Food intake in ruminants

After Blaxter K L, Wainman F W and Wilson R S 1%1 Animal Production 3 51. 

Measures Species Form of roughage Percentage difference 

Adapted from Greenhaigh J F D and Reid G W 1973 Animal Production 16 223. 

The breakdown of food particles in the rumen and its effect on intake has been the subject of much research. In practice it is possible, as explained above, to overcome the resistance of plant cell walls by mechanical or chemical treatment, but the processes involved are expensive, may have undesirable side effects (e.g. mechanical treatment reducing digestibility), and cannot be applied to grazed forages. In the longer term the aim is to identify new forage species or breed new varieties that are broken down more rapidly in the rumen. 

There are some foods that are eaten in lesser quantity than would be expected from their digestibility or cell wall content. These include some types of silage, 

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Formulation of a ration or diet requires knowledge of the nutrient requirements of the animal (discussed in this part) and the nutritional value of the foods (discussed in Part 3) and, in order to combine these two, the amount of foods the animal can consume. Therefore, Chapter 17 gives details of factors affecting food intake in both monogastrics and ruminants and the methods used to predict food intake. 

Provenza, F.D. 1995. Postingestive feedback as an elementary determinant of food preference and intake in ruminants. J. Range. Manage. 48 2-17. 

Much of the safety support comes from the fact that CIA is already consumed in the human diet, especially in ruminant derived products, without apparent adverse effects. However, the isomeric composition of naturally occurring CLA is different from the CIA available as food supplements. Ruminant meat and dairy products contain mainly the cis-9,trans- 1 CLA isomer, whereas commercial CLA preparations generally contain equal proportions of the cts-9,trans- and the trans-l0,cis-l2 CLA isomer. There is a growing body of evidence that these two isomers have very distinct biological functions (1). In addition, the intake of CLA from natural dietary sources was calculated to range from 150—400 mg/day (2, 3), whereas the recommended intake for commercial CLA supplements varies from 1 to 3.4 g/day. The safety assessment of CLA based on the natural occurrence in the diet is therefore limited. 

The practical question that logically follows this discussion of dietary CLA and human health is the following What is the daily consumption of CLA when American adult humans consume a typical diet Because CLA occurs in much greater amounts in animal-derived than in plant-derived foods and because foods derived from ruminant animals have greater concentrations of CLA than do foods from nonruminant animals, the blend of foods in the diet from plant and animal sources directly influences CLA intake. The principal isomer that occurs in animal-derived foods is cis-9,trans- CLA. Most of the CLA in human plasma and tissues is thought to be derived from the diet the amount synthesized within humans who consume typical diets is less significant (93). Because the CLA content of different foods is highly variable (ruminant vs. nonruminant sources and plant vs. animal sources) and because environmental conditions influence the CLA content of foods, the estimation of typical CLA intakes by humans is problematic. 

The synthesis by rumen microorganisms of all members of the vitamin B complex and of vitamin K has already been mentioned (see Chapter 5). In ruminants receiving foods well supplied with B vitamins, the amounts synthesised are relatively small, but they increase if the vitamin intake in the diet decreases. The adult ruminant is therefore independent of a dietary source of these vitamins, but it should be remembered that adequate synthesis of vitamin B 2 will take place only if there is sufficient cobalt in the diet. 

Many nutrient deficiencies influence fertility indirectly, through their effects on the general metabolism of the animal. For example, phosphorus deficiency in grazing ruminants, which has often been associated with poor fertihty, appears to affect reproduction because it restricts many metabolic processes, hence food intake and the general plane of nutrition. However, there is also some evidence that phosphorus deficiency has a direct effect on reproduction through suppressing oestrous cycles. 

Ill-health can reduce the intake of both ruminants and non-ruminants. Contrary to popular opinion, infestations of gastrointestinal parasites tend to reduce intake, presumably because the interference with digestive function overrides any metabolic stimulus arising from a reduction in the absorption of nutrients. There is also evidence that stimulation of the animal s immune system, as happens with parasite infestations, may be responsible for a reduction in food intake. Infestations with external parasites, such as ticks, also reduce intake. 

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