Fat source and calcium level effects on finishing steer performance, digestion, and metabolism

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Abstract
A 111-d finishing study evaluated animal growth and carcass characteristics using 138 steers (366 kg) in a randomized complete block design with a 2 × 3 factorial arrangement of treatments. The dietary treatments consisted of no supplemental fat or 3.5% tallow or soybean oil soapstock (SS) fed with .6% and .9% dietary Ca. Fat increased DMI (P < .05) but interacted with Ca level (P < .05) for gain/feed and ADG. All diets containing fat or .9% Ca were converted more efficiently to gain man the .6% Ca, no supplemental fat diet (P < .05). The .9% Ca interacted with fat source to decrease gain (P < .05) and tended to decrease efficiency in the tallow diet but improved efficiency (P < .05) and tended to improve gain in the no-fat diet. In the SS diet, .9% Ca had no effect on ADG, DMI, or efficiency of gain. Fat addition increased backfat (P < .10) and interacted with Ca on hot carcass weight, final weight, and dressing percentage (P < .05). Feeding fat increased the proportion of 18:0 (P < .02) and decreased the proportion of 16:1 fatty acids (P < .06) in intermuscular fat. A replicated 3 × 3 Latin square design, using six Holstein steers (349 kg) fed three diets, with no supplemental fat or 3.5% SS or tallow with 1.0% Ca, was used to explore the effects of fat sources when fed with high Ca on digestion and metabolism. Ruminal fluid pH was higher (P < .10) when steers were fed fat. Adding fat did not affect (P > .10) duodenal or ileal pH, VFA proportions or total concentration, or ruminal liquid volume or flow rate. Liquid retention time was shorter and liquid rate of passage was higher (P < .05) with dietary fat addition. Adding fat did not affect site or extent of starch or DM digestion. There was net synthesis of 16:0, 18:0, and 18:1 fatty acids in the rumen. When steers were fed tallow, synthesis of 16:0 and 18:0 fatty acids in the rumen was lower (P < .10) than when steers were fed SS. Feeding fat tended to decrease (P = .11) bacterial N flowing at the duodenum but did not affect nonbacterial N or total N. Fat addition seems to affect ruminal kinetics, and the effects may vary with fat source, particularly relative to fatty acid synthesis and digestion.

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