In vitro studies on microbial efficiency from two cuts of ryegrass (Lolium perenne, cv. Aberdart) with different proportions of sugars and protein

An in vitro study investigated effects on microbial efficiency of feeding ryegrass diets differing in water soluble carbohydrates (WSC) and crude protein (CP) levels. A 1st cut of a diploid ryegrass (Aberdart) with high WSC-low CP (Cut1), a 2nd cut with low WSC-high CP (Cut2) contents and the two cuts, supplemented with mainly casein or fructans + sucrose (Cu1P and Cut2S) to achieve similar WSC and CP levels, were used. Dietary levels of WSC, CP and neutral detergent fibre (NDFom) were (in g/kg dry matter): 298, 100, 464 (Cut1), 101, 179, 484 (Cut2), 290, 181, 401 (Cut1P) and 290, 181, 351 (Cut2S), respectively. The in vitro system (SIMCO) was a repeatedly fed semi-continuous culture allowing stratification of vessel contents and selective retention of feed particles. The estimated NDFom in sacco degradation rate (h−1) and indigestible NDFom (INDFom) was 0.042 and 0.117 for the 1st cut and 0.086 and 0.270 for the 2nd cut. Digestibility of NDFom in SIMCO tended to be higher in the 1st cut than in the 2nd cut diets. Fermentation patterns were in normal rumen ranges. Ribonucleic acid (RNA) and volatile fatty acid (VFA) efficiencies (g/kg truly digested organic matter) differed among treatments (P=0.005 and P=0.018, respectively). Statistical significance at P<0.05 was only achieved for Cut1P vs Cut2 and Cut2S (RNA efficiency) and for Cut1P vs Cut2S (VFA efficiency). Molar proportions of acetate were lower and propionate higher in 1st cut, compared to 2nd cut diets (P<0.05). Mean retention time (MRT) of the liquid fraction was fixed at 10.4 h, but MRT of INDFom was long, varying from 95 to 303 h and strongly influenced by the daily supply of INDFom in the different diets, which was approximately twice as high in the 2nd as compared to the 1st cut diets. This resulted in a strong negative relationship between RNA efficiency and MRT. Microbial N and VFA efficiency in the SIMCO system is likely to be more dependent on solids retention time than on the relatively wide range of WSC and CP levels used in this study. To evaluate diets with respect to microbial efficiency, fermentation system must have the possibility to separate effects substrates from digesta retention under near rumen-like conditions.