Genetic variation in alfalfa for ruminal protein degradability

Abstract

Protein in alfalfa is extensively degraded in the rumen and is often wasted due to excessive ammonia formation. The objective of this study was to determine if there are genetic differences in ruminal degradability of alfalfa forage protein. Two replicates of field-grown forage were taken at each of two different cuttings from 22 alfalfa entries (Medicago spp.), including 19 of M. sativa plus three of M. falcata (total 88 samples). The forage was analyzed for acid detergent fiber (ADF), total N and acid detergent insoluble N (ADIN), and for degraded N at 0-h, potentially degradable protein, rate of protein degradation, and net protein escape (total escape minus ADIN) using a ruminal inhibitor in vitro system. There were no differences in ADF, total N or ADIN among the 22 alfalfa entries. Significant differences among entries were detected for the four parameters determined using the in vitro system: N degraded at 0-h (P < 0.002), degradable protein (P < 0.0025), degradation rate (P < 0.001), and net protein escape (P < 0.001). There were trends for the 19 entries of M. sativa to have larger degraded N fractions and smaller degradable protein fractions than the three entries of M. falcata. The 19 entries of M. sativa germplasm had degradation rates ranging from 0.21 to 0.25 h−1, whereas those for M. falcata averaged 0.18 h−1. Estimated net protein escapes from the rumen averaged 171 and 212 g kg−1 total nitrogen for M. sativa and M. falcata, respectively (24% greater for M. falcata). Estimated net protein escapes for the three M. falcata entries were greater than 18 of 19 M. sativa entries. The consistency of these differences suggests that conventional plant breeding techniques could be used to develop lines of alfalfa forage with improved protein utilization in ruminants. Key words: Alfalfa, Medicago sativa L., Medicago falcata L., ruminal protein escape