|Hutchison, Jana - Edwards|
Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/6/2006
Publication Date: 12/1/2006
Citation: Kuhn, M.T., Hutchison, J.L., Wiggans, G.R. 2006. Characterization of Holstein heifer fertility in the United States. Journal of Dairy Science. 89(12):4907-4920. Interpretive Summary: Fertility has become a topic of utmost interest to the dairy industry due to a decline in fertility over the last 40 years. This decline in fertility has been due largely to increased milk yield. Loss in fertility has now become a costly problem for US dairy producers. Considerable research has been done on cow fertility but very little research has been done on fertility in heifers, especially in the US. Animal Improvement Program Laboratory's new reproductive record, however, has resulted in the acquisition of breeding information for heifers and has afforded new opportunities for reproductive research, including heifer fertility. Factors affecting Holstein heifer fertility were identified in this research, which can assist management as well as mating decisions. Genetic parameters were also estimated and indicated that the genetic component of heifer fertility is quite low. Nonetheless, results also indicated that selection on the currently available genetic evaluations for cow fertility will also improve heifer fertility.
Technical Abstract: The overall object of this research was to characterize US Holstein heifer fertility. This included investigation of factors influencing heifer fertility and estimation of heritability, as well as correlations with cow fertility and first lactation milk yield. A secondary objective was to compare linear and logistic model estimates of fixed effects. Data consisted of Holstein heifers, which were artificially inseminated, with their first breeding after 2002. Herds were required to have at least 60 breedings across the 3 years of data and an overall average conception rate (CR) between 20 and 80%. After edits there were a total of 537,938 breedings of 362,512 heifers in 2,668 herds from 41 states used for analysis. After edits, the overall average conception rate for US Holstein heifers was 57%. Linear and logistic model estimates for all factors were nearly identical. Year of breeding accounted for the most variation in heifer CR, with heifer age and month of breeding being the next most important factors. Conception rate in heifers is maximal at an intermediate age of 15 to 16 months. Heifers at 26 months of age and older have roughly a 10% lower CR than heifers bred at younger ages. Although month of breeding affected heifer CR, effects are less than for cows. In contrast to cow fertility, heifer CR is nearly as good in the hotter summer months as in cooler months. Approximately 88% of US herds had a 40 to 70% heifer CR. Heritability estimates of heifer CR on first service were 0.5% from the linear model and 1.0% from the threshold model. Genetic correlation estimates of heifer CR on first service with cow CR on first service and with first lactation milk yield were 0.39 and –0.19, respectively. Selection on either the currently available US daughter pregnancy rate evaluations for cow fertility or on cow CR will improve heifer fertility. Furthermore, heritability of heifer CR is lower than for cow CR and reporting of heifer breedings is currently less complete than for cow breedings. Thus, there are currently no immediate plans to implement a US genetic evaluation for heifer CR.