| Three anaerobic digestion trials were conducted to evaluate methane production from feedlot manure. As organic matter content of manure decreased, degradation of manure and methane production also linearly decreased (P ≤ 0.02). Quality, or organic matter content of manure, had a greater impact on anaerobic digestion than diet (with and without distillers grains) fed to cattle. Minerals in manure are concentrated in effluent removed from anaerobic digesters.;Knowing mineral retention within cattle allows for more accurate calculation of mineral excretion in manure. Three serial slaughter trials measured mineral retention in growing and finishing cattle. During a growing phase, beef cattle gaining 1.10-1.18 kg/d retained 6.8 g Ca, 3.9 g P, 0.24 g Mg, 1.2 g K, and 0.73 g S/100 g protein gained. Retention of minerals during the growing phase was not affected by diet fed to cattle (P ≥ 0.20). Finishing beef cattle, gaining 1.63-2.02 kg/d retained 7.9-17.3 g Ca, 3.2-6.2 g P, -0.027-0.41 g Mg, 0.48-2.9 g K, and 0.29-1.9 g S/100 g protein gained. Retention of Mg, K, and S during the finishing period was greater (P ≤ 0.02) for cattle grown slowly (< 0.6 kg/d) during the growing phase. Retention of minerals was also measured in Holstein steers serially slaughtered every 28 d after 226 days on a finishing ration. Mineral retention was not affected by addition of zilpaterol hydrochloride in the finishing ration when retention was expressed relative to protein gain (P ≥ 0.14). Retention of Ca, P, Mg, K, and S linearly decreased (P < 0.01) over days on feed when expressed as g/d. Expressing retention relative to protein gain resulted in no differences across days on feed (P ≥ 0.11) averaging 14.4 g Ca, 7.5 g P, 0.45 g Mg, 1.3 g K, and 1.0 g S/100 g protein gained. In all trials, expressing mineral retention relative to protein gain allowed for better comparisons of mineral retention across a range of weights and ADG. |
