Intake and digestive responses by ruminants fed fresh temperate pasture supplemented with increased levels of sorghum grain: A comparison between cattle and sheep

Abstract

The objective of this study was to compare the response of cattle and sheep fed a temperate pasture to increasing levels of sorghum grain supplementation on feed intake, digestibility, rumen fermentation and microbial protein synthesis. Twenty four cattle and 24 sheep housed in metabolism cages and fed a fresh temperate forage ad libitum were randomly assigned within each species to 1 of 4 treatments (n=6 of each species per treatment): non-supplemented or supplemented with ground sorghum grain at 5, 10 or 15g/kg of their body weight. The grain proportion in the supplemented diets varied from 0.14 up to 0.43 (SEM=0.012) and was similar for both species at all supplementation levels. Feed intake was differentially affected by supplementation in cattle and sheep (P<0.01). The increasing supplementation level linearly increased total dry matter intake in cattle (P<0.01) and decreased this variable in sheep (P≤0.02). Increased supplementation levels decreased forage intake in both species; however, this effect was more pronounced in sheep than in cattle (P<0.01). Although, organic matter (OM) and neutral detergent fiber digestibility did not differ between species, acid detergent fiber (ADF) and starch digestibility were differentially affected by supplementation (P≤0.03). In cattle, increased supplementation levels did not affect ADF digestibility and linearly decreased starch digestibility (P<0.01), whereas in sheep, ADF digestibility linearly decreased (P<0.01) and starch digestibility remained constant as supplementation increased. The ruminal parameters were similarly affected by supplementation in both species. While the pH and acetate to propionate ratio decreased, the total volatile fatty acids concentration increased with increased supplementation levels (P<0.01). However, cattle had a higher pH and acetate to propionate ratio than sheep (P<0.01). Microbial protein synthesis linearly increased in cattle (P=0.02), whereas it decreased in sheep at increased supplementation levels (P<0.01). The efficiency of microbial protein synthesis (MPSE) decreased similarly in both species with supplementation level, but cattle had higher MPSE than sheep (P<0.01). In conclusion, the increase in supplementation level led to differential responses in cattle and sheep fed temperate forage. In cattle, the increase of sorghum in the diet was effective in enhancing feed intake and digestive utilization of the diet. However, sorghum grain supplementation fed to sheep resulted in a high ruminal fermentation that reduced fiber digestibility and total OM intake.