Heat treatment at harvest was used to investigate the effects of proteolysis on silage composition and digestion by sheep. Four alfalfa (Medicago sativa) silages were prepared, two from mid-bloom and two from pre-bloom crops from the same field. Mid-bloom alfalfa was conserved with formic acid as two unwilted silages, either without (unwilted control; UWC) or after heat treatment (unwilted heated; UWH) applied as steam for 1 min. Pre-bloom alfalfa was ensiled either after 24 h wilting (wilted control; WC) or after heating in a crop dehydrator for 2 min (wilted control; WH). Heated treatments were inoculated with Lactobacillus plantarum. Eight wethers, cannulated in the rumen and duodenum, were given the silages to determine the effects of heat treatment on digestion. Heat treatment inhibited protease activity and reduced protein catabolism in the silo. In unwilted silages, heat treatment had no effects (P greater than .05) on OM or N digestion, but it reduced (P less than .05) CP degradability in the rumen. In wilted silages, heat treatment reduced (P less than .05) apparent OM digestion in the rumen and increased (P less than .05) the proportion of N intake flowing to the intestines as non-ammonia N (NAN). Efficiency of microbial protein synthesis also was increased (P less than .01). Absorption of N posterior to the duodenum was increased (P less than .05) in WH compared to WC, but there was no effect (P greater than .05) of heat treatment on apparent total tract N digestibility.(ABSTRACT TRUNCATED AT 250 WORDS)