Genetic methods of improving dairy cattle for the south: a review and prospects from Regional Project S-49

An overview of heat stress relief with global warming in perspective

Global warming seems more probable, whether as gradual warming or increased frequency of warmer episodes. The productivity of cattle in temperate countries will decline unless counteracting steps are adopted. The probability of pre-emptive breeding for maintaining temperate breed performance coupled with heat stress tolerance is too low to be adopted for counteracting warming. The expected warming will mostly involve temperature increases. These will indirectly affect radiant heat gain in animals owing to reduced radiant heat dissipation from the body by convective heat loss, which results in an increased sensitivity to incoming radiant heat at higher air temperatures. These necessitate an emphasis on increasing convective heat loss by structure design and forced air flow by fans. Convective heat loss diminishes with increasing air temperatures. Evaporative heat loss remains the alternative. Evaporative cooling of the ambient requires partial enclosing of the space surrounding the animals and is limited by the humidity in ambient air. An alternative was developed of coupling forced ventilation with wetting of animal surface. The exchange of ambient air flowing on animal surface makes the evaporation practically independent of air humidity and the loss of heat from animal surface practically independent of the surface to air temperature gradient. The coupling of forced ventilation with wetting combination may be attained in various parts of the dairy farm, the holding area of the milking parlour, the feeding trip and the resting area. Each of these requires differing structural and technological adaptations. Climate and farming systems vary between locations which require specific solutions.

Prepartum and peripartum reproductive performance of dairy heifers freshening at young ages

Reproductive performance of five breeds (81% Holstein or Jersey) in the Florida Agricultural Experiment Station herd was evaluated from data collected over 20 yr. Data represented 1144 parturitions after gestations greater than 250 d with all twin births excluded. Number of services per conception was not affected by age at first insemination. Mean age of exposure to first service was 414 d; mean age at first parturition was 25.8 mo. Overall, 39.7% of heifers calved at 23 mo or less. Intermediate ages, 24 to 27 mo, comprised 33.9% of heifers, whereas 26.4% of heifers calved at greater than or equal to 28 mo. Overall incidences of problems at parturition were retained fetal membranes, 3.6%; dystocia, 3.8%; metritis, 10.5%; and stillbirth, 11.5%. Frequencies of stillbirth and retained fetal membranes apparently were not affected by age, but metritis increased linearly with age at first parturition. Holsteins experience higher incidences of all problems than did Jerseys. Heritability (from paternal half-sib correlation) of birth weights and gestation lengths were .31 and .24; genetic correlation was .14. Heritabilities of occurrence of stillbirth, retained fetal membranes, metritis, and dystocia were negative and considered to be 0. Heritabilities of age at first service, days from first service to conception, number of services, and age at first parturition were .22, .11, .05, and .43. Dairy producers can consider reducing age at first parturition to at least 22 mo, under environmental and management conditions of this study, without increased frequencies of problems at parturition.