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The objective of the study was to determine the influence of short and long term exposure to heat stress on the CR of lactating dairy cows in different natural service and artificial insemination (AI) breeding programs. Furthermore the relationship between breeding type and parity was determined. The retrospective study was conducted on a commercial dairy farm in Sachsen-Anhalt, Germany from May 2010 to October 2012. The herd consisted of 1,150 Holstein dairy cows with an average milk production of 10,345 kg. The barn was positioned in a NE-SW orientation with open ventilation and a mechanical fansystem. After 35 d post
partum cows were treated with a presynchronization program (PGF 2α-14d-PGF2α). Cows that showed estrus after the second injection of PGF2α received natural service by a bull or AI with frozen-thawed or fresh semen. Pregnancy diagnoses were performed 35 to 42 d after the day of breeding. Ambient temperature and relative humidity within the barn were recorded using a Tinytag Plus II logger (Gemini loggers Ltd, Chichester, UK) and used to calculate the temperature-humidity-index (THI) according to the equation reported by the NRC (1971): THI = (1.8 × AT + 32) – ((0.55 – 0.0055 × RH) × (1.8 × AT – 26)). Short term heat stress was defined as a mean THI≥73 at the day of breeding and long term heat stress was defined as a mean THI≥73 in the period from d 21 to d 1 before day of breeding. The dataset contained 5,192 breeding records from 1,537 lactating dairy cows on a single dairy farm. The overall conception rate (CR) obtained was 33.0%. Multiparous cows bred by AI with frozen-thawed semen were 22% less likely to get pregnant than primiparous cows. Cows bred by AI with frozen-thawed semen exposed to long term heat stress were 63% less likely to get pregnant than cows not exposed to heat stress. Cows bred by AI with frozen-thawed semen receiving ≥4 services were 15% less likely to get pregnant than cows receiving 1 service. Multiparous cows bred by AI with fresh semen were 67% less likely to get pregnant than primiparous cows. Cows bred by AI with fresh semen exposed to short term heat stress were 80% less likely to get pregnant than cows not exposed to heat stress. The present study indicates, especially CR of cows inseminated with fresh semen are negatively affected by short term heat stress and cows inseminated with frozen-thawed semen are negatively affected by long term heat stress. The CR of cows bred by natural service were not affected by short and long term heat stress. Therefore climate conditions should be considered in the selection of breeding strategies to
optimize AI and resulting CR.