Importance of detail in per cow production

This study examined the effect of herd replacement rate and replacement heifer attributes (live weight, first-lactation production, longevity) on physical and financial performance of a 300 ha, 750 cow dairy farm, categorised as a DairyNZ “System 3” with 10-20% of the feed bought in and dry (non-lactating) animals grazed off-farm. A resource optimisation linear program (LP) model, “GSL”, was used to analyse the effect on farm profit (cash surplus) of varying cow age-group production (kg MS/cow/yr). Initially, the model was constrained to 750 cows, average production 380 kg MS/mature cow/yr; flexible drying off and culling dates. Common prices were assumed for milk, surplus calves and cull cows. Grazing off costs depended on age groups of heifers and cows. Initial herd (H) parameters were: replacement rate 25%; loss rate 5%; average cow life 6 years (H:25/5/6). Heifer production as a percentage of a mature cow was: 2 yr old 70%; 3 yr old 85%; 4 yr old 95% (P:70/85/95). The LP model purchased 442.1 t feed, produced 238,600 kg MS/yr, and profit (cash surplus) $643,274. When the LP’s only constraints were production of 380 kg MS/mature cow, H:17/3/8, P:90/100/100, and optimised for all other parameters, herd size was 621 cows; purchased feed 29.7 t. Total production was 225,470 kg MS; profit was $836,488. Cow longevity and age-group performance increased. Using averages from farm data ignores important detail and the opportunity to distinguish where diminishing returns apply in an economic analysis of a production system.