The effect of an exogenous amylase on performance and total-tract digestibility in lactating dairy cows fed a high-byproduct diet

The objective of this experiment was to determine the performance and digestibility response of lactating dairy cows fed a reduced-starch diet containing a commercial amylase product. Treatments consisted of a normal-starch total mixed ration (NS-), a reduced-starch total mixed ration (RS-), and a reduced-starch total mixed ration with exogenous amylase (RS+) added to the concentrate. Treatments were assigned according to a replicated 3 x 3 Latin square design with 28-d periods. Twenty-three cows completed the study. Starch concentrations in NS-, RS-, and RS+ total mixed rations were 27.7, 23.5, and 22.7%, respectively. Effects of treatment on intake, milk production, milk composition, and total-tract apparent nutrient digestibility were evaluated during the last week of each period. Effects of amylase on in vitro starch digestibility of the NS- and RS- grain mixes were also measured. We hypothesized that the reduction in dietary starch in the RS- ration would decrease diet digestibility and limit milk production compared with NS- due to a decrease in available energy, and that RS+ would alleviate some of this decrease by increasing nutrient digestibility. Contrary to this hypothesis, the RS- diet did not affect intake or milk production relative to the NS- diet, except for increased milk urea nitrogen and a tendency for a decrease in milk protein yield. This lack of response is attributed to both low milk fat concentrations across treatments and greater than predicted dietary energy content preventing the energy deficit that was intended to occur with the reduced-starch rations. Cows fed the RS+ ration had the lowest production performance, with reduced milk, fat-corrected milk, protein, and lactose yields relative to cows fed NS-. Cows fed RS+ also had reduced lactose yield and tended to have reduced milk and fat-corrected milk relative to cows fed RS-. Despite the negative effects of the RS+ treatment on performance, exogenous amylase did increase both in vitro and in vivo measurements of digestibility. Although amylase increased nutrient digestibility, this did not translate into improved milk performance, likely due to the relatively high energy content of experimental diets compared with cow requirements.