Effects of Environmental Heat on Conception Rates in Lactating Dairy Cows: Critical Periods of Exposure

Abstract

Environmental heat can reduce conception rates (the proportion of services that result in pregnancy) in lactating dairy cows. The study objectives were to identify periods of exposure relative to the service date in which environmental heat is most closely associated with conception rates, and to assess whether the total time cows are exposed to high environmental heat within each 24-h period is more closely associated with conception rates than is the maximum environmental heat for each 24-h period. A retrospective observational study was conducted in 25 predominantly Holstein-Friesian commercial dairy herds located in Australia. Associations between weather and conception rates were assessed using 16,878 services performed over a 21-mo period. Services were classified as successful based on rectal palpation. Two measures of heat load were defined for each 24-h period: the maximum temperature-humidity index (THI) for the period, and the number of hours in the 24-h period when the THI was >72. Conception rates were reduced when cows were exposed to a high heat load from the day of service to 6 d after service, and in wk -1. Heat loads in wk -3 to -5 were also associated with reduced conception rates. Thus, management interventions to ameliorate the effects of heat load on conception rates should be implemented at least 5 wk before anticipated service and should continue until at least 1 wk after service. High autocorrelations existed between successive daily values in both measures, and associations between day of heat load relative to service day and conception rates differed substantially when ridge regression was used to account for this autocorrelation. This indicates that when assessing the effects of heat load on conception rates, the autocorrelation in heat load between days should be accounted for in analyses. The results suggest that either weekly averages or totals summarizing the daily heat load are adequate to describe heat load when assessing effects on conception rates in lactating dairy cows.

Determination of potential risk of heat stress of cattle in four locations of Central and Southern Chile

“…In Nebraska, USA, Amundson et al (2006) reported a decrease in conception rates at THI greater than 72.9, but at the same time reported a positive effect of wind speed on conception rate. Similar responses were reported by Morton et al (2007) in Australia. In addition, cows under heat stress have reduced duration and intensity of estrus, altered follicular development, and impaired embryonic development (Jordan 2003).…”

Factors associated with 56-day non-return rate in dairy cattle

“…The warmest month during the survey was June 2009, with mean maximum temperatures ranging from 19-23ºC, and mean relative humidity (RH) from 62-91% (Meteogalícia, 2010). Upper critical temperatures for lactating cows can be as low as 24-27ºC, with high RH (Morton et al, 2007). These temperatures could be reached inside the farms only occasionally.…”

Do Dairy Cattle Need Protection against Weather in a Temperate Climate? A Review

“…The data used were mean values calculated on resp. 727, 25505, 11, 17 and 200 records (Hansen & Aréchiga, 1997;Morton et al, 2007;Nabenishi et al, 2011;Tao & Dahl, 2013;Zimbelman et al, 2007). One can assume normality of distribution of the original population, based on the central limit theorem stating that the distribution of the sum (or average) of a large number of independent, identically distributed variables will be approximately normal, regardless of the underlying distribution (Stigler, 1986).…”

“…Hansen and Aréchiga (1997) found that embryos were most sensitive to a high body temperature one or two days after conception, but this effect could be extended to six days after insemination (Morton, Tranter, Mayer, & Jonsson, 2007). Berman (2011) suggested breed differences, i.e.…”

“…In fact, interactions explained relatively more the observed variation in test data than main factors such as dry air temperature or dew point temperature (Brown-Brandl, Jones, & Woldt, 2005). However, also postponed physiological effects from a few days up to two to six weeks after the climatological event have to be considered (Jordan, 2003;Morton, Tranter, Mayer, & Jonsson, 2007;D. L. Smith, T. Smith, Rude, & Ward, 2013).…”