Comparison of Forage and Concentrate Digestibility in Sheep and Cattle

The objectives were to determine the efficacy of sheep as a digestibility model for cattle using two diets, forage or concentrate-based, with modern genetics and management. Diets, with ingredients of grass hay, corn grain, soybean meal, and urea, were formulated to contain either 80% forage or 80% concentrate on DM basis to compare digestibility between wethers and steers. Twelve Suffolk wethers were blocked into 2 blocks with 6 wethers in each block. Wethers within a block were divided into 2 groups (A and B) with 3 wethers in each group. Wethers in group A were fed 80% forage diet while group B wethers were on 80% concentrate diet. Six Angus steers were also blocked in 2 groups (A and B) with 3 steers in each group. In period 1, steers from group A were fed 80% forage diet while group B steers were on 80% concentrate diet. Steers were fed in a switchback design. Wethers and steers were fed for ad libitum intake. After 3 weeks of adaptation on diets, feed intake, refusals, and feces were collected and measured. Feed and fecal DM, OM, NDF, and ADF were analyzed. Refusals were analyzed for DM. Data were analyzed using Proc Mixed in SAS with diet, species, and block as fixed effects and then with Proc Corr in SAS. There was an interaction (P<0.01) between species and diet for DM and OM digestibility. When fed the concentrate-based diet, DM and OM digestibility did not differ in wethers and steers; however, when fed forage-based diet wethers digested less (P<0.05) than steers. Although lambs and steers had similar (P>0.05) OM and DM digestibility when fed concentrate-based diet, OM and DM were not correlated (OM r=0.45, P>0.45; DM r=0.32, P>0.36). There were no interactions (P≥0.83) for fiber digestibility; NDF and ADF digestibility were greater (P<0.05) in steers than wethers. Due to differences between NDF digestibility in steers and wethers, NDF digestibilities between species were not correlated when fed either concentrate (r=0.76, P=0.13) or forage-based diets (r=0.10, P=0.85). Similarly, ADF digestibilities between species were not correlated when fed concentrate (r=0.40, P=0.50) or forage-based diets (r=-0.02, P=0.97). Present-day sheep and cattle fed concentrate-based diets appear to have more digestibility parameters in common than when they are fed forage-based diets. For that reason, sheep used a model for cattle digestibility when forage-based diets are fed should be considered with caution.