MODEL-PREDICTED VALUE OF ENZYME-TREATED ALFALFA SILAGE FOR LACTATING DAIRY-COWS

A model containing a mechanistic rumen submodel sensitive to variation s in feed carbohydrate fractions was used to determine the potential v alue for lactating dairy cattle (Bos taurus) of treating alfalfa (Medi cago sativa L.) silage with enzymes that reduce neutral detergent fibe r (NDF) concentration. The net carbohydrate and protein system used he re integrates nutrient intake, ruminal fermentation, intestinal digest ion, absorption, and metabolism with cattle requirements. For each of a series of potential effects of enzyme treatment, the treatment value per ton of silage was determined either from a reduction in diet cost s or an increase in milk production calculated from increasing the sup ply of metabolizable energy (ME). Balanced rations were formulated fro m corn (Zea mays L.) silage, alfalfa silage, ground dry com grain, hig h moisture ear corn, soybean [Glycine max Merr. L.] meal, and heat-tre ated soybean meal for early- and mid-lactation cows. Potential reducti ons in NDF effected by enzyme treatment were considered to be 2, 4, or 6 percentage points. In Scenario 1, diets were rebalanced as NDF was reduced, while maximizing the use of alfalfa silage and minimizing the use of corn grain. In Scenario 2, diets were held constant, and the i ncrease in ME as a result of enzyme treatment was used to calculate a potential increase in milk production. In Scenario 3, dry matter intak e (DMI) was increased to maintain a fixed intake of effective NDF (eND F), and the ME-allowed increase in milk production was calculated. Add itionally, an increase in the rate of fiber digestion from 7 to 9%/h w as considered in Scenario 3. Reduction in diet costs in Scenario 1 res ulted in treatment values of less than $1/ton silage. Increases in mil k production in Scenario 2 resulted in treatment values of $2.60 to $4 .80/ton silage for a 4 percentage point reduction in NDF. In the most optimistic cases (Scenario 3), treatment values were as high as $40/to n with a 4 percentage point reduction in NDF. The model predicts that a production response is necessary to obtain substantial economic bene fit from enzyme treatment of silage. Treatment values were higher with a larger NDF reduction, with a higher milk price, and with more matur e alfalfa.