By Todd Bilby
STEPHENVILLE, Texas – Heat stress (HS) negatively impacts all aspects of dairy cattle production. Milk production decline and reproduction losses during the summer substantially impact the economic potential of dairy farms.
The annual economic impact of HS on American animal agriculture has been estimated at $2 billion, with the dairy industry alone accounting for $900 million of this loss. Heat stress occurs over a wide combination of solar radiation levels, ambient temperatures, and relative humidity. This is further aggravated by metabolic heat production (generated by the cow herself).
Generally, it is assumed that a cow becomes more sensitive to HS as milk production increases due to elevated metabolic heat production. The dairy industry continues to focus on selecting for production traits which, in turn, may increase the dairy cow’s susceptibility to HS, further intensifying the summer decline in milk production and reproduction.
In addition, selecting for milk yield reduces the thermoregulatory range of the dairy cow (Berman et al., 1985).
Breeds predominantly used in the U.S. dairy industry were developed in temperate climates, and are most productive between the temperatures of 41 and 59 degrees Fahrenheit. Cows experience a loss in production when temperatures increase from 59 to 77 degrees Fahrenheit (Hahn, 1985).
However, dramatic reductions are observed when the temperature exceeds 77 degrees F. Consequently, strategies should be initiated to lessen the severity of HS on both reproduction and milk production to improve cow performance and farm profitability
Traditionally, dry pregnant cows are provided little protection from HS because they are not lactating and it is incorrectly assumed they are less prone to HS. Additional stressors are imposed during this period due to abrupt physiological, nutritional, and environmental changes. These changes can increase the cows’ susceptibility to HS and have a critical influence on postpartum cow health, milk production and reproduction.
The dry period is particularly crucial since it involves mammary gland involution and subsequent development, rapid fetal growth, and induction of lactation. Heat stress during this time period can affect endocrine responses that may increase fetal abortions, shorten the gestation length, lower calf birth weight, and reduce follicle and oocyte maturation associated with the postpartum reproductive cycle.
Many studies reporting subtle effects of HS on subsequent fertility were published more than 20 years ago when the average milk yield was much less than it is today.
In addition, our cooling systems and knowledge of proper cooling (when, where, and to what extent) to reduce HS has increased substantially.
A study conducted in Saudi Arabia on three different farms observed an improvement in peak milk production (90.9 vs. 87.2 lbs), decreased services per conception (3.1 vs. 3.7 services), and reduced culling for reproductive failure (7.7% vs. 19%) for dry cows evaporatively cooled vs. shade only (Wiersma and Armstrong, 1988).
More recently, Avendano-Reyes et al. (2006) concluded that cooling dry cows with shades, fans, and water spray vs. cows with only shade decreased services per conception and days open, and increased milk yield during the postpartum period.
In 2006, Urdaz et al. observed that dry cows with feed line sprinklers, fans and shade compared to cows with only feed line sprinklers had an increased 60 d milk yield with no difference in body condition score (BCS) changes, incidence of postparturient disorders, or serum nonesterified fatty acid concentrations. In this study, reproductive parameters were not measured; however, cooling dry cows with shades, fans, and sprinklers compared with only sprinklers improved total 60 d milk production by 185.5 lb/cow, and increased estimated annual profits by $2,131/cow (based on milk only).
The problem of carryover effects from summer HS to fall fertility may be accentuated due to HS during the dry period. It is well known that a period of approximately 2 months is needed for low autumn fertility to be restored to the level prevailing in the winter. It takes approximately 40-50 days for antral follicles to develop into large dominant follicles and ovulate (Roth et al., 2001). If HS occurs during this time period both the follicle and oocyte inside the follicle become damaged. Once ovulation occurs, the damaged oocyte has reduced chances of fertilizing and developing into a viable embryo.
Cooling dry cows may reduce HS effects on the antral follicle destined to ovulate 40-50 d later, which coincides with the start of most breeding periods, and possibly increases first service conception rates.
The greatest opportunity to reduce the negative effects of HS during both the pre- and postpartum periods is through cooling. As mentioned above, cooling dry cows with feed line sprinklers, fans and shades proved to be beneficial for reducing services per conception, reproductive culls, days open, and increasing milk yield with a significant return on investment compared to cows with either shades alone or feed line sprinklers alone. (Wiersma and Armstrong, 1988; Avendano-Reyes et al., 2006; Urdaz et al., 2006).
In addition to proper cooling, changing management decisions may help reduce the severity of HS in areas of intermittent heat waves. For instance, at dry-off, many cows receive vaccines that can cause a fever spike which, when coupled with HS, can cause body temperature to rise above normal (101.3-102.8 degrees F).
In the 2006 California heat wave, many cows died (not only in the fresh pen as expected) within the first few days of dry off (personal unpublished observations). Possibly, during severe heat waves it would prove beneficial to delay vaccinations at dry-off if the dry pen does not contain adequate cooling.
■ To contact Dr. Todd Bilby, e-mail him at firstname.lastname@example.org or call 254-968-4144.