RUMEN MICROBIAL GROWTH IN SHEEP FED THREE DISTINCT QUALITY HAYS

Abstract

Forages usually fed to sheep in tropics have low values of crude protein (CP) content and/or low availability of protein. The aim of this work was to identify different profiles of microbial protein (MP) yield for sheep fed hays with distinct CP levels. Three hays (LUC - Lucerne, SIG - signalgrass and TIF - Tifton-85), six animals (Santa Inês wethers fitted with rumen and duodenum cannulas) placed in a double Latin square were used to evaluate transit rates of microbial protein and in vivo microbial synthesis. In vitro microbial protein synthesis was evaluated in a complete factorial statistical design. The parameters studied were rumen pH and ammonia-N, microbial protein transit using 15N as marker and microbial protein synthesis by in vivo (purine derivatives urinary excretion) and in vitro (32P incorporation) techniques. Rumen pH values did not present significant differences for treatments (hays) and these values were in a level considered as satisfactory to good microbial development. As studied hays presented great variations on CP levels, the concentration of ammonia-N was higher for animals fed LUC (191 g CP/kg DM) and lower for animals fed SIG (29 g/kg DM), therefore presented no difference for ammonia-N concentration (P > 0.05) between SIG and TIF (75 g CP/kg DM). The nitrogen assimilation rate (k1) by rumen microorganisms was much faster (P < 0.01) for treatment LUC, which showed that, in this treatment, nitrogen was more available than in others. MP produced from LUC had a faster passage (k2) from rumen to duodenum (P < 0.01). Animals fed LUC were those that presented higher MP supply (P < 0.05). Although there was no difference (P = 0.12) between both treatments with lower protein levels (TIF and SIG), there is a visible trend of that with lower protein content, the results were even more unsatisfactory, regarding the MP flow. The analysis of variance of the results of net MP production after 8 h incubation with 32P showed that there was an effect of inoculum (P = 0.0089), but there was no effect neither of substratum nor of substratum*inoculum interaction (P > 0.05). Feeds with low proteic content had prejudicial effect on utilization of nutrients by sheep and on maintenance of a healthy rumen environment. This fact was highlighted by variations on rumen ammonia-N concentrations. The microbial synthesis capacity was identified more efficiently by purine derivatives urinary excretion technique. The in vitro technique of 32P incorporation did not distinguish microbial growths from tested feeds degradation. In vitro technique showed strong dependency of inoculum quality.