Estimating silage energy value and milk yield to rank corn hybrids

This paper provides a revised summative energy equation and applies it to estimate the energy value of corn (Zea mays) silage. Estimating the energy value of corn silage is important, because energy is the primary nutrient contributed by corn silage to dairy cattle rations. Estimated energy intake from corn silage was used to estimate milk yield from corn silage by dairy cows. The milk yield estimate was used to rank corn hybrids in silage evaluation and breeding programs. The revised (MILK2000) forage quality (milk Mg-1) and yield (milk ha(-1)) indices were evaluated relative to MILK1995 indices in corn silage hybrid performance trials. A previously published summative energy equation (Weiss, 1996), with crude protein, fat, non-fiber carbohydrate (NFC), and neutral detergent fiber (NDF) fractions and corresponding digestibility coefficients, was adapted for corn silage as follows: the crude protein and fat fractions were not altered, the NEC fraction with constant digestibility was replaced with starch and non-starch NFC fractions, the starch digestibility coefficient was varied in relationship to whole-plant dry matter (DM) concentration and kernel processing, and the NDF digestibility coefficient based on lignin concentration was replaced by a 48 h or maintenance intake in vitro measurement of NDF digestibility (NDFD). Our summative approach integrates known differences in starch digestibility, as affected by whole-plant DM concentration and kernel processing, and NDFD into estimates of the energy value of corn silage. It also provides a framework for the future incorporation of laboratory measures of starch digestibility into estimates of the energy value of corn silage. For the MILK2000 model, we used our net energy for lactation estimates along with DM intake estimated from NDF concentration and NDFD to estimate milk Mg-1 of corn silage DM. Corn hybrid checks characterized by high and low milk Mg-1 were selected on the basis of above average forage yield and either low or high NDF concentration. These low and high NDF check hybrids were then included in 61 trials conducted between 1995 and 2000. The frequency with which low NDF hybrids had greater estimated milk Mg-1 than high NDF hybrids was 0.90-0.93. Both maturity at harvest and NDFD strongly influenced the relative hybrid rankings with MILK2000 versus MILK1995. MILK2000 provides an index for evaluating relative performance among hybrids and could be applied to ranking hybrids tested in performance trials. (C) 2003 Elsevier B.V. All rights reserved.