INTRODUCTION
Ruminant animals fed conventional diets typically consume limited amounts of fat. Nevertheless, induction of essential fatty acid deficiencies in ruminants has been difficult to demonstrate (Palmquist et al., 1977). The ruminant’s ability to make very efficient use of essential fatty acids (Noble, 1984) has likely contributed to relatively modest interest in lipid nutrition of ruminants. Because of its caloric density, the primary function of fat in diets consumed by ruminants is to provide energy (NRC, 2007).
Interest in the area of supplementing ruminant animal diets with fat has increased over the past decade. To illustrate this point, we listed the manuscripts published in the Journal of Animal Science in which investigators reported outcomes of fat supplementation for ruminants since Dr. Hess began his career at the University of Wyoming (Table 1). The average number of papers published annually from July 1996, through December 1999, nearly doubled from January 2000, through July 2007. The lowest number of manuscripts published on the topic since the turn of the decade was 9, which was comparable to the greatest number of papers published in a single year among the previous 3.5 yr. The increase in interest noted above stemmed from the concept that manipulation of ruminant animal diets via supplementation with lipid is a means to influence a variety of physiological processes or alter fatty acid composition of food products derived from ruminant animals. The primary objective of this review is to summarize research conducted by the authors on the inclusion of fat in diets fed to various classes of ruminant livestock.