Influence of Particle Size of Cottonseed Hulls and Bermudagrass Hay on Intake, Sorting Behavior, and Rumen Fluid Parameters of Feedlot Lambs

Effects of particle size (PS) of cottonseed hulls (CSH) and bermudagrass hay (BGH) in females Rambouillet lambs (n = 40) feedlot diets on intake, sorting behavior, ruminal fluid parameters, serum urea nitrogen (SUN), and in vitro dry matter digestibility (IVDMD) were evaluated. Lambs were individually fed a total mixed ration (TMR) with 800 g/kg concentrate and 200 g/kg roughage (CSH or BGH) during two feedlot experiments of 25 and 21 days, respectively, following a completely randomized design with 10 replications. The roughages were ground using a hammer mill with 9.52 mm (coarse), 6.35 mm (medium), or 4.76 mm (fine) screen while the control treatment diets used unground CSH or chopped BGH. Increasing PS of CSH quadratically decreased (P < 0.001) the rejection for particles retained in the sieve of 1.18 mm and tended (P = 0.07) to linearly increase preference for particles retained in the sieve of 8 mm. Increasing PS of BGH linearly increased (P < 0.001) preference for particles retained in the sieve of 1.18 mm. Refusals acid detergent fiber (ADF) quadratically increased (P = 0.001) with increasing CSH PS, but no differences were found for neutral detergent fiber (aNDF) and crude protein (CP). No differences were observed in ADF, aNDF, and CP of BGH refusals. Rumen fluid pH was not affected by PS. No differences in rumen fluid total volatile fatty acids (VFA) concentration and VFA proportions were observed in CSH diets despite a trend (P = 0.06) of a linear increase in valeric acid when PS was increased. Total VFA quadratically increased (P = 0.01) with BGH PS, while no differences were found in VFA proportions. Rumen ammonia concentration tended (P = 0.09) to quadratically increase when CSH PS increased. The SUN did not vary with PS. The IVDMD of CSH and the respective TMR quadratically decreased (P = 0.03) and increased (P = 0.002) with rumen fluid from lambs receiving increasing PS, respectively, while diets with BSH had its TMR IVDMD quadratically increased (P = 0.002). Dry matter intake (kg/d) did not vary with increasing PS but proportional intake (% of BW) decreased linearly (P = 0.007) in CSH diets. Roughage PS must be considered in high-energy diets for fattening lambs due to its effect on sorting behavior, digestibility, and rumen fluid parameters.