Use of sunflower meal or fish meal as protein supplement for high quality fresh forage diets: ruminal fermentation, microbial protein synthesis and sites of digestion

Abstract

Four Holstein heifers with ruminal and duodenal cannulas were used in a cross-over design with the objective to evaluate if, in fresh forage diets, the ruminal N requirements for microbial protein synthesis would be met by forage protein and also if total protein flow at the duodenum could be increased by the use of a low degradable protein (LDP) in the concentrate. Diets consisted of fresh alfalfa forage (CP=19.4%, NDF=44.5%, IVDMD=65.5%) offered ad libitum and 4.5 kg dry matter (DM) of either a highly degradable protein (HDP; CP=18%, IVDMD=82.4%) or a low degradable protein concentrate (LDP; CP=19.9% and IVDMD=83.8%). Concentrates contained ground corn, wheat bran, salt mix and either sunflower meal (SFM) (24% of DM; HDP) or fish meal (FM) (14% of DM; LDP). Cows were fed three times per day. Chromic oxide (Cr 2O 5) was used as an external marker for digesta flow estimation. Total diet compositions were: OM=91.4 and 90.3%, and CP=19.4 and 20.0% for HDP and LDP, respectively. Total OM and CP intake averaged 10.5 kg OM per day and 364 g N per day ( P>0.10). Apparent ruminal OM digestibility was 36.5% of OM intake ( P=0.87). Although ruminal ammonia concentration was higher ( P=0.02) when HDP was fed (25.3 mg/dl versus 19.5 mg/dl), the proportion of total N consumed reaching the duodenum did not differ with the type of supplement fed ( P=0.45) (average: 98.5% of intake). Duodenal flow of non-ammonia N (NAN), microbial N (MN) and dietary N were not different among treatments (average: 354, 281 and 74 g per day). Efficiency of microbial protein synthesis (MPS), as g MN/kg OM apparently or truly digested in rumen, was not affected ( P>0.10) by treatments (average: 73.6 and 42.0). Ruminal pH (average: 6.1) and total VFA concentration (average: 159.5 mmol/dl) were also unaffected by the source of protein used. According to these results, the use of SFM or FM as a source of HDP and LDP, respectively, did not alter either MPS or total CP flow to the duodenum, despite lower rumen ammonia concentration when FM was fed in the concentrate.