Interaction of tallow and hay particle size on ruminal parameters

Four nonlactating ruminally cannulated Holstein cows were used in a 4 x 4 L atin square experiment with 4 21-d periods to determine if the effects of d ietary fat would be affected by hay particle length. Treatments consisted o f two levels of tallow (0 and 5%) and two hay particle lengths (short-cut a nd 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 ratio n (TMR) once per day. Samples of the 0 and 5% tallow TMR were ground and in cubated in situ in polyester bags for 24 and 48 h. Ruminal samples were tak en 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 d etergent fiber (NDF) were not affected by tallow or by hay by tallow intera ctions. There was a trend for tallow to improve total tract digestibility o f crude protein (CP) (70.2 vs. 74.7%). After 48 h of ruminal incubation, ta llow significantly decreased the digestibilities of DM, ADF, and NDF. No ha y length by tallow interactions for DM, NDF, ADF or CP digestibilities occu rred after 24 or 48 h. Tallow increased concentrations of propionate and de creased concentrations of acetate and valerate and the acetate-to-propionat e ratio. Total volatile fatty acids increased when tallow was added to diet s with short-cut hay, which suggests that when unprotected fat is added to diets with a high level of hay, a shortcut hay length may be advantageous. This result may be due to shorter rumen retention time of feed particles, w hich reduces the time for fatty acids to exert antimicrobial effects. Or, i t may because the increased surface area of the hay particle provides more area for microbial attachment and increased fermentation.