Abstract
Abstract The objective of this study was to evaluate effects of feeding supplemental isoacids in a growing diet on parameters of rumen fermentation and in situ DM disappearance. Eight mature ruminally cannulated beef heifers (Angus crossbred; mean BW 618 ± 31 kg) were used in a replicated 4 x 4 Latin square design. Heifers were housed in individual pens and fed the following treatment diets: no supplemental isoacids (ISO) in the basal diet (Control); 7.2 g of 2-methylbutyric acid· animal-1· day-1 (ISO1); 7.2 g of 2-methylbutyric + 7.2 g isobutyric acids· animal-1· day-1 (ISO2); and 7.2 g 2-methylbutyric + 7.2 g isobutyric + 7.2 g isovaleric acid· animal-1· day-1 (ISO3). Basal diet consisted of (DM basis) 26.9% wheat hay, 35.5% steam flaked corn, 22.6% wet corn gluten feed, 12.3% DDG, and 2.7% mineral/vitamin supplement. Each experimental period lasted 28 d, with 21 d for diet adaptation and 7 d for data and sample collection. On day 1 of each sampling period ruminal fluid was collected 2 hours before and 1, 3, 7 and 10 hours after morning feeding. The pH of ruminal contents was immediately measured with an electrode pH meter. Urine samples (approximately 200 mL) were collected on day 2 of each sampling period 4 h after the feeding and filtered to remove particles. Urine was analyzed for creatinine, uric acid, and allantoin. Bags were placed into rumen for 72, 48, 36, 24, 12, 8, 4 and 0 hours and all bags were removed at same time on the end of incubation (hour 0), rinsed with tap water and dried at 55°C. Data were analyzed using the MIXED procedure of SAS. A contrast was observed (P = 0.029) for overall rumen pH between control versus isoacid treatments (6.44 vs 6.31). Uric acid, allantoin, total purines and intestinal flow of microbial nitrogen were not affected by treatments (P ≥ 0.10) with overall means of 10.47, 328.80, 339.27 mmol/d and 246.64 g N/d, respectively. Overall, no effects were observed for in situ dry matter disappearance (P ≥ 0.10). However, at hour 4 differences between ISO2 and ISO3 (P = 0.02; 44.89% vs 38.45%) and ISO2 and ISO4 (P = 0.05; 44.89% vs 39.75%) were observed. No effects of in situ DM disappearance was observed (P ≥ 0.10). Supplementing beef heifers with isoacids may reduce rumen pH as a result of increased ruminal volatile fatty acid production; however, supplemental ISO in this study did not affect microbial nitrogen synthesis or dry matter in situ disappearance. Further analyses are currently in progress.
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