An overview of heat stress relief with global warming in perspective

Upper limit of thermal stability and subsequent rise of thermoregulatory functions as affected by forced ventilation were examined. Rectal temperature, respiratory frequency, ear skin temperature, body weight, and milk yield were recorded biweekly July to March over 2 yr for 170 Israeli-Holstein cows (305-day milk yield 9000 kg/cow) at air temperatures 10 to 36 degrees C. Cows were in an open shelter. One side was force ventilated over 2.5 m along the stanchions (air velocity 1.5 to 3 m/s) from 0500 to 2200 h. Control side mean air velocity was .5 m/s. Within the 10 to 24 degrees C range, rectal temperature was not affected by air temperature or forced ventilation but increased by .02 degrees C/kg fat-corrected milk in animals producing above 24 kg/day. Between 26 and 36 degrees C rectal temperature increased with air temperature in both groups; rate of rise was halved by forced ventilation. In this range of air temperature, rectal temperature increased with rising milk yield, as in the lower air temperature range, in both high-producing and lower-producing cows in forced ventilation. Body weight or parity did not have significant effects. Mean ear skin temperature was higher for control animals, but its rate of increase with air temperature was similar in both groups. Forced ventilation reduced mean respiratory rate. An upper critical temperature is 25 to 26 degrees C and is independent of milk yield or acclimatizational state of cows exposed to the natural sequence of climate.

The SLICK haplotype (http://omia.angis.org.au/OMIA001372/9913/) in cattle confers animals with a short and sleek hair coat. Originally identified in Senepol cattle, the gene has been introduced into Holsteins. The objectives of the current study were to determine (1) whether lactating Holsteins with the slick hair phenotype have superior ability for thermoregulation compared with wild-type cows or relatives not inheriting the SLICK haplotype, and (2) whether seasonal depression in milk yield would be reduced in SLICK cows. In experiment 1, diurnal variation in vaginal temperature in the summer was monitored for cows housed in a freestall barn with fans and sprinklers. Vaginal temperatures were lower in slick-haired cows than in relatives and wild-type cows. In experiment 2, acute responses to heat stress were monitored after cows were moved to a dry lot in which the only heat abatement was shade cloth. The increases in rectal temperature and respiration rate caused by heat stress during the day were lower for slick cows than for relatives or wild-type cows. Moreover, sweating rate was higher for slick cows than for cows of the other 2 types. In experiment 3, effects of season of calving (summer vs. winter) on milk yield and composition were determined. Compared with milk yield of cows calving in winter, milk yield during the first 90 d in milk was lower for cows calving in the summer. However, this reduction was less pronounced for slick cows than for wild-type cows. In conclusion, Holsteins with slick hair have superior thermoregulatory ability compared with non-slick animals and experience a less drastic depression in milk yield during the summer. Copyright © 2014 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.

Environmental heat stress, present during warm seasons and warm episodes, severely impairs dairy cattle performance, particularly in warmer climates. It is widely viewed that warm climate breeds (Zebu and Sanga cattle) are adapted to the climate in which they evolved. Such adaptations might be exploited for increasing cattle productivity in warm climates and decrease the effect of warm periods in cooler climates. The literature was reviewed for presence of such adaptations. Evidence is clear for resistance to ticks and tick-transmitted diseases in Zebu and Sanga breeds as well as for a possible development of resistance to ticks in additional breeds. Development of resistance to ticks demands time; hence, it needs to be balanced with potential use of insecticides or vaccination. The presumption of higher sweating rates in Zebu-derived breeds, based upon morphological differences in sweat glands between breeds, has not been substantiated. Relatively few studies have examined hair coat characteristics and their responses to seasonal heat, particularly in temperate climate breeds. Recently, a gene for slick hair coat has been observed that improved heat tolerance when introduced into temperate climate breeds. No solid evidence exists that hair coat in these lines is lighter than in well-fed warm climate-adapted Holsteins. Warm climate breeds and their F1 crosses share as dominant characteristics lower maintenance requirements and milk yields, and limited response to improved feeding and management. These characteristics are not adaptations to a feed-limited environment but are constitutive and useful in serving survival when feed is scarce and seasonal and high temperatures prevail. The negative relationship between milk yield and fertility present in temperate climates breeds also prevails in Zebu cattle. Fertility impairment by warm conditions might be counteracted in advanced farming systems by extra corporeal early embryo culture. In general, adaptations found in warm climate cattle breeds did not increase heat dissipation capacity, but rather diminished climate-induced strain by decreasing milk production. The negative relationship between reproductive efficiency and milk yield, although relatively low, also appears in Zebu cattle. This association, coupled with limited feed intake, acting over millennia, probably created the selection pressure for a low milk production in these breeds.