Fat supplementation in animal production—ruminants

Abstract

In 198 I, 204 ooo tonnes of fat were used by the UK Compound Food Industry in the production of foods for farm animals (Ministry of Agriculture, Fisheries and Food, 1982). Of this, it has been estimated that 65000 tonnes (Meggison, 1982) were used in the production of ruminant rations (mainly for the dairy cow), the remainder being used for pigs and poultry. Since approximately 5 million tonnes of food were produced for ruminants, this is an average rate of addition of 13 g/kg diet, although, of course, this average conceals very wide variations. Is a further increase in the use of fat desirable? There are basically two reasons for the addition of fat to the diet of the ruminant animal, first, to act as an energy source and, second, to influence the composition of the final animal product in a particular desired direction. With the assistance of the microbes in the rumen, the ruminant is able to digest forage materials with a high cellulose content. Many of these materials are of relatively low digestibility and, as a result, the food intake of the animal is often limited by the bulk of the food rather than by its energy content, as is the case in simple-stomached animals (Baile, 1979). Therefore, there may be cases in which it is desirable to increase the mean concentration of metabolizable energy (ME) in the food and the easiest way to do this is to add fat to the diet. For example, a 600 kg dairy cow eating 9 kg/d of silage dry matter (ME content 1 1 MJ/kg) and 9 kg/d of concentrates (ME content 13 MJ/d) should give a milk yield of 28.3 l/d with a fat content of 4% and a solids-not-fat content of 8.5%. If 0.5 kg of fat is substituted for 0.5 kg concentrates, the yield of the same milk should increase to 31.0 l/d; i.e. by approximately 9%. This calculation is based on assumptions that the addition of fat does not affect the total food intake or the digestibility or metabolizability of the basal diet and that the ME content of the fat is 34 MJ/kg. It is proposed to discuss each of these assumptions in turn. Food intake Johnson & McClure (1973) investigated the effect of adding either animal fat (HEF) or maize oil directly to grass at the time of ensilage: in some silages, 10 g ground limestone/kg were also added. Without limestone, the addition of up to 120 g HEF/kg reduced the voluntary intake by up to 27% in sheep and up to 5570 in steers. When limestone was added, the reduction in intake was less at the lower levels of addition of HEF but the same at the higher levels and, overall, the addition of limestone did not affect the food intake. When maize oil was added at