Influence of herd environment on health and fertility and their relationship with milk production.

High levels of milk production in dairy cattle can have negative side effects on health and fertility traits. This paper explores the relationships among milk yield, health, and fertility traits both across and within herd environments on a national scale. A total of 456,574 lactations from 3904 herds recorded from 1995 to 1999 in The Netherlands were analyzed. Herd environment was defined by 41 variables derived from production records and the annual national agricultural survey. Principal components analysis reduced this set to 4 components: intensity, defined as average production per cow, average fertility, farm size, and relative performance indicating whether herds had good (poor) health and fertility despite a high (low) production. Both fertility and health were better for some traits in high-intensity herds and for other traits in low-intensity herds. In high-intensity herds, somatic cell count (SCC) levels were lower, drops in production occurred more often, and first service took place earlier but with lower success. High fertility occurred more often in herds located on sandy soils and in those that had lower SCC levels, had fewer drops in production and higher cow survival. On large farms, drops in production were less frequent and fertility was somewhat better. The within-herd analysis showed that the relationship of milk yield with health and fertility was stronger in herds with high production, fertility, or both. In herds with poor relative performance, there was no difference in production levels between animals with good health or fertility and those with poor health or fertility.