Characterization of in situ nitrogen and fiber digestion and bacterial nitrogen contamination of hay crop forages preserved at different dry matter percentages.

Alfalfa, red clover, orchardgrass and timothy were harvested in the vegetative stage, wilted and stored as hay, or ensiled in small batch silos (20 kg) at 60, 40 or 20% (direct cut) dry matter and were analyzed for compositional differences. A ruminally cannulated lactating cow, consuming 50% of her dry matter intake from hay crop silage, was used to measure in situ dry matter, N, neutral detergent fiber and acid detergent fiber disappearance. Diaminopimelic acid was used as a bacterial marker to correct for bacterial N contamination for in situ residual N. Fibrous components tended to become concentrated as percent dry matter at preservation decreased, presumably associated with leaching of water solubles during storage. For most forages, as dry matter percentage of preservation decreased, water soluble dry matter and N increased, with a concomitant increase of ruminally nondigested dry matter. Specific trends in coefficients of digestion associated with forage type or preservation dry matter percentage were not observed for dry matter, N, neutral detergent fiber or acid detergent fiber. Correction for contamination by bacterial N decreased lag time in digestion and altered rates of N digestion compared with noncorrected rates. Linear and quadratic bacterial N contamination profiles were observed with time of ruminal incubation. Rate of digestion of N was highly correlated with fibrous component concentration, and to a lesser extent to rate of neutral and acid detergent fiber digestion. Dry matter percentage at preservation had a variable effect on ruminal digestion rate of dry matter and N, which varied with forage type and had no effect on neutral detergent and acid detergent fiber digestion rates. Correction for bacterial N contamination should be considered when establishing N digestion rates for forage by the in situ technique.