Evaluation of a simplified in vitro method to estimate the rumen protein degradation

Abstract

This study aimed to evaluate the reliability of a simplified method to estimate the ruminal degradability of crude protein (CP), based on ammonia N release in vitro, compared to the in situ conventional method. Two preliminary trials were conducted to evaluate the dynamics of ammonia N production and uptake by rumen microorganisms and its relationship with gas and purine bases (PB) accumulation in vitro as an indicator of microbial biomass. In Trial 1, starch, cellulose and casein samples were incubated and gas production and both ammonia N and PB accumulation were measured. No significant correlation was found between gas, ammonia N and PB accumulation. In Trial 2, casein and soybean meal samples were incubated with rumen inoculum added with 10 mg/L of (¹⁵NH₄)₂SO₄ and gas production, ammonia N accumulation and ¹⁵N concentration in the incubation medium were measured. Ammonia N uptake represented only 6 % of the total ammonia N released throughout the 48 h of fermentation of casein or soybean meal. The ¹⁵N concentration drastically decreased up to 24 h of incubation. Trial 3 evaluated whether the in vitro CP degradation parameters of ten moderate to high-protein feedstuffs were related to those obtained by the in situ conventional method. There was a positive linear relationship (P < 0.05) between methods for estimating the potentially degradable CP (R² ≥ 0.80) and the fractional rate of CP degradation (kd; R² = 0.56), with the in vitro kd being, on average, 66 % lower than the in situ. Effective CP degradability (ECPD) of feedstuffs estimated in both methods was positively related (R² = 0.93; P < 0.05). The simplified method based on ammonia N release in vitro, without correction for ammonia N uptake by microorganisms, may be used to estimate the extent or ruminal protein degradation of feedstuffs with moderate to high CP content.