

fraction of NDF that consists solely of cellulose and lignin. The NDF content of forage is related to

the forage dry matter intake and ADF content with the animal's capacity to digest forage. Thus, an

increase in NDF content will decrease intake, while an increase in ADF will decrease digestibility of

the feed and the energy available for milk production.

Polysaccharides that do not form part of NDF basically include starch, pectins and fructans.

These polysaccharides are highly digestible but present some differences in the fermentation

process that occurs in the rumen, and therefore the concept of soluble fibre was developed, which

includes fructans and pectins but not starch.

The fermentation of fructans and pectins produces volatile fatty acids (e.g. acetic, propionic

and butyric acid), whereas the fermentation of starch not only produces volatile fatty acids but

also lactic acid (Figure 3). This aspect is very important due to its effect on rumen pH, as will be

discussed below.

HOW DOES ONE COMPARE FORAGE QUALITY?

An animal's capacity to ingest and digest forage depends on the latter's quality, and types of

forage are classified according to relative feed value (RFV) (Linn & Martin, 1989) and relative

forage quality (RFQ) (Moore & Undersander, 2002).

The RFV index classifies forage quality. The RFV is an index to classify forage quality,

combining the ingestibilidad and digestibility of forage. The RFV is expressed as a percentage of

the value that would have an alfalfa of reference (RFV = 1. 29), whose content of NDF and FAD

would be 53 and 41%, respectively. It is calculated according to the following equation:

calculating forage intake potential and digestibility according to the following equation:

RFV = DMI x DDM/1.29

Where:

Dry matter intake (DMI) = 120 / %NDF

Digestible dry matter (DDM) = 88.9 - (0.779 x %ADF)

According to RFV, 5 categories of alfalfa are considered: Supreme, Premium, Good, Fair and

Utility (see Figure 4). The bigger the content of NDF and FAD the lower intake and digestibility and

worse quality of fodder.

NDF and ADF content primarily depends on the phenological stage of the plant

(development of buds, leaves and lateral buds, elongation of stems and rosette, development of

vegetative parts, emergence of floral organ, bloom, fruit formation, and ripening of fruits and

seeds) at the time of harvest (Bosworth & Stingler, 1992), although it may also be influenced by

other factors such as genetic variety, season and number of cutting, environmental conditions

(temperature, water regime) and harvest and drying conditions. Note that alfalfa is an important

source of protein (CP) in the dairy cow's diet, and that protein content is usually inversely related

to NDF content; thus, an increase in NDF is associated with a reduction CP content.

Relative forage quality (RFQ) is estimated using the following equation:

RFQ = DMI x TDN/ 1.23

Where:

DMI: Dry matter intake as a percentage of live weight

TDN: Total digestible nutrients (DM%)

DMI is calculated:

DMI =[(120/%NDF)+(In vitro digestibility at 48 h of NDF-45)*0.374]/(135*100)

TDN=(0.98*NFC)+(0.93*CP)+(2.25*0.97*CF)+(Dvitro*NDF)-7

Where:

NFC: non-fibrous carbohydrates (%DM); CP: crude protein (%DM); CF: crude fat (%DM);

Dvitro: In vitro digestibility at 48 h of NDF; NDF: neutro detergent fibre (%DM).

This concept is basically the same as RFV and even generates similar values; however, the

method uses total digestible nutrients (TDN) rather than ADF to calculate dry matter digestibility.

In addition, it also uses different prediction equations to estimate DMI. However, the values of

RFV and RFQ are similar only when the value of the NDF digestibility presents an average value. If

the value of alfalfa that we want to assess moves away from the mean value, the RFQ would be a

more appropriate index. This is so because the RFV system assume that the digestibility is

inversely proportional to the content of FAD and, this relationship, however is not fully linear, and

as a result, either the relationship between RFV and % of NDF.

WHAT ROLE DOES INSOLUBLE FIBRE PLAY IN THE DAIRY COW'S DIET?

The rumen is where fermentation takes place, and it contains various microorganisms

(bacteria, protozoa and fungi) which digest the feed ingested by the animal. The feed is basically

fermented in three stages: colonisation by rumen microorganisms, dissociation of cell wall

polysaccharides, and hydrolysis and fermentation of intracellular components (Russell & Hespell,

1981).

Fermentation of structural and non-structural carbohydrates (NDF components and starch,

respectively) basically produces volatile fatty acids (e.g. acetic, propionic and butyric acid), which

are absorbed by the rumen mucosa and used as precursors in the synthesis of glucose, amino

acids, long-chain fatty acids, etc., for maintenance of the animal and milk production.

Structural carbohydrates are less digestible by rumen microorganisms, especially if lignin

content is high, and favour the development of populations of cellulolytic bacteria, which mainly

produce acetic acid as a product of metabolism. With an appropriate feed particle size, the higher

the NDF content, the longer the time required for mastication during intake and rumination and

the greater the production of saliva (Welch & Smith, 1970). Saliva has various functions, one of

which is to act as a buffer for variations in rumen fluid pH caused by the production of acids during

fermentation.

However, non-structural polysaccharides such as starch generally ferment very rapidly in the

rumen and produce a higher amount of propionic and lactic acid.

In contrast to forage, cereals have a high starch content and do not favour rumination due to

their NDF lower content and smaller particle size, thus also reducing saliva production and pH

buffer capacity. Therefore, as the percentage of grain in the ration is increased and the percentage

of forage is decreased, the production of propionic acid increases and pH decreases. In broad

terms, forage — and therefore NDF — content in the ration is directly proportional to the acetic

acid content and pH value of rumen contents, and inversely proportional to the propionic acid

content (Figure 5). High production cows consuming starch-rich rations with a very low proportion

of forage, or containing forage which has been chopped too finely, may develop a condition

known as rumen acidosis.

WHAT IS RUMEN ACIDOSIS?

Acute rumen acidosis is a condition characterised by a prolonged period of time during

which rumen pH is below 5, and is generally and primarily associated with a sharp rise in intake of

easily fermentable carbohydrates (starch), a substrate that induces increased bacterial growth,

produces high amounts of volatile fatty acids and decreases rumen fluid pH. In turn, this decrease

in pH favours the growth of lactic acid-producing bacteria (Streptococcus bovis and Selenomonas

ruminantium first and Lactobacilli second), while reducing the growth rate of bacteria that

consume this acid (mainly Megasphera elsdenii). The result is a greater accumulation of lactic acid

and volatile fatty acids, with a consequent decrease in rumen pH (Figure 6).

A very steep and prolonged drop in pH will cause metabolic acidosis (Owens et al., 1998).

Due to the high osmotic pressure of the rumen, water from the blood capillaries is released into

the gastrointestinal tract, causing diarrhoea. The clinical signs of acute acidosis include anorexia,

abdominal pain, tachycardia, lethargy and even death.

Intermittent periods of reduced pH will lead to subclinical acidosis. This causes the rumen

papillae to atrophy, triggering ruminitis and fibrosis characteristic of acidosis. These changes in the

rumen induce a reduction in feed intake, partially mitigating episodes of acidosis. However, these

will recur once the animal again consumes high amounts of feed (Nocek, 1997; Enemark et al.,

2002; Oetzel, 2003).

DOES RUMEN ACIDOSIS AFFECT MILK YIELD AND COMPOSITION?

The most obvious and visible symptoms of subclinical acidosis include cyclical variations in

feed consumption and a reduction in digestibility, in particular of the fibrous component of the

diet.

If prolonged over time, a reduction in intake can cause a reduction in milk production.

Acidosis is also associated with other conditions that affect production. For example, laminitis, an

inflammation of connective tissue in the hoof, has been associated with a drop in systemic pH and

subsequent tissue inflammation during episodes of acidosis (Nocek, 1997). More recently, Gozho

et al. (2005) associated this inflammation with toxins from bacteria that enter the rumen through

wall surfaces damaged by the effect of acidosis and subsequently enter and spread through the

bloodstream. Animals with laminitis spend more time lying down, resulting in a lower intake and

lower milk production.

It is also common for cows consuming starch-rich rations to produce milk with a lower fat

content, and milk composition can even be altered. This phenomenon is known as low milk fat

syndrome or milk fat depression, and occurs in nutritionally healthy animals in optimal physical

condition with a positive energy balance. It has a multifactorial origin, but one of the main causes

is a reduction in digestion of cellulose and consequently, in the proportion of acetic acid, the

precursor of short-chain fatty acids (4-16 carbon atoms). In addition, an increase in rumen

propionic acid due to high starch diets promotes gluconeogenesis in the liver and insulin release,

hindering an adequate supply of precursors for fat synthesis in the mammary gland.

Recently, milk fat depression in dairy cows has also been linked with some of the fatty acids

present in milk, intermediaries in the biohydrogenation phenomena that occur in the rumen,

especially those with double bonds at the 10 carbon position in conjugated linoleic acid (trans-10

cis-12 CLA and cis-10 trans-12 CLA), which have an inhibitory effect on fat synthesis in the

mammary gland (Shingfield et al., 2010).

WHAT PERCENTAGE OF FIBRE SHOULD BE INCLUDED IN THE DAIRY COW'S RATION?

Given the differences between forage and concentrates as regards fibre content, the

National Research Council (NRC, 2001) introduced recommendations for dairy cattle rations that

establish fibre requirements for both types of feed (Figure 9).

These recommendations are based on NDF and non-forage carbohydrate (NFC) content in

the diet. Thus, the higher the forage NDF content, the lower the percentage of NDF required in the

ration as a whole and the higher the percentage of carbohydrates provided by the concentrate,

and vice versa.

In the case of dairy cattle, acidosis is often the consequence not only of a high content of

highly fermentable carbohydrates, but also of the administration of a diet with a deficient particle

size, which renders the fibre ineffective. It is therefore necessary to pay attention not only to fibre

content but also to particle size since this determines whether the fibre provided fulfils its function

or not. This essential aspect merits detailed analysis, and will therefore be discussed in subsequent

articles.

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