Digestibility of nitrogen in heat-damaged alfalfa.

In order to partition indigestible N in heat-damaged forages into its various chemical fractions, alfalfa (41.1% dry matter), stored in an upright stave silo, was not sealed, and heating was promoted. Silage was labeled as unheated silage (temperature below 20 degrees C); 35 degrees C silage (temperature near 35 degrees C, 5220 degrees C d); and 60 degrees C silage (temperature near 60 degrees C, 10,800 degrees C d). Total amino acid content of 60 degrees C silage was lower and acid detergent insoluble amino acid content was higher than in 35 degrees C or unheated silages. Apparent N digestibilities, determined using lactating dairy cows, were 47.5, 48.4, and 63.6% for 60 degrees C, 35 degrees C, and unheated silages. Estimates of true N digestibility were 76, 76, and 90%; true digestibility coefficients for amino acid N were 86, 92, and 95%; and true digestion coefficients for nonamino acid N were 67, 45, and 69% for 60 degrees C, 35 degrees C, and unheated silages. Two possible causes for depressed N digestibility of 60 degrees C silage were 1) the ratio of poorly digested nonamino acid N to highly digested amino acid N was higher in 60 degrees C silage than in 35 degrees C or unheated silage, and 2) amino acid N in 60 degrees C silage was digested less than amino acid N in 35 degrees C or unheated silage. Reasons for low digestibility of N in 35 degrees C silage are less clear, but evidence suggests formation of indigestible N complexes in digesta.