Relationship between in situ rumen protein degradability and chemical composition of alfalfa hays

The variation in protein degradation kinetics and effective degradability of ten alfalfa hays was studied to evaluate the prediction of these parameters from chemical composition of the hays. The alfalfa hays were selected on the basis of the stags of maturity to obtain a wide range of variation in their chemical composition. Rumen degradation of crude protein (CP) was measured by the nylon bag technique on four cannulated wethers fed one of the tested alfalfa hays: in two equal meals at an intake level of 80 g dry matter.kg(-1) LW0.75. Four of the studied hays with different vegetative state were mordanted with chromium (Cr) and rumen outflow rates determined from faecal Cr concentrations after single dose. The neutral detergent insoluble nitrogen (NDIN) showed a wide variation among hays, from 6.2 to 29.8 % of total N. In six of the ten hays the existence of a lag time was verified with values ranging from 2.70 to 6.75 h. The length of this period was positively related to the NDIN content of hays (R-2 = 0.57; P < 0.05). The NDIN was also found to be the best predictor of effective degradability of protein (R-2 = 0.77; P < 0.001), which ranged from 61.1 to 74.7 %. The soluble and insoluble potentially degradable protein fractions represented from 35.9 to 48.1 % and from 38.7 to 55.2 % of the total CP, respectively. Values of the degradation rate were highly variable among hays, ranging from 0.061 to 0.151 h(-1). The non-degradable protein fraction also showed a wide variation from om 3 to 20 %. The best single predictor of this last fraction was the acid detergent lignin content (R-2 = 0.83; P < 0.001), but the prediction was improved when considering as the second variable the cellulose expressed as a fraction of the cellular wall (R-2 = 0.92; P < 0.001). ((C) Elsevier / Inra).