Book Description

Simulation of a dairy herd over a period of 15 years was used to examine replacement strategies among six operationally practical systems of culling cows which whould maximize income over feed costs. The strategies or criteria used to remove the lower ranking cows to a relatively constant herd size were: (1) Mature Equivalent (M.E.) milk production, (2) M.E. gross milk income, (3) actual milk production, (4) actual gross income, (5) income over feed cost, and (6) present value of expected gross income of cow and her subsequent replacements. Two each of prices of milk, fat differential, feed, and operational costs totaled 2 4 x 6 x 2 replications per trial or 192 replications. The following were treated as stochastic variables: (a) variation in milk production and milkfat percentage between cows and between lactations of the same cow, (b) chance of a calf being a heifer or bull, (c) chance of involuntary death or removal of cows and youngstock, and (d) chance of month of the year of involuntary removals. The mean and variance of the base herd approximated the average Michigan DHIA Holstein population in 1966. The sire value, approximating the best bulls used in A.I. was identical for any given year through all strategies and replications, but improved over time at the rate of 130 1b milk per year. Culls or cows removed because of low in each respective strategy were removed at the most profitable point to cull in their respective lactations by equating the milk income of the marginal month with the sun of the month's feed costs and monthly operational charges. Practical use of this simple method of determining when to cull cows within a lactation was demonstrated. A complete factorial design to analyze the generates data showed no significant differences between strategies under alternative combinations of prices in average income including salvage over feed cost per cow discounted to the present. This income was affected by the level of milk price (P