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Departments of Agricultural Economics and Animal Science, Texas A&M University, College Station 77843
ABSTRACT
Value of days open was estimated by budgeting simulation of a commercial herd situation for Holstein cows of alternative milk yields with varying calving intervals. Values for days open were differences of income over feed cost per cow per day among alternative calving intervals of 12, 13, and 15 mo. Producing abilities for milk of cows corresponding to a 13-mo calving interval were 5900, 6800, 7700, and 8600 kg 3.5% milk per 300-day lactation. Conditions of herd management were a fixed policy of 65 days dry, herd size of 120 cows, group feeding with income-maximizing rations solved by linear programming, and a herd age structure of 30% of cows <36 mo and 70% of cows 
36 mo.
Effect was little on annual income over feed cost in the short run for modest increases or decreases of calving interval. Depending on milk producing ability, minor positive or negative differences occurred by increasing calving interval from 12 to 15 mo. However, there was a positive value per day open ($.21 to $.40) for all milk producing abilities when calving interval was increased from 12 to 13 mo and a consistently negative value per day open (–$.04 to –$.23) when it was increased from 13 to 15 mo. Thus, the 13-mo calving interval appears to be close to optimal. Costs associated with changes in either direction from 13 mo were small enough not to be a major management issue when cows were grouped and fed according to milk yield and when dry period was 65 days.
1 Technical article 18782 of the Texas Agricultural Experiment Station, College Station 77843. Project 2491, a contribution to Southern Regional Project S49, Genetic Methods of Improving Dairy Cattle for the South.
2 Ministerio de Agricultura-Instituto Nicaraguense de Reforma Agraria (MIDA-INRA), Dirección General de Producción de Leche, Apartado 54-47, Managua, Nicaragua.
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