In vitro ruminal undegradable proteins of alfalfa cultivars

The quality of alfalfa would be greatly improved by an increase in its ruminal undegradable protein (RUP) concentration. Protein degradation rate (PDR), in vitro dry matter digestibility (IVDMD), leaf weight ratio (LWR), dry matter yield (DMY), total nitrogen (TN), in vitro RUP (expressed on both TN, RUP-TN, and dry matter basis, RUP-DM), acid detergent fiber (ADF), and neutral detergent fiber (NDF) concentrations were determined in 27 alfalfa cultivars. Cultivars were seeded in triplicate on 2 consecutive years and evaluated during the 2 subsequent production years with two harvests per year. Protein degradation rate and RUP-TN were determined using a ruminal inhibitor in vitro system. Data were averaged for spring growth, summer regrowth, and both harvests across 2 production years. Each of the three data sets was analyzed by ANOVA followed by a principal component analysis (PCA) on the ANOVA means. For the four-harvest data, cultivar differences were highly significant (P < 0.001) for all variates except for PDR (P = 0.07) and PUP-TN concentration (P = 0.010). The first PCA axis was largely defined positively by RUP-DM, IVDMD, TN, LWR, and RUP-TN, but negatively with ADF, NDF, PDR, and DMY. The second PCA axis defined a contrast between PDR versus RUP-TN, DMY, ADF, and NDF. Five cultivars were distinctive with high or low PCA scores in all three PCA. Rangelander and Heinrichs, along with Ultra, had low PDR; the first two cultivars had low DMY whereas Ultra was a medium-yielding cultivar. In contrast, Algonquin and Oneida VR had high PDR and medium DMY. While the first principal component (PC) indicated a general trend that low PDR and high RUP were associated with low-yielding cultivars, the second PC identified specific cultivars with both low PDR and high DMY. Therefore, selection for low PDR and high DMY is feasible.