Diet by supplemental fat interactions in dairy cattle

A series of studies were conducted in an attempt to more clearly identify diet by supplemental fat interactions in dairy cattle. The objectives were: (1) to investigate the effects of two fat sources with different degree of unsaturation on rumen fermentation and performance of dairy cows when corn silage (CS) is the sole forage source; (2) to determine if replacing CS made from high-yielding grain hybrids with alfalfa silage will alleviate the negative effects of supplemental tallow on rumen fermentation, dry matter intake (DMI) and milk fat percentage; (3) to investigate if the length of chop of processed CS influences the impact on supplemental tallow on cow performance; and (4) to determine if the alfalfa forage preservation method (silage vs. hay) or particle length affects on DMI, milk fat production and formation of trans fatty acids in the rumen when feeding CS-based diets supplemented with tallow.;Experiment 1 used 15 cows in a replicated 5 x 5 Latin square and treatments were 0% fat (control), 2% tallow (T), 2% choice white grease (CWG), 4% T, and 4% CWG with 50% CS (DM basis). Cows fed supplemental fat had lower DMI and produced less milk fat than control cows, regardless of the source and amount of fat fed.;In experiment 2, 18 cows were assigned to a replicated 6 x 6 Latin square design in a 2 x 3 factorial arrangement of treatments with 0 and 2% T and 3 forage treatments: (1) 50% DM as CS; (2) 37.5% CS and 12% alfalfa silage (AS); (3) 25% CS and 25% AS. Replacing CS with AS partially alleviated the negative effects of tallow on DMI and milk fat yield.;Experiment 3 employed 16 cows in a replicated 4 x 4 Latin square design with treatments arranged in a 2 x 2 factorial design with 0 and 2% T and CS harvested either at 19- or 32-mm theoretical length of cut. Tallow supplementation had a negative impact on performance regardless of the CS particle length.;In experiment 4, 15 cows were used in a replicated 5 x 5 Latin square design. Treatments were (DM basis): (1) 50% CS without T; (2) 50% CS and 2% T; (3) 25% CS, 25% AS and 2% T; (4) 25% CS, 25% chopped alfalfa hay (AH) and 2% tallow; and (5) 25% CS, 25% long-stem AH and 2% T. Cows fed CS plus T produced the least milk fat. No effect of alfalfa preservation method was observed on cow performance. Feeding chopped AH appears to be more beneficial than long-stem AH in supporting milk fat production.;These results suggest that negative effects of tallow on cow performance are not unique to corn silage-based diets. Tallow-induced milk fat depression could not be totally reversed by the addition of AS to the diets. Inclusion of alfalfa either as silage or hay seems to affect the response to supplemental tallow similarly. Alfalfa hay but not corn silage particle length appears to affect the milk fat yield response to supplemental tallow.