Avaliação de métodos para estimar a degradação ruminal das proteínas: in situ, in vitro/gases e Rusitec

Abstract

The objective of this study was to evaluate ruminal degradation parameters of the nitrogen compounds estimated in the in situ, in vitro / gas and Rusitec methods, in order to identify an overestimation of ruminal degradability estimated by the in situ method. Samples of a diet composed of corn silage (60%), corn grain (18.6%), soybean meal (20.2%), phosphate-bicalcium (0.2%), calcitic limestone (0.5%), common salt (0.2%) and urea (0.3%) were incubated in assays using the in situ, in vitro / gas and Rusitec methods. In situ and in vitro methods, degradation rates (kd) and protein degradability were estimated. In the Rusitec method, three trials were carried out for 9 days each, using five fermenters. Each fermenter received 10 g of the diet daily in nylon bags with porosity of 41 μm. The liquid effluent was collected daily and aliquots were collected for further analysis of ammoniacal N, peptidic N and aminoacid N. The linear effect of the rate of passage of the liquid fraction obtained in the Rusitec assay was analyzed using a mixed model, which included fixed effects of passage rate, random effects of period and fermentation vessels, using the MIXED procedure of SAS. The differences between the degradability parameters estimated by the in situ and in vitro methods were verified by the "F" test. The in situ method showed significantly higher values of degradability (P <0.05) compared to the in vitro method. The N escape in the Rusitec method presented a linear effect increasing with the increase of the passage rate, this shows a considerable escape of N, reaching values of 67% of the N that disappeared from the nylon bags at rates of 7% / h. In conclusion, with the present study it was possible to verify that N fractions of the diet incubated in nylon bags escape ruminal degradation without undergoing the action of the bacteria and this escape is accentuated by the increase of the passage rate.