Beginning with the 2000 Wisconsin Corn Hybrid Performance Trial, ranking of corn hybrid quality was estimated using MILK2000. Previously, MILK91 and MILK95 estimated dry matter intake using NDF, and estimated NE_L (Mcal/lb) using acid detergent fiber or in vitro true digestibility. MILK2000 estimates dry matter intake using neutral detergent fiber and cell wall digestibility (the base dry matter intake is increased or decreased 0.374 lb. per 1% unit change in cell wall digestibility above or below the average cell wall digestibility of the trial). NE_L (Mcal/lb) is estimated using a modified summative energy equation. MILK2000 accounts for the effects of whole-plant DM content and kernel processing on starch digestibility. A MILK2000 spreadsheet can be obtained at www.wisc.edu/dysci.

In the silage hybrid trials, corn silage was analyzed using near infra-red spectroscopy equations derived from previous work of Jim Coors and Joe Lauer (UW-Madison). Plot samples were dried, ground and analyzed for crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), in vitro cell wall digestibility (CWD), in vitro digestibility (IVD), and starch. An example of the results is presented in Table 1.

MILK2000 silage performance indices, milk per ton and milk per acre (see Table 1 and Figure 1), were calculated using an adaptation by Eric Schwab and Randy Shaver (UW-Madison Dairy Science Department) of the MILK95 model (Undersander, Howard and Shaver; J. Prod. Agric. 6:231-235). In Milk2000, the energy content of corn silage was estimated using a modification of a published summative energy equation (Weiss and co-workers, 1992; Anim. Feed Sci. Technol. 39:95-110). In the modified summative equation, CP, fat, NDF, starch, and sugar plus organic acid fractions were included along with their corresponding total-tract digestibility coefficients for estimating the energy content of corn silage. A regression equation developed from literature data was used to predict total tract starch digestibility from whole-plant dry matter content. The lab measure of CWD was used for the NDF digestibility coefficient. Digestibility coefficients used for the CP, fat, and sugar plus organic acid fractions were constants. Dry matter intake was estimated using the samples NDF content and CWD assuming a 1350 lb. cow consuming a 30% NDF diet. Using National Research Council (NRC, 1989) energy requirements, the intake of energy from corn silage was converted to expected milk per ton.

Because the cows maintenance energy requirements were partitioned against the total diet in MILK2000 rather than against only corn silage as was done in MILK95, there was a base increase in our new estimate of milk per ton which was of equal value across all hybrids and did not influence ranking. Milk per acre was calculated using milk per ton and dry matter yield per acre.