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Abstract
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.
