Evaluation of the ruminal effects of supplemental fat in lucerne hay and maize silage rations using rumen evacuation and ruminal metabolite concentrations

Four mature, rumen-cannulated Hereford × Brahman yearling steers (455 kg) were used in two 4 × 4 Latin square design experiments. Dietary treatments consisted of 3.5 kg per animal day−1 (dry matter basis) of maize silage (Experiment 1) or medium quality lucerne hay (Experiment 2) supplemented with either a maize-soyabean meal control (CON), 5% added fat as rolled whole soyabeans (WSB), 10% added fat as rumen-protected fat (RPF), or 5% added fat as WSB + 5% added fat as RPF (WSB-RPF). After 9-day dietary adjustment periods, the rumen of the steers were completely evacuated 24 h after feeding and samples of rumen liquor collected 3 and 24 h after feeding. In both experiments, steers fed CON generally had less total dry matter (DM) and fiber components remaining in the rumen 24 h after feeding than steers fed supplemental fat. However, the degree of apparent inhibition of ruminal fermentation seemed to depend upon forage type. Steers fed maize silage supplemented with fat (Experiment 1) had 43% more DM remaining in the rumen compared with those supplemented with CON. In contrast, steers fed lucerne hay supplemented with fat (Experiment 2) had only about 5% more DM remaining than those supplemented with CON. Furthermore, although the acetate:propionate ratio was reduced when fat was added to maize silage rations, supplemental fat seemed to have little effect on the acetate:propionate ratio when steers were fed lucerne hay. The RPF source used in these experiments seemed to be only partially inert and negatively affected ruminal fermentation. Based on rumen evacuation and ruminal acetate:propionate ratios, when WSB and RPF were fed together (WSB-RPF) the deleterious effects of RPF on rumen fermentation seemed to decline. Overall, the data are interpreted to suggest that single-time-point complete rumen evacuation represents a viable means for assessing the effects of source and level of supplemental fat on ruminal fermentation.