Microcomputer simulation of management practices affecting timing of net income in dairy cattle

Microcomputer simulation was used to evaluate effects of all combinations of two levels of involuntary culling, heifer rearing, and sire selection against dystocia in heifers on timing and magnitude of net income in dairy cattle. Time to cumulative payoff of expenses and net income per day of herdlife were measured for herds and individual cows. Net income was accumulated monthly, and expressed per day of life and per day to 96 mo. Twenty herds of 80 cows were simulated for 20 yr under eight options in the herd study. More than 1000 cows with complete herdlives, from a single herd, were individually simulated for each of eight options in the cow study, with no voluntary culling of cows.

For the herd study, milk yield per cow averaged 6838 kg ± 858 kg/yr, and net income per cow was $671 i $193/yr. Mean time to payoff was 60.0 mo, and mean net income to 96 mo, including salvage value, was $.36/d. Heifers calving at 26 mo had rearing expenses of $1030, time to payoff of 54.6 mo, and net income of $.432/d, compared with rearing expenses of $1200, time to payoff of 70.0 mo, and net income of $.285/d for heifers calving at 32 mo. Options with 12% involuntary culling paid off 2.3 mo earlier and had $.081morenetincomeperdaythan24$.485/d). Options with 32 mo age at first calving, 24% involuntary culling, and random mating had latest payoff (74.3 mo) and lowest net income (S.246/d). Herds and years had large effects on time to payoff and net income due to differences in herd production and genetic trend of sires for production over time. Including salvage value for cows surviving 96 mo increased net income $.064/d for herds, and $.25/d for cows. Twenty-one percent of the value of increased milk yield was attributed to increased feed costs.

For the cow study, cost of rearing heifers was $141 ± $127, mean time to payoff was 53.4 mo, and mean time to cumulative profit was of 56.5 mo. Cumulative profit represented positive cumulative net income for 12 consecutive mo, which included 69% of cows with time to payoff. Net income was $.19/d for all cows, $.46/d for cows with a first calving, $.60/d for cows surviving 96 mo, and $.85/d for cows surviving 96 mo including salvage value. Heifers calving at 26 mo paid off expenses at 47.1 mo, compared with 60.6 mo for heifers calving at 32 mo. Heifers calving at 26 mo cost $.07/d more to raise to first calving, but paid off by 70 d in milk into their second lactation, compared with payoff by 140 d in milk of the third lactation for cows calving at 32 mo. Regressions of time to payoff and net income per day at 96 mo on cow production were -.0077 d/kg and .00028 $/kg, respectively. The regression of time to payoff on PD Dollars was -.0035 d/PDS, and the regression of net income per day on PD Dollars was .00072 S/PDS for cows that calved. Differences did not exist in time to payoff between levels of involuntary culling and selection against dystocia.

Heifer rearing was most important in this study due to large differences in time to payoff and net income as age at first calving changed. Sire selection against dystocia in heifers was least important due to the mating program used, with intermediate differences in payoff and net income between levels of involuntary culling.