Records representing 19,266 Holstein cows from Arizona DHIA data over a 5-yr period were analyzed to determine the effects of season and lactation number on milk production and reproduction. Seasons were winter (December, January, and February), spring (March, April, and May), summer (June, July, and August), and fall (September, October, and November). Traits analyzed by least squares ANOVA were 305-d FCM, complete lactation milk, calving interval, and services per conception. All sources of variation were significant except the interaction between lactation number and season of calving for complete lactation milk. Milk production was depressed for cows calving in summer and fall. First lactation cows had lowest milk production, and highest production occurred in either lactation 4 or 5. Cows calving in spring and summer had reduced reproductive performance, as measured by calving interval and services per conception. First lactation cows had lowest values for both reproductive traits. Previous days dry was negatively related to milk production for spring calvings but was positively related for all other seasons. Cows with higher milk production had reduced reproductive performance. Partial regression coefficients for calving interval and services per conception were 12 d and .25 services per conception per 1000 kg of 305-d FCM, respectively. Despite the negative effects of thermal stress, milk production and fertility in this study were not depressed as severely as in previous research reported from Arizona. Calving schedules may be adjusted to minimize the adverse effect of heat stress.

A total of 648 purebred Holstein and 319 backcross Holstein × Jersey dairy cattle were compared for production, reproduction, health, linear type, and growth traits. Animals were born between 2003 and 2009 and were housed in the University of Wisconsin-Madison Integrated Dairy Facility. All animals had Holstein dams; lactating dams were mated to unproven Holstein sires to produce purebred (control) Holsteins or to unproven F(1) Jersey × Holstein crossbred sires to produce backcross animals, whereas nulliparous dams were mated to proven Holstein sires to produce purebred (other) Holsteins. Traits were analyzed using mixed linear models with effects of season of birth, age of dam, sire, birth year of sire, days in milk, lactation, and linear type score evaluator. Control Holsteins had greater 305-d milk yield (12,645 vs. 11,456 kg), 305-d mature equivalent milk yield (13,420 vs. 12,180 kg), peak daily milk yield (49.5 vs. 46.4 kg), total lactation milk yield (11,556 vs. 10,796 kg), and daily fat-corrected milk yield (43 vs. 40 kg) compared with backcrosses. Days open and services per conception as a heifer or cow did not differ between control Holsteins, other Holsteins, or backcrosses. The proportion of first-parity births that required assistance was less in control Holsteins than in backcross cows (3.7 vs. 11.2%). The incidence of scours or respiratory problems in calves did not differ between control Holsteins, other Holsteins, and backcrosses, nor did the incidence of mastitis, injury, or feet problems. Control Holstein heifers were heavier (629 vs. 557 kg), with greater hip height (145 vs. 139 cm), body length (167 vs. 163 cm), heart girth (205 vs. 198 cm), and hip width (54 vs. 53 cm) at 22 mo of age. On a 50-point scale for linear type traits, Holsteins were larger in stature compared with backcrosses (41 vs. 28), had wider rumps (37 vs. 33), and wider rear udders (34 vs. 32). Results of this study suggest that backcross Holstein × Jersey cattle have decreased production but fail to demonstrate an advantage in health and reproduction compared with purebred Holsteins.