Graduate Thesis Or Dissertation
 

Influence of supplement composition on utilization of low-quality, cool-season forages by beef cattle

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  • Two studies were conducted to evaluate the influence of supplement composition on intake and digestibility of a low-quality (< 6% CP), cool-season forage, as well as cow performance. Treatments included a non-supplemented control (CON), corn (approximately 8% CP), corn and urea (LU = corn + 0.09 mg/kg BW urea, approximately 27% CP; HU = corn + 0.17 mg/kg BW urea, approximately 43% CP) and a positive control of SBM (approximately 51% CP). In Experiment 1, 5 ruminally cannulated Angus x Hereford steers (560 ± 79 kg of BW) were used in an incomplete 5 x 4 Latin square with four 28-d periods to compare the effects of urea addition to a corn-based supplement on forage intake, digestibility and ruminal fermentation characteristics. Forage intake and digestibility were not influence by supplementation (P > 0.10); however, intake was greater for SBM than HU (P = 0.01). Ruminal NH₃-N increased with supplementation (P < 0.01), increased linearly with urea inclusion (P < 0.01) and was greater for HU than SBM (P < 0.01), However, ruminal NH₃-N for non-supplemented steers was 1.61 mM, within the range believed to support optimal growth of rumen microbes in vivo, suggesting that ruminally available-N was not limiting forage utilization. Total volatile fatty acid concentration was not influenced by supplement composition (P > 0.10). In Experiment 2, 80 late gestation (approximately 190 d pregnant) Angus x Hereford cows (507 ± 10 kg) were stratified by age, BCS, and BW and randomly allotted to the treatments described in Experiment 1 (20 pens; 4 cows/pen; 4 pens/treatment). Cow BW and BCS change were improved with supplementation (P < 0.01) and with increasing urea inclusion (P < 0.01), but did not differ between the HU and SBM treatments (P > 0.10). Cow BUN (P = 0.05), glucose and NEFA were not influenced by supplementation (P > 0.10); supplementation increased IGF-I (P < 0.01) and tended to increase insulin (P = 0.07). Blood variable concentrations did not differ between HU and SBM. These results suggest that a starch-based energy supplement fed at less than 0.5% of BW in conjunction with urea is an acceptable management alternative to supplementation with natural protein for ruminants consuming low-quality, cool-season forages.
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