Hot weather causes heat stress in dairy cattle. Although effects are more severe in hot climates, dairy cattle in areas with relatively moderate climates also are exposed to periods of heat stress. The resultant decrease in milk production and reproductive efficiency can be offset by implementation of a program consisting of cooling through shades, ventilation and spray, and fans. The economic benefit should be determined before installation of equipment to reduce heat stress.

Dairy Herd Improvement data from a large, commercial dairy farm were used to evaluate the relationships of first lactation milk and fat yield and age at first calving on passively acquired immunity in Holstein heifers. Total serum Ig concentration was measured 24 to 48 h postpartum (mean = 25.5 mg/ml, SD = 19.2) on calves allowed to suckle their dams freely through 24 h postpartum. No additional colostrum was supplied and no attempt was made to ensure adequate colostrum intake. Calves were raised in Tucson, AZ until 6 mo of age, then transferred to Idaho to complete the growing phase and to be bred. Heifers were returned to Tucson during their 7th or 8th mo of gestation. Serum Ig concentration, measured shortly after birth, was an important source of variation for mature equivalent milk (b = 8.5 kg/Ig unit) and mature equivalent fat (b = .24 kg/Ig unit) production in the first lactation but did not affect age at first calving. It is impossible to ascertain from these data whether Ig concentration at birth was directly or indirectly related to production; regardless, supplying an adequate amount and concentration of colostrum at the appropriate time to dairy heifers may enhance their future productivity.

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.

The influence of acute exposure to thermal stress on the secretion of progesterone, estradiol, cortisol, and LH was monitored in 14 lactating Holstein cows. Eight cows were maintained throughout the summer in a refrigerated air-conditioned tie stall barn. An additional eight cows were maintained in outdoor corrals with access only to shade. Rectal temperatures and respiration rates of cows under heat stress conditions were elevated above the cows maintained under air conditioning. Cows in both environments exhibited similar serum concentrations of progesterone and estradiol throughout the estrous cycle. Serum cortisol concentrations were higher in heat-stressed cows compared to cows maintained under cooling. The number of LH pulses on d 5 of the estrous cycle was greatly reduced in the heat-stressed cows compared to the cows under cooling. No differences in the number of pulses of LH were observed on d 12 of the estrous cycle between cows in the two environments. Data suggest that heat stress can suppress anterior pituitary release of LH without having a measurable influence on ovarian steroid hormone secretion.

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