Digestibility of Nitrogen in Heat-Damaged Alfalfa

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Abstract
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.degree. C); 35.degree. C silage (temperature near 35.degree. C, 5220.degree. C d); and 60.degree. C silage (temperature near 60.degree. C, 10,800.degree. C d). Total amino acid content of 60.degree. silage was lower and acid detergent insoluble amino acid content was higher than in 35.degree. C or unheated silages. Apparent N digestibilities, determined using lactating dairy cows, were 47.5, 48.4, and 63.6% for 60.degree. C, 35.degree. 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.degree. C, 35.degree. C, and unheated silages. Two possible causes for depressed N digestibility of 60.degree. C silage were 1) the ratio of poorly digested nonamino acid N to highly digested amino acid N was higher in 60.degree. C silage than in 35.degree. C or unheated silage, and 2) amino acid N in 60.degree. C silage was digested less than amino acid N in 35.degree. C or unheated silage. Reasons for low digestibility of N in 35.degree. C silage are less clear, but evidence suggests formation of indigestible N complexes in digesta.