Genetic correlations among fertility traits and lactation persistency within and across Holstein herds with different milk production during the first three lactations

Genotype by environment interaction (G x E) effects for lactating cows' fertility traits may differ with lactation number when accounting for herd production level. Our objective here was to investigate the genetic correlations among three fertility traits in cows and lactation persistency within and across herds with different milk-production levels by using lactation records of Japanese Holstein cows (475,446 records for first lactation, 378,540 for second, and 265,560 for third). Herds with data were categorized into three groups (LOW, MM, and HIGH) based on the average herd-year effects in each herd for 305-day milk yield at the first lactation. The fertility traits evaluated were conception rate at first insemination (CR); number of inseminations (NI); and days open (DO). Lactation persistency (PER) was defined as the difference between milk yields at 240 and 60 days in milk. Genetic parameters were estimated within each lactation by using a multiple-trait model that considered the traits of different herd milk-production groups (HPGs) as separate traits. The genetic correlations among fertility traits were estimated by using a three HPG x three-trait animal model; for those between fertility traits and PER we used a three HPG x two-trait (each fertility trait and PER) animal model. The genetic correlations among fertility traits were strong, whereas those between fertility traits and PER were weak and undesirable; most of the values did not differ significantly among HPGs or lactations. Some of the genetic correlations of CR or NI among different HPGs for the second and third lactations were lower than those for the first lactation. These findings suggest that G x E effects on a cow's ability to conceive with respect to herd production level are larger in later lactations than in the first lactation.