A decade of developments in the area of fat supplementation research with beef cattle and sheep.

Supplementing ruminant animal diets with fat has been investigated as a means to influence a variety of physiological processes or to alter fatty acid composition of food products derived from ruminant animals. Several digestion experiments have been conducted with beef cattle and sheep to elucidate the effects of supplemental fat on utilization of other dietary components. Negative associative effects are not likely to be observed in ruminants consuming forage-based diets with supplemental fat at < or = 2% of DMI. Inclusion of supplemental fat at < or = 3% of DM is recommended to obtain the most benefit from the energy contained within the fat and other dietary components in high-forage diets. For ruminants fed high-concentrate diets, supplementing fat at 6% of diet DM is expected to have minimal impacts on utilization of other dietary components. Although there is greater potential to supply the ruminant animal with unsaturated fatty acids from dietary origin if fat is added to high-concentrate diets, incomplete ruminal biohydrogenation of C18 unsaturated fatty acids results in an increase in duodenal flow of 18:1 trans fatty acids regardless of basal diet consumed by the animal. The biohydrogenation intermediate 18:1 trans-11 (trans-vaccenic acid) is the likely precursor to cis-9, trans-11 CLA because the magnitude of increase in CLA content in tissues or milk of ruminants fed fat is much greater than the increase in CLA presented to the small intestine of ruminants fed fat supplements. Duodenal flow of trans-vaccenic acid is also substantially greater than CLA. Increasing unsaturated fatty acids status of ruminants imparts physiological responses that are separate than the energy value of supplemental fat. Manipulating maternal diet to improve unsaturated fatty acid status of the neonate has practical benefits for animals experiencing stress due to exposure to cold environments or conditions which mount an immune response. Supplementing fat to provide an additional 16 to 18 g/d of 18:2n-6 to the small intestine of beef cows for the first 60 to 90 d of lactation will have negative impacts on reproduction and may impair immune function of the suckling calf. Consequences of the suckling animal increasing its intake of unsaturated fatty acids because of manipulation of maternal diet warrants further investigation.