Objective 1 was to investigate effects of heat stress and breed on milk and component yield for Holstein and Jersey cows on the same farm. Objective 2 was to determine the effects of breed on udder health as measured by somatic cell count (SCC) during times of heat stress. Data were collected from DHIA records of 142 Jersey cows and 586 Holstein cows from the University herd at Mississippi State University. During heat stress Jersey milk yield and 4% fat corrected milk (FCM) increased (P<0.01). Holstein milk yield and FCM decreased during heat stress (P<0.01). Heat stress affected somatic cell count (SCC) although effects varied by intensity of heat stress. Breed did not have an affect on SCC. Milk fat and protein percentages declined for both breeds in heat stress conditions. Milk fat but not milk protein of Jersey cows increased as stress increased from mild to severe.

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.