Efficacy of sheep as a digestibility model for cattle when fed concentrate-based or forage-based diets

The objectives were to determine the efficacy of sheep as a digestibility model for cattle feeding two diets, forage or concentrate based, under current genetics. Twelve Suffolk wethers were blocked into two periods with six wethers in each period. Within each period, wethers were fed a forage-based diet (n = 3) or a concentrate-based diet (n = 3). Six angus steers were also fed a forage-based diet (n = 3) or a concentrate-based diets (n = 3) in switchback design with two periods. All animals were adapted to diets for a minimum of 3 wk, then feed intake, refusals, and feces were collected. Feed and fecal dry matter (DM), organic matter (OM), neutral detergent fiber (NDF), acid detergent fiber (ADF), and starch were analyzed. Refusals were analyzed for DM. Data were analyzed using Proc Mixed in SAS with diet and species as fixed and period as a random effect. Dry matter intake as percentage of body weight for each animal within each period was used as a covariable. 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 were similar between wethers and steers (P > 0.18); however, when fed the forage-based diet, DM and OM digestibility was less (P < 0.01) for wethers than steers. Like DM and OM, an interaction (P < 0.05) between species and diet was present for starch digestibility. When fed the forage-based diet, starch digestibility did not differ (P = 0.66) between wethers and steers; however, when fed concentrate-based diet, wethers had a greater starch digestibility (P < 0.05) than steers. There was no interaction (P > 0.45) between species and diet for NDF and ADF digestibility. Regardless of the diet fed, NDF and ADF digestibilities were greater (P < 0.05) in steers than wethers. Present day sheep were not a good model for cattle when fed forage-based diets, but sheep were an acceptable model for cattle when fed concentrate-based diets.