Effect of heat processing on ruminal degradability and intestinal disappearance of nitrogen and amino acids in Iranian whole soybean

The effects of heat processing (roasting and steeping) on nitrogen (N) fractionations, ruminal degradation and intestinal digestibility of N and amino acids (AA) in two Iranian whole soybean cultivars (Sahar and Williams) were determined using two ruminally fistulated and two intestinally cannulated Holstein steers by nylon bag techniques. The seeds were roasted at 140 to 145 °C using a drum roaster. A fraction of the seeds were cooled immediately and the rest were held in isolated barrels for 45 min (steeping). The non-protein N (NPN) and buffer soluble N (BSN) fractions in heat processed soybeans were found to be reduced significantly (P < 0.01) compared to raw seeds. Soluble (a) and insoluble potentially degradable (b) fractions of N, showed a similar pattern in raw and heat processed soybeans (in both cultivars), but rate of degradation (c) was reduced dramatically in heat processed seeds (67 and 70% in Sahar and Williams cultivars, respectively). The ruminal degradability of N and AA of heat processed soybeans was reduced significantly (P < 0.001) compared to raw seeds. However, among the individual AA some variation was found. Intestinal digestibility of N and AA (total and individual) was improved significantly (P < 0.001) by roasting (16.9 and 12.3% for N and total AA, respectively) and steep-roasting (25.0 and 18.5% for N and total AA, respectively). Intestinal digestibility of N or AA, measured with or without ruminal pre-incubation, was the same. Total tract disappearance of N was not significantly different between raw and roasted seeds. Total tract disappearance of total AA was higher for raw than roasted seeds, due to higher protein degradability of raw seeds. Roasting and steep-roasting were effective methods of changing the site of digestion from rumen to small intestine and therefore the amount of digestible undegraded AA in the small intestine can be increased. There was no significant difference between the two soybean cultivars with respect to N and total AA ruminal degradability, intestinal digestibility and total tract disappearance. The interaction between cultivar and heat processing was also not significantly different.