Chapter 6

CONTROL OF FEED INTAKE IN RUMINANTS

6.1 SOME PRACTICAL OBSERVATIONS 

6.1.1 Animal behaviour

The productivity of ruminants is determined by many factors, but two of the most important are what and how much they eat. While ruminants will consume a wide variety of feeds, some of which initially may be quite unpalatable, they are highly selective and appear to enjoy some feeds more than others. Animals usually eat green pastures or cereal grains with apparent relish, consuming other feeds only relatively slowly and without apparent interest.  It appears that ruminants recognise both unpleasant and pleasant sensations associated with feed either prior to or during eating.  They often continue to consume some feeds even though these may cause metabolic upsets. Good examples of this are the continued ingestion of thermo-ammoniated straws even though this results in ``hysteria," and the continued consumption of molasses in cases of ``molasses toxicity" (see Chapter 8).

Ruminants, like all animals, are individualistic and where selection is possible the intake of different feeds varies among animals. Animals grazing dry pasture vary their intake of supplement considerably, from none to an appreciable amount (see Figure 6.1). In a flock of sheep grazing dry pasture, only 50% of the flock consumed urea/molasses blocks (Nolan et al. 1975). About 75% of a herd of cattle grazing dry pasture took readily accessible molasses (R A Leng, unpublished observation). Under tethered-husbandry systems, as occur in India, all cattle and buffalo will eventually consume molasses/urea blocks, although some animals need 14 to 21 days before they will consume a significant amount of the mixture (see Chapter 11).

Figure 6.1 Cattle grazing on unimproved, dry native pasture (low in N) had highly variable intakes of supplement (meat meal/soy bean mixture). Among the animals that were consuming the supplement there was a significant relationship between intake and growth rate. The intake was measured using the tritiated water technique (Source: R Williams and R A Leng, unpublished data).

Appetite for particular feeds obviously varies between individuals. Grazing ruminants that have not been given supplements previously take some time to, or may never, adjust to supplements.  However, once they have been induced to consume a certain feed they will accept it again even after a number of years.

Under grazing conditions there is usually a wide variety of pasture plants available, but only some are selected and these are not necessarily abundant in the pasture. Quite often plants that comprise only a small proportion of the herbage in a pasture make up a large proportion of the animals' diet, particularly under extensive grazing conditions. When grazing pressure is not high animals select mainly the leaves of pasture plants.  However, when less forage is available, the scope for selection is obviously less.

Particularly in the tropics, cattle grazing extensively tend to graze only part of the available area, and continue to graze this part, keeping the pasture very short, apparently preferring to live on young re-growth. It has been demonstrated that cattle return to areas that have been cut, particularly when the pasture is green, and they graze these areas in preference to those that have not been cut.

In many tropical countries, farmers are aware of this behaviour and use high stocking rates to maintain pasture in an immature state. Under these circumstances supplementary forage is usually given at night to meet the `shortfall' of feed. One problem resulting from the concentration of stock on small areas is that parasite burdens may become extremely heavy. 

6.1.2 Selection

Sheep and goats graze more selectively than large ruminants. For instance, sheep on low-N pasture (average 0.8 g N/100 g dry matter) selected herbage that contained between 1.5 and 2 g N/100g dry matter (Langlands and Sansom 1976). Cattle are less able to graze selectively than small ruminants because they take larger bites and the way in which they prehend the plants is not conducive to selection. The capacity of sheep to select a diet of high N content is illustrated in Figure 6.2, which shows that rumen ammonia levels were consistently and considerably higher in sheep than in cattle on the same communal grazing land in Ethiopia, both during the dry season and after the first rains.

Figure 6.2 Ammonia level in rumen fluid of cattle and sheep grazing on communal pastures in the Debre Berhan area of Ethiopia during 1985 (Source: A Neguissie, R A Leng and T R Preston, unpublished data)

 Ingestion of nutrients which balance a deficient diet may also give pleasant sensations to the animal. There is evidence that some animals can balance their diet by selecting from a choice of feeds. For example, within seconds of nibbling, rats are able to recognise a diet with thiamine added, suggesting that the pleasant sensation is immediate and must be due to rapid diffusion of nutrients into the bucal nerve cells (McClymont 1967). However, there are other cases in which animals do not differentiate between deficient and adequate feeds.

Groups of mixed species of grazing animals often use pasture more efficiently than a single species. Cattle eat coarser grasses than sheep; goats prefer to browse trees and shrubs that are highly unpalatable to sheep. The differences in palatability, and therefore relative edibility, of different plants to livestock are important in natural ecosystems and also in communal grazing. Disturbances of natural ecosystems can lead to problems of overgrazing. Where cattle are introduced at the expense of wildlife (in particular the browsers) bush and shrub encroach upon the grazing areas. Goats are often grazed with cattle and sheep in order to control scrubland. 

6.1.3 Dysphagia 

Dysphagia is the ingestion of materials that are, as far as can be determined, nutritionally inert or even harmful and which are not normally consumed. It has been reported that phosphorus deficiency in cattle leads to the animals chewing bones and ingesting sticks and often wire.It has also been reported that sheep, cattle and horses exhibit unusual behaviour such as soil licking, although these activities are not necessarily associated with any nutritional deficiencies.

6.2 FACTORS IN FEED THAT AFFECT INTAKE

There are many factors that affect feed intake, of which smell is often the most important. Animals may reject feed without tasting it.  For instance, the smell of dung reduces the intake of pasture by cattle, but if the grass around the faeces is cut and carried the animal will eat it readily. This behaviour has probably developed to protect the animal against infestation by intestinal parasites. Unpalatable pasture appears to give off volatile materials, since animals reject it without tasting. In the same way, animals will only eat small amounts of mouldy feeds and diseased plants, such as rust-affected grasses, even under pen-feeding.

Feeds that are dusty tend to cause irritation of the nose and eyes of animals and decrease feed intake. Damping these materials increases intake and has important application in developing countries. In northern India farmers always add water to chopped wheat straw (wheat bhoosa) prior to feeding, which appears to increase intake of straw.

Chopping straw into short lengths tends to increase intake of the straw. Fine grinding and pelleting also increases intake of straw but has little applicability in developing countries because of the high energy costs associated with this form of processing.

6.3 ANIMAL FACTORS & FEED INTAKE

6.3.1 Physiological state 

Undoubtedly ruminants increase their feed intake in response to an increase in demand for energy or protein or both. The increased need for energy for exercise and for countering extreme cold stress increases feed intake and flow of digesta through the rumen and the intestinal tract (Orton et al. 1985b; Kennedy and Milligan 1978).

Feed intake is high in:

• Young growing animals and older animals that need to restore depleted body tissue

• Adult ruminants in the last trimester of pregnancy when the foetus is growing most rapidly

• Lactating animals

• Animals undertaking heavy work.

There is now considerable evidence that the expression of maximum feed intake is dependent on an appropriate balance of nutrients in the products of digestion. This is illustrated in Figures 6.3 and 4.17. In high yielding dairy cows fed a diet consisting mainly of concentrates based on maize grain, supplementation with bypass protein increased intake of the basal diet, giving commensurate increases in milk yield (Figure 6.3).  Similar responses were obtained in cows when molasses was substituted by maize grain, which induced major changes in the pattern of VFA production (Figure 4.17). In the former case the responses were apparently induced by increasing the protein-to-energy ratio (P/E) in absorbed products; and in the latter case by an increase in the proportion of glucogenic precursors relative to either energy or protein or both (see Chapter 4).

Figure 6.3  Increasing the percentage of crude protein in the ration of dairy cows increased dry-matter intake and milk yield (Source: Clay and Satter 1979 as modified by Oldham 1980).

It thus appears that nutrient demand is a major stimulus to the ``feeding" centres of the hypothalamus and that, in practical feeding of ruminants, it is nutrient imbalance that primarily limits the level of feed intake and therefore productivity. 

6.3.2 Major factors involved in decreasing feed intake

There are three basic stimuli associated with digestion and metabolism that arise from food seeking and ingestion, and which either solely or in combination inhibit the feeding centres of the hypothalamus and therefore limit feed intake. These are: 

• Absorption and metabolism of nutrients: when the end-products of digestion are imbalanced to meet a particular productive function, there will be an excess of C2-energy which must be expended as heat. The animal reduces its feed intake as a consequence of this imbalance, particularly in hot climates

• Distension of the digestive tract: although distension of the tract can limit intake, this has often been overemphasised and with many feeds deficiencies of nutrients (mainly amino acids) are the first limiting factors

• Fatigue: ruminants become fatigued in seeking, ingesting, chewing and ruminating their feed. In animals given fibrous feeds, twice as much digesta passes through the rumination cycle as is eaten, which implies that rumination may be a major cause of fatigue. Feed intake is restricted by the need to ruminate and the time taken for this.

In animals on high-energy diets, the rate at which VFAs are absorbed from the rumen may limit feed intake. This was demonstrated in sheep by infusing VFAs into the rumen whenever a sheep began to eat. Infusion of VFAs invariably reduced the meal size (Figure 6.4). Infusing butyrate had much less effect on the size of the meal than infusing either propionate or acetate.

Figure 6.4 Injections of VFAs as the animal starts a meal depresses feed intake. Acetate and propionate infusions depressed intake more than butyrate (Source: Baile and Mayer 1970).

Infusion of acetate into the jugular vein had little or no effect on feed intake, showing that it is the absorption of acetate across the rumen wall that causes cessation of feeding in animals on high quality diets.  Thus, on high-concentrate diets the level of acetate in the rumen appears to be a primary ``controller" of feed intake. This requires that there are receptors in the rumen wall that release humoral agents that affect the feeding centres of the hypothalamus.

It appears that increasing the volume of the rumen and therefore reducing the concentration of VFAs in rumen fluid stimulates feed intake on such diets.

6.3.3 Palatability and unpalatability

Grazing ruminants select some pasture plants and leave others. Plants have therefore been described as being more palatable or less so. However, palatability is only demonstrated for short periods if the variety and availability of feed are limited. Even relatively unpalatable materials (in a free-choice feeding system) are eaten when no other feeds are available. Many feeds that would not be selected in a free-choice feeding system can be used as a basal diet (see Chapter 8) and quite high levels of productivity can be achieved provided that the principles of nutrient balance are observed.

The apparent ability of animals to select the more digestible components of a diet is well known. The data in Table 6.1 show marked increases in intake when goats were offered 50% more dry straw than they normally consumed. Similar findings were reported for sheep given oat straw supplemented with urea (Figure 6.5). This effect is important, since in a mixed livestock operation the productivity of small ruminants can be increased by providing them with more straw than they can eat, allowing them to select the more nutritious leaf material, and the residues can be given to mature draught animals.

Figure 6.5  Relationship between amount of fresh feed offered (oat straw plus urea) and feed intake by sheep (Source: J O Iful, F Muyekho, A Tolera, T N Asanji and T R Preston, unpublished data)

Similar selection responses were demonstrated when goats and sheep had access to varying amounts of leucaena foliage (including branches), given as a supplement to a guinea-grass diet. Increases in the availability of tree foliage led to selection of leaves in preference to bark (Figure 6.6).

Figure 6.6 As the amount of leucaena foliage on offer was increased, both sheep and goats selected leaf more than petiole, which was preferred to bark.  The animals also had free access to guinea grass ( Panicum maximum) (Source: Mani 1984).

6.3.4 Effects of exercise on feed intake 

Although there has been little research on the effects of exercise on feed intake, it is anticipated that ruminants will increase their feed intake in response to exercise, even when fed low-digestibility forages.

Horses given a diet containing 6% protein ate less and grew more slowly than horses on a diet containing 12% protein. When energy expenditure was increased, feed intake also increased and the exercised horses on the low-protein diet grew at the same rate as the exercised horses and non-exercised horses on the high-protein diet (Figure 6.7).

It is hypothesised that the energy/protein ratio in the nutrients available for metabolism was altered by expending energy in exercise. This has implications for draught animals, in which the increased feed intake associated with work may result in a better balance of protein to energy (P/E) in the nutrients available for anabolism because some of the energy is used in exercise. 

6.3.5 Animal genotype and voluntary feed intake 

Where nutritional and environmental constraints are absent, potential feed intake is determined by the animal's genetic potential for production. At the same time the amount of feed consumed determines the productivity that is achieved (eg. milk yield is directly related to feed intake). Selection for high yield in dairy cows has in fact led to the selection of animals of large body size with a capacity to consume large amounts of feed. Some Friesian cows are reported to consume up to 6.5% of their body weight and produce at peak lactation in excess of 80 litres of milk per day.

One consequence of this appears to be a high basal metabolic rate in animals selected for high productivity. Such animals are inappropriate in developing countries since the resources available will support only moderate levels of production. On a given feed intake (which may be limited by nutrient imbalance) animals with a higher metabolic rate have much less energy and protein available for production.

Another important characteristic of cows that have been selected for high milk yields in temperate countries is their capacity to mobilise body reserves and to partition nutrients directly into milk production rather than to maintenance of body tissue and reproductive function (Figure 6.8).

Figure 6.8  The partitioning of nutrients in Friesian cows of high (``good") and low (``bad") genetic merit for milk production. Both animals were fed the same diet (Source: Bines and Hart 1978).

The failure to ingest sufficient nutrients may be due to one or a combination of two factors that commonly occur in developing countries. These are: 

• Heat stress resulting from the animal's inability to transfer sufficient of the heat produced by digestion and metabolism from its body to the surrounding hot and humid atmosphere. The immediate reaction on the part of the cow is to stop eating

• The type of diet offered, which may be based on fibrous crop residues.

An animal in this situation may produce a reasonable lactation yield by mobilising body reserves---this will depend on the extent of the under-nutrition---but, because of the resultant poor body condition she is more susceptible to disease and she is also likely to be anoestrous for at least the period of the lactation. The resulting longer calving interval transforms the perhaps respectable lactation yield into a poor annual milk yield and therefore a poor lifetime performance.

Cows with some Bos indicus genes may have greater tolerance to some of the diseases that are endemic in developing countries and may also be able to disperse more body heat than cows with a purely Bos taurus genome. As a result, the former are usually much more productive than the latter in the long-term, although their lactation yields may appear to be relatively modest.

The differences in voluntary feed intake among ruminant species are related to basal metabolism. Intake is also affected by the interaction between the balance of nutrients in the absorbed products of digestion and environmental factors.  Although Bos indicus cattle have a lower basal rate of metabolism, and therefore lower potential productivity (Frisch and Vercoe 1979), they do not reduce their feed intake as much as Bos taurus cattle when subjected to stress brought about by poor nutrition, disease or heat. In stressful environments, they can out-produce the ``temperate" breeds (Frisch and Vercoe 1979).

Within the Bos taurus breed, and also among buffalo, there are individuals with a high capacity for growth and milk production. Exceptional animals have been reported to have peak lactation yields of up to 25 litres/day or more and an annual milk production of upto 3000 litres. It is not known whether these animals combine their high voluntary feed intake with a low basal rate of metabolism and still maintain their resistance to stress. One possibility is that they are able to adjust their basal metabolic rate: to increase it under favourable conditions but to allow it to fall in periods of high nutritional stress. 

6.3.6 Rumen distension and nutrient imbalance 

The amount of feed eaten by sheep and cattle is often determined by the rate of absorption of the soluble components of the feed and the rate of passage through the rumen of both soluble and insoluble particulate digesta. The composition of a diet determines an animal's voluntary intake. Feed intake is reduced by nutrient imbalance. The first limiting nutrient may be ammonia in the rumen or essential amino acids in the animal. Supplementing the diet with bypass protein often increases the intake of feeds that are deficient in protein (Leng et al. 1977). The degree to which intake is increased by bypass protein supplements depends upon the digestibility of the feed and environmental temperature.

Ruminants eat less straw than hay, and less mature pasture than immature pasture. The intake of these feeds can, however, be modified by supplements of bypass protein together with fermentable nitrogen (Chapter 7). The effects on intake of so-called 'quality of a hay' were demonstrated in early work by Blaxter and his colleagues (Table 6.2) and attributed to differences in digestibility.

That digestibility may not be the first constraint to feed intake is demonstrated by a number of studies (eg. see Table 6.3). The studies of Lindsay and Loxton (1981) on the effects of supplements on the intake of mature grass hay indicate that nutrient imbalance is the primary limitation to feed intake. In animals given a 70% digestible diet based on sugarcane pith, the effects of supplements on intake of the basal diet were directly related to animal performance (Figure 8.14).

Ruminants must produce more heat when nutrients are imbalanced. In particular, ruminants consuming diets that support a high acetate fermentation (low propionate), and which are also low in protein, may have insufficient glucose to meet the obligatory needs of fat synthesis, and be forced to produce considerable heat through metabolic regulation. This would have a major effect in hot or hot/humid areas, where an animal would already have difficulty maintaining body temperature, the net result beiing a depression in feed intake. In temperate countries, the high heat production could be beneficial in countering cold stress. However, it is apparent that ruminants under tropical conditions would require more glucose than in temperate countries in order to use efficiently their diet.

These contrasting interpretations illustrate the confusion that is brought about by referring to low-digestibility and/or low-N forages as 'low quality' forages. More often than not the limitation to intake of these forages is nutrient imbalance and the influence of low digestibility only becomes apparent when the nutrients have been balanced. The data in Table 6.4 show that the feed intake and nitrogen balance of sheep fed a basal diet of teff hay increased in response to supplementation with legume, the digestibility of which was similar to that of the basal diet. This demonstrates that often, where low intake of a forage is attributed to distension of the rumen, it is more likely to be related to a limiting nutrient.

In ruminants the association between low intake and low digestibility of feed is due to the slow rate of digestion in the rumen and therefore the slow rate of breakdown of feed particles.  Rumen fungi may play an important role in this process. It has been demonstrated that when a large population of fungi colonises fibre the tensile strength of the fibre decreases, which may accelerate the breakdown of the feed particularly during rumination (Akin et al. 1983). 

6.3.7 Fatigue 

Fatigue may be very important in controlling appetite and therefore productivity of ruminants on diets of crop residues, dry pastures or fibrous agro-industrial byproducts. When a large amount of muscular effort is needed to ingest the feed, fatigue-induced stimuli can inhibit intake. The biological basis of fatigue is obscure, but the continuation of any physical activity eventually results in fatigue. With grazing animals, as pasture density decreases the rate of biting increases as does the time spent grazing (Allden 1962) but apparently only upto a maximum of 13h/d. Fatigue probably contributes to limiting grazing time (McClymont 1967). In animals on low-digestibility feeds in which there is no nutrient imbalance, ruminal distension and fatigue are probably the major stimuli that interact to inhibit appetite.

The various factors that contribute to appetite control can be represented as a balance of facilitatory and inhibitory stimuli (Figure 6.9, modified from McClymont 1967).

Figure 6.9  The balance of factors that influence voluntary feed intake in ruminants.

 6.3.8 Acetate clearance and feed intake control

Ruminants have to match the rate of absorption of VFAs from the rumen with their utilisation in metabolism. Weston (1966) showed that acetate clearance (see Chapter 4) was highly correlated with the intake of lucrene chaff by sheep. Undoubtedly, the clearance rate of acetate is affected by the balance of nutrients available, particularly the ratio of acetate/propionate and acetate/amino acids (see Chapter 4, Figure 4).  Therefore the effects of the balance of nutrients on feed intake may be mediated by the ability of the animal to clear acetate at a sufficient rate to maintain blood levels within physiological limits. It can be hypothesised that acetate clearance is a major integrating factor that ultimately allows the expression of feed intake, since in general the environmental and physiological factors that increase feed intake increase the rate of metabolism of acetate (eg. work and cold stress, lactation). The factors that generally decrease feed intake are either associated with an increased acetate availability relative to other nutrients (eg. low N diets) or a decreased utilisation of acetate relative to other nutrients (eg.  heat stress).

The increase in feed intake in cold stressed animals is associated largely with an increased contraction rate and movement of digesta from the rumen (Kennedy et al. (1986).

Even if acetate clearance is only associative and not causative it still appears to be the metabolic parameter which is most highly correlated with feed intake (Weston 1966). It is difficult, therefore, to ignore acetate clearance in this respect as intake generally is one of the best indices of feed quality and digestibility.

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