Animal Feed Science and Technology 96 1/2 1 13 Amsterdam, Netherlands: Elsevier Science B.V..

Abstract

A three-step in vitro procedure was developed by Calsamiglia and Stern [J. Anim. Sci. 73 (1995) 1459] to estimate intestinal protein digestion after 16 h incubation in the rumen. The objective of our work was to modify this procedure to give an accurate estimation of N digestibility without the use of surgically modified animals by replacing the rumen incubation step by an in vitro protease enzyme incubation, and the use of small nylon bags for samples instead of the time-consuming filtration step. The results were compared with those from mobile nylon bag studies or ADIN determinations. In addition, the effect of heat treatment of grains and soyabean on protein digestibility was investigated. Briefly, samples were weighed into small nylon bags, sealed, and a maximum of 30 bags were placed in a 2.4 1 bottle containing borate-phosphate buffer at 39 degrees C. After 1 h, protease solution (protease type xiv, Streptomyces griseus) was added. After 4 h, the bags were removed and rinsed thoroughly. Half of the bags were dried at 50-55 degrees C for 48 h and the dry weight was recorded. The rest of the bags were incubated for 1 h in pepsin solution, then NaOH and pancreatin solutions were added and the bags were incubated for 24 h at 39 degrees C. Bags were rinsed thoroughly and dried at 50-55 degrees C for 48 h. The dry weight of the bags was recorded and the bag residue and the original feeds were analysed for N. Samples were barley, oats, wheat and soybeans, heat-treated for various times, and were obtained from other trials for which mobile bag digestibility values were determined with or without 16 h rumen incubation. Mobile bag digestibility values without rumen incubation ranged from 77 to 97%. Regression analysis of the relationship of N digestibility (mobile bag) and N digestibility (in vitro) for all grain samples resulted in a highly significant relationship (r2=0.95, P<0.0001). When soyabean samples were included in the analysis, the r2 was only 0.02. Residual trypsin inhibitor activity of some of the samples interfered with mobile bag digestibility (no rumen incubation), while there was no interference of trypsin inhibitor with the in vitro digestibility determinations, demonstrating a benefit of the proposed method. There was also no interference for mobile bag digestibility when the samples were first incubated in the rumen for 16 h. This in vitro procedure closely simulated the physiological conditions in the animal allowing a prediction of intestinal N digestibility without the use of surgically modified animals. The method was sensitive to heat damage of the samples and could be used to rapidly evaluate the nutritional value of grain and soyabean feedstuffs for ruminants.