Effects of Days Open Days Dry, and Season of Freshening on Income over Feed Cost and 305-Day Mature Equivalent Milk Yield, for Three Different Production Levels11Published as Paper No. 10737, Journal Ser., Nebraska Agric. Res. Div., Univ. of Nebraska, Lincoln, NE 68583-0908

Days open, days dry, and season of calving were examined to maximize Income Over Feed Cost (IOFC) and 305-d mature equivalent milk production for three different production levels. Models with all fixed effects were analyzed using SAS® PROC GLM for 50,743 Holstein cows from 4687 herds calving in yr 1990 and 1991. Production level of cow was not important to determine optimum days open to maximize 305-d milk yield, but was important for days open to maximize IOFC. For 305-d milk yield, days open of 91 to 105 d, 76 to 90 d, and 61 to 75 d were optimum for cows calving in season 2 (February through April), season 3 (May through July), and season 4 (August through October) respectively. For IOFC, optimum days open was 15 d higher for average and above average production levels for seasons 2 and 3 than for below average production level. Optimum days open to maximize IOFC for calving season 4 was 15 d higher for above average production level than for below average and average levels. Cows calving in season 2 had highest optimum days open. Conclusive results could not be obtained for season 1 as less than 10% of records in the data set belonged to that season and of those only 10% had an open period shorter than 150 d. On average, cows calving in season 3 yielded highest IOFC, which was $667, $482, and $276 more than cows calving in season 1 (November to January), season 2, and season 4 respectively. The 305-d milk yield was not different for the four seasons of calving.