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Digestibility of Nitrogen in Heat-Damaged Alfalfa¹

Department of Dairy Science Ohio Agricultural Research and Development Center, The Ohio State University, Wooster 44691

W. L. Shockey

US Dairy Forage Research Center United States Department of Agriculture Agricultural Research Service Ohio Cluster, Wooster 44691

³ Reprint requests.

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

¹ Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University, Journal Article Number 16-86.

² Present address: Department of Animal and Range Sciences, North Dakota State University, Fargo 58105.