The objectives of this research were to evaluate breed differences for heat-stress resistance using age at first calving and first calving interval, and to assess breed by region interactions for seven regions of the United States for survival-related traits up to five years of age in Brown Swiss, Holstein, and Jersey cows. Age at first calving and first calving interval were studied in farms with two breeds, with Holstein and Brown Swiss or Holstein and Jersey cows. The survival-related traits were analyzed in farms with one or two breeds. Seven regions within the United States were defined: Northeast, Northwest, Central north, Central, Central south, Southwest and Southeast. The fertility traits were also analyzed in seven individual states: Wisconsin, Ohio, Oregon, California, Arizona, Florida, and Texas. Brown Swiss were older than Holsteins at first calving (833 ± 2.4 d vs. 806 ± 2.0 d in regions, and 830 ± 3.1 d vs. 803 ± 2.4 d in states), but Holsteins and Brown Swiss did not differ for first calving interval. Jerseys were younger than Holsteins at first calving and had shorter first calving intervals (P < 0.01). In data from individual states, Holsteins housed with Brown Swiss were older at first calving than Holsteins housed with Jerseys (800 ± 2.7 d vs. 780 ± 2.5 d). Holsteins housed with Jerseys had slightly shorter first calving intervals than Holsteins housed with Brown Swiss, and the interaction of "type of Holstein: with season of the first calving was highly significant (P < 0.01). Region and season effects were smaller for Jerseys than for Holsteins, thus, Jerseys showed evidence of heat-stress resistance with respect to Holsteins. Management modified age at first calving in Holsteins, depending on the type of herd they were located in. Longer calving intervals might have been partly due to voluntary waiting period to breed the cows. The survival-related traits were evaluated up to five years of age. They consisted of stayability, number of completed lactations, days lived, herd-life, and total days in milk. For herds with one breed, the order for stayability to five years of age, from longer to shorter-lived breed was: Brown Swiss, Jersey and Holstein, but for the ratio of days in milk to herd-life the order was: Holstein, Jersey and Brown Swiss, and for the ratio of days in milk to days lived, it was: Jersey, and Holstein and Brown Swiss tied. This last ordering was the same for number of lactations completed by five years of age. The results for two-breed herds were similar since Brown Swiss and Jerseys had larger (Chi-square P < 0.01) probabilities of living past five years of age than Holsteins, and for days in milk and number of lactations completed, Jerseys had higher values than Holsteins (P < 0.01), but Holsteins and Brown Swiss tied in some analyses. Breed by region interaction was always significant. If all other conditions were assumed equal, Jerseys would give fastest returns by five years of age. The overall conclusion is that Jerseys performed better for the traits analyzed, all of them highly influenced by environmental conditions. / Ph. D.

With the objective of estimating genetic parameters for stayability to 48 and 60 months of age (STAY48 and STAY60) and their association with milk production in the first lactation (P305), age at first calving (AFC) and first calving interval (FCI) data from 3,344 cows from six herds located in the states of Minas Gerais, São Paulo and Paraiba were analyzed. The (co) variances were estimated by Bayesian approach, using bi-characteristics animal models. For STAY48, STAY60 and AFC systematic effects farm-year-season of birth were include with model. For P305 and FCI, the model used included the systematic effects of farm-year-season of calving and age at calving (linear and quadratic) as a covariable. In addition to these systematic effects for all traits were also included the effects of random additive genetic and residual. The mean estimates of heritability were 0.25 ± 0.08, 0.32 ± 0.07, 0.31 ± 0.05, 0.28 ± 0.05 and 0.08 ± 0.04 for STAY48, STAY60, P305, AFC and FCI, respectively. The a posteriori estimates of genetic correlations between STAY48 and P305 AFC and FCI were 0.90 ± 0.10, -0.55 ± 0.24 and 0.64 ± 0.33. The estimated genetic correlations between STAY60 and P305, AFC and FCI were 0.94 ± 0.07, -0.54 ± 0.21 and 0.83 ± 0.18. The genetic and phenotypic correlations estimated between stayability to 48 and 60 months of age and milk production in the first lactation indicate that production in the first lactation may be used as a selection criterion for longevity. Similarly, the selection to reduce the age at first calving can cause, for a correlated response female. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Com o objetivo de estimar parâmetros genéticos para stayability aos 48 e 60 meses de idade (STAY48 e STAY60) e suas associações com produção de leite na primeira lactação (P305), idade ao primeiro parto (IPP) e primeiro intervalo de partos (PIDP), foram analisados dados de 3.344 vacas pertencentes a seis rebanhos localizados nos estados de Minas Gerais, São Paulo e Paraíba. Os componentes de (co)variância foram estimados por abordagem Bayesiana em análises bi-características. Para as características STAY48, STAY60 e IPP, foram incluídos os efeitos sistemáticos de fazenda-ano-estação de nascimento. Para P305 e PIDP, o modelo utilizado incluiu os efeitos sistemáticos de fazenda-ano-estação de parto e idade da vaca ao parto com covariável (efeitos linear e quadrático) como covariável. Além desses efeitos sistemáticos, para todas as características estudadas, foram considerados os efeitos aleatórios genético aditivo direto e residual. As herdabilidades médias estimadas foram 0,25±0,08, 0,32±0,07, 0,31±0,05, 0,28±0,05 e 0,08±0,04 para STAY48, STAY60, P305, IPP e PIDP, respectivamente. As estimativas a posteriori das correlações genéticas obtidas entre STAY48 com P305, IPP e PIDP foram de 0,90±0,10, -0,55±0,24 e 0,64±0,33. As correlações genéticas estimadas entre STAY60 com P305, IPP e PIDP foram de 0,94±0,07,-0,54±0,21 e 0,83±0,18. As correlações genética e fenotípica estimadas entre a stayability aos 48 e 60 meses de idade e produção de leite na primeira lactação indicam que a produção na primeira lactação pode ser usada como critério de seleção para longevidade. De forma semelhante, a seleção para reduzir a idade ao primeiro parto pode provocar, por resposta correlacionada, em melhorias na longevidade das fêmeas.