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Resumen
Growing, finishing, and metabolism trials were conducted with heifers to evaluate estimates of microbial CP (MCP) supply from allantoin in spot urine samples. Dietary treatments were expected to create different MCP flows in each trial. In the growing trial, three supplemental protein sources differing in degradability of protein were fed with and without steep liquor (SL). Inclusion of SL did not change protein efficiency with AminoPlus ® and Soypass® being higher than soybean meal (SBM). Estimates of MCP and microbial efficiency were increased with inclusion of SL and increased with increasing level of supplemental protein. In the finishing and metabolism trial, corn bran inclusion and supplementation of SBM instead of urea in high-moisture corn diets were expected to increase MCP efficiency and flow. Corn bran and SBM only impacted performance early in the feeding period for finishing heifers. Estimates of MCP and microbial efficiency were unaffected by corn bran or SBM addition with high coefficients of variation for individual heifers. For heifers in the metabolism trial, microbial efficiency and MCP flow were lower for the control diet than diets containing corn bran or SBM. There was little diurnal variability in MCP estimates and day to day variability was due to changes in digestible intake. Microbial efficiency was well correlated with ruminal pH measurements, and MCP estimates followed estimates from the NRC model. Results of these trials indicate a high degree of individual animal variability for estimates of MCP; however, diurnal and day to day variability appears to be small. Additionally, estimates for MCP flow and efficiency were feasible and related to performance and digestion parameters. More work is needed to obtain accurate and precise values, but spot sampling of urine shows promise as a non-invasive technique to estimate microbial flow and efficiency.
