Effect of forage and concentrate intake level on rumen degradability of protein sources having different in vitro rates of N solubilisation

Abstract

The rates of N solubilisation of soya bean meal (SM), ensiled lucerne (EL), brewers' grains (BG) and fishmeal (FM) were measured by means of a protease solution of Streptomyces griseus. The unavailable N estimated with a dynamic model was 28.3, 58.9, 66.7 and 52.3 of the total N in SM, EL, BG and FM, respectively. In situ N and non-protein organic matter (NPOM) degradability of the 4 protein sources were measured in the rumen of 6 fistulated cows. The in situ experiments measured the influence of dry matter intake (DMI) and forage: concentrate ratio (F:C) on degradability of protein sources with hay (H) or maize silage (MS) based diets. Two levels of DMI, equal to about 1 and 2% (DMI 1 and DMI 2) of live weight (LW) and two F:C (35:65 and 70:30, on DM basis), were studied for each diet. Increasing DMI significantly ( P < 0.05) reduced the mean effective degradability of N and non-protein organic matter (NPOM) in H but not MS diets. However, the reduction varied with protein source (BG > SM > FM > EL for N and FM > BG > EL > O < SM for NPOM). Increasing F:C significantly ( P < 0.05) caused the mean effective degradability of N to rise in MS but not H diets (SM > EL > BG > FM), and reduced the mean effective degradability of NPOM in H but not MS diets (FM > BG > EL > SM). Compared with hay, maize silage caused a significant ( P < 0.01) decrease in the effective degradability of both N and NPOM values in all the protein sources. Some agreement was found between initial solubility measured in vitro or in situ. However, the in situ solubility values with increasing incubation times, become less comparable with those obtained in situ.