The main objective was to evaluate different models to improve the genetic analysis of days open (DO) in dairy cattle. Thus, three studies were conducted: 1) to evaluate the use of a linear random regression model for DO; 2) to study the genetic variation of calving success (CS); and 3) to evaluate the joint analysis of DO and CS by a threshold linear animal model.

The objective of the first study was to investigate the feasibility of using a linear Legendre random regression model for the analysis of female fertility as expressed by DO. The analysis provided insight about the kind and amount of variation over the trajectory for age at calving from 24 to 90 mo. An aging and genetic component of female fertility was discovered. Heritability ranged from 0.07 to 0.10 from 24 to 72 mo. Permanent environmental correlations between fixed ages at calving changed from positive to negative values with greater distance between days open in young cows and days open in older cows; r(24,36) = 0.88, r(24,72) = -0.65. The random regression animal model is a better alternative to a repeatability animal model for estimation of variance components of DO. However, the assumption of genetic correlation of unity between adjacent parities is not realistic. Further research is needed to evaluate the advantages of using random regression model on the genetic analysis of DO.

The objective of the second study was to investigate genetic parameters for calving success (CS) at different parity groups. Three definitions of CS were evaluated: CS-B had two categories (1=success and 2=failure), CS-T (1=success, 2= failure with opportunity to calve, and 3=failure without opportunity to calve), and CS-T2 (1=success, 2= failure without opportunity to calve, and 3= failure with opportunity to calve). Data from 1236 Holstein females from parity 0 to 5 were analyzed with threshold animal model. Posterior means of heritability for CS-B from parity 0 to 5 were slightly lower than those for CS-T. Posterior means of heritability for CS-B were 0.16(.08), 0.08(.03), 0.23(.07), 0.23(0.07), 0.24(.08) and 0.17(.08) for parity 0 to 5, respectively. Posterior means of heritability for CS-T were 0.17(.08), 0.10(.03), 0.21(.06), 0.22(0.07), 0.28(.08) and 0.21(.08) for parity 0 to 5, respectively. Posterior means of heritability for CS-T2 were 0.10(.07), 0.09(.04), 0.29(.06), 0.27(0.08), 0.24(.08) and 0.30(.09) for parity 0 to 5, respectively. Posterior means of genetic correlations between adjacent calving opportunities for CS-B, CS-T, and CS-T2 were variable in sign and magnitude with high posterior standard deviations. Our results indicate that CS-B, CS-T, and CS-T2 have substantial genetic variation to allow genetic improvement of female fertility. Some advantages of using CS for genetic evaluation of sires for female fertility is that it includes fertility performance of both heifer and lactating cows and it takes into account censored records for days open.

The objective of the third study was to investigate genetic parameters for both DO and CS at different parity groups. The joint analysis of DO with CS-B, CS-T, and CS-T2 were done by a linear-threshold animal model. Data from 1236 Holstein females from parity 0 to 4 were used. Posterior means of heritability for CS-B, CS-T, and CS-T2 were low to moderate. Posterior means of heritability for CS-B from parity 0 to 4 were similar than those for CS-T and CS-T2. Posterior means of CS-B heritability were 0.14(.04), 0.09(.03), 0.28(.07), 0.25(0.06), and 0.26(.06) for parity 0 to 4, respectively, CS-T heritability posterior means were 0.13(.04), 0.09(.03), 0.26(.06), 0.24(0.06), and 0.28(.06) for parity 0 to 4, respectively, and CS-T2 heritability posterior means were 0.15(.04), 0.09(.03), 0.28(.06), 0.26(0.06), and 0.26(.06) for parity 0 to 4, respectively. Posterior means of heritability for DO from joint analysis with CS-B, CS-T and CS-T2 were similar, and they increased from low to moderate with parity. Posterior means of genetic correlations between DO and CS in the same parity were positive and varied from low to moderate, but they had large posterior standard deviations. CS is a categorical trait with similar genetic variation as DO and it is analyzed jointly with DO to make effective use of the genetic correlation between these two traits in lactating cows. The joint analysis can identify cows with greater genetic merit for conception at an earlier stage of lactation and greater success at maintenance of pregnancy.