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Nitrogen excretion from beef cattle for 6 cover crop mixes as estimated by a nutritional model
The objective of this study was to predict fecal and urinary N excretion for different maturities of beef cattle using model simulations based on compositional analysis of cover crop mixes. Two replicates of six forage mixes containing differing legumes with and without rape were grown under dryland conditions at Dakota Lakes Research Farm near Pierre SD in 2010 and 2011. Samples were analyzed for DM, CP, soluble CP, ADF, NDF, acid detergent insoluble CP, neutral detergent insoluble CP, lignin, starch, simple sugars and crude fat. Data was entered into the Large Ruminant Nutrition System model (LRNS) for estimation of N excretion. Two animal scenarios were analyzed: a non-lactating, pregnant beef cow and replacement beef heifer. Dry matter intake was set to 2.33% of BW for all simulations. The experiment was a split-plot arrangement of a randomized complete block with animal type and cover crop mixture as the whole plot and harvest date as the sub-plot. Forage mix and harvest date were treated as fixed factors and replications were considered random. Student’s t-test was used to separate mean effects when an F test was significant (P=0.05). Crude protein concentrations of forage mixes were always at least 14% but CP content was affected by month within year (P < 0.01). Predictions of N utilization, except kg/d of fecal N excreted, were affected by forage mix. Fecal N excretion ranged from 33.6 to 43.7% and urinary excretion from 52.5 to 58.8 % of N intake. Predicted N intake varied by month within year due to varied CP concentration of the mixes. Differences in N intake resulted in difference in the amount of predicted N excreted (kg/d) for cows, but not heifers. The predicted percentage of N intake excreted in feces did not differ by animal maturity. There was an animal maturity by month within year interaction for urinary excretion both when expressed as total kg/d excreted and percentage of N intake. Urinary N excretion varied from a low of 43.8 % of intake for heifers to a high of 66.4% for cows. These types of estimates may be useful to make assessments about N flows in crop-livestock systems.
Keywords:
forage, nitrogen, beef cattle