Interaction of tallow and hay particle size on ruminal parameters

Four nonlactating ruminally cannulated Holstein cows were used in a 4 x 4 Latin square experiment with 4 21-d periods to determine if the effects of dietary fat would be affected by hay particle length. Treatments consisted of two levels of tallow (0 and 5%) and two hay particle lengths (short-cut and long-cut) in a 2 x 2 factorial. Diets contained alfalfa hay, corn silage, and concentrate [1:1:2, dry matter (DM) basis] fed as a total mixed ration (TMR) once per day. Samples of the 0 and 5% tallow TMR were ground and incubated in situ in polyester bags for 24 and 48 h. Ruminal samples were taken on day 21 at 0800 h and at 2-h intervals until 1600 h. The total tract digestibilities of acid detergent fiber (ADF) and neutral detergent fiber (NDF) were not affected by tallow or by hay by tallow interactions. There was a trend for tallow to improve total tract digestibility of crude protein (CP) (70.2 vs. 74.7%). After 48 h of ruminal incubation, tallow significantly decreased the digestibilities of DM, ADF, and NDF. No hay length by tallow interactions for DM, NDF, ADF or CP digestibilities occurred after 24 or 48 h. Tallow increased concentrations of propionate and decreased concentrations of acetate and valerate and the acetate-to-propionate ratio. Total volatile fatty acids increased when tallow was added to diets with short-cut hay, which suggests that when unprotected fat is added to diets with a high level of hay, a short-cut hay length may be advantageous. This result may be due to shorter rumen retention time of feed particles, which reduces the time for fatty acids to exert antimicrobial effects. Or, it may because the increased surface area of the hay particle provides more area for microbial attachment and increased fermentation.