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Objectives: Reduced reproductive performance of dairy cows in combination with reduced libido and semen quality of bulls during periods of heat stress can cause important economic losses to the dairy industry. Therefore, the breeding type used in periods of heat stress should be considered to maximize reproductive performance. 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) breedingprograms. Furthermore the relationship between breeding type and parity was determined.
Materials and Methods: 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 NESW orientation with open ventilation and a mechanical fan-system. The voluntary waiting period was set at 55 d postpartum. Between 35 and 49 d cows received an initial injection of 25 mg PGF2α (Dinoprost, Dinolytic, Zoetis Deutschland GmbH, Berlin, Germany) and a second injection of 25 mg PGF2α 2 weeks later to regress the corpora lutea. Cows that showed estrus after the second injection of PGF2α were artificially inseminated or received natural service by a bull. Pedometers (Milkline, Gariga di Podenzano, Italy) and visual observation were used to detect estrus. Inseminations were performed within 12 h after detection of estrus with frozen-thawed or fresh semen. Pregnancy diagnoses were performed 35 to 42 d after the day of breeding with transrectal ultrasonography by the herd veterinarian. 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 x AT + 32) - ((0.55 - 0.0055 x RH) x (1.8 x 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.
Results: 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% and average open days were 129.3 d. Inseminations with frozen-thawed and fresh semen were conducted in 80.6% and 5.6% of the cows, respectively and 13.8% of the cows were bred by natural service. Minimum, maximum, and mean temperaturehumidity index during the study period were 34.6, 83.7, and 60.4 ± 8.5%, respectively. 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. There were no significant interactions between heat stress and the variables parity and number of services. 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. There were no significant interactions between heat stress and parity. In the multivariable logistic regression model investigating cows bred by natural service, the variables heat stress (P = 0.21), parity (P = 0.98) and number of services (P = 0.24) had no significant association to CR.
Conclusions: The present study indicates, that the likelihood to get pregnant is reduced by short and long term heat stress depending on the breeding type. Especially CR of cows inseminated with fresh semen are negatively affected by short term heat stress and cows nseminated 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. During periods of short term heat stress the application of fresh semen needs to be improved.