Effects of energy and protein supplementation on microbial-N synthesis and allantoin excretion in sheep fed guinea grass

Abstract

An experiment was conducted to determine the effects of two types of protein, soybean meal (SBM) and fish meal (FM); and two types of energy supplements, corn flour (CF) and paper pulp (PP) on in vivo digestibility of organic matter (OM), rumen fermentation pattern and dilution rate, rumen microbial-N synthesis and flow of organic matter and nitrogenous compounds through the duodenum in four Merino rams with an average weight of 54.4±4.5 kg. The relationships between duodenal purine flow and urinary allantoin and duodenal protein/energy (MJ rumen VFA per day) ratio were also investigated. The experiment was conducted in a 4×4 Latin square design with a 2×2 factorial arrangement of dietary treatments. The animals, fitted with both rumen and duodenal cannulae were housed in individual crates and fed chopped fresh guinea grass ad libitum twice daily, 100 g molasses and one of the four dietary supplements: (i) 170 g FM+268 g PP (FM+PP); (ii) 170 g FM+268 g CF (FM+CF); (iii) 200 g SBM+200 g PP (SBM+PP); and (iv) 200 g SBM+200 g CF (SBM+CF). Each supplement, at varying rates of rumen degradability, was formulated to provide similar amount of N and gross energy. The results showed that rumen pHs were similar, ranging from pH 5.8 to 6.0, for all animals fed the different dietary supplements. Rumen ammonia concentration was significantly ( p<0.05) higher in animals fed SBM (170.2–190.7 mg N l −1) than in animals fed FM supplement (166.8–170.2 mg N l −1). Rumen VFA concentrations were similar (94–103 mM) but molar proportions of acetate and propionate were respectively lower and higher in treatment SBM+CF, leading to a significantly decreased acetate:propionate ratio in this treatment (3.6) compared to SBM+PP (4.3). There were no significant differences between treatments in rumen fluid dilution rate and rumen volume. Duodenal OM flow was similar in PP supplemented diets, but differed significantly ( p<0.05) between protein supplements in the CF diets. Organic matter digestibility in the rumen was significantly ( p<0.05) higher in the CF+FM diet. Moreover, animals fed SBM supplement showed higher total tract OM digestibility than animals fed FM supplement. The flow of nitrogenous compounds to the duodenum, i.e., total-N, non-ammonia-N and rumen-undegradable-N were not significantly affected by either protein or energy supplements. Microbial-N flow tended ( p<0.08) to be higher in sheep fed CF supplement (average of 10.2 g per day) than in those fed PP supplement (average of 8.1 g per day). Urinary allantoin excretion was low (0.30–0.42 mmol per day/kg BW 0.75). A positive linear correlation ( r=0.73, p<0.005) between urinary allantoin (μmol per day/kg BW 0.75) and duodenal purines (μmol per day/kg BW 0.75) was observed. The efficiency of rumen microbial-N synthesis based on OM truly digested in the rumen was significantly ( p<0.02) higher in sheep fed CF supplement (15.2–16.6 g N kg −1 OMTDR) than in those fed PP supplement (12.2–12.8 g N kg −1 OMTDR). Differences in microbial protein:energy ratio or total duodenal protein:energy ratio among dietary treatments were not significant.