The overarching aim was to examine the relationship of rectal temperature at fixed time artificial insemination (FTAI) on pregnancy outcomes in a typical breeding season with expected pregnancy rates approaching 50% using Bos indicus and Bos taurus cattle. This represents a continuum of steps to test the hypothesis that elevated body temperature at or around insemination is functionally important to maximize pregnancy outcomes. Rectal temperature of Bos indicus cattle at FTAI ranged from 37.0 to 40.9 °C; 60.6% were hyperthermic. Positive factors impacting pregnancy outcomes were rectal temperature at FTAI, body condition, and estrus patch scores. Rectal temperature at FTAI was positively associated with pregnancy outcomes (P < 0.0001); per each 1 °C increase pregnancy odds increased 1.9 times (95% CI: 1.4 to 2.6). Highest pregnancy outcomes occurred with rectal temperatures exceeding 40 °C (P = 0.0004). Rectal temperature before FTAI in Bos taurus cattle ranged from 37.8 to 41.8 °C; 43.3% were hyperthermic. Factors impacting pregnancy were rectal temperature at FTAI, estrus activity, parity, and ambient conditions on day of FTAI. Rectal temperature of Bos taurus cattle at FTAI was positively associated with pregnancy (P = 0.0286); odds increased 1.45 times (95% CI: 1.0 to 2.0) per each 1 °C increase. Highest pregnancy outcomes occurred with rectal temperatures at FTAI exceeding 40 °C (P = 0.057). Moreover, positive relationship of rectal temperature at FTAI to pregnancy persisted in estrual females (71.25% of total; P = 0.0408; OR 1.5; 95% CI: 1.0 to 2.2). Mindful that 1) elevated temperatures observed in Bos indicus and Bos taurus cattle directly promote meiotic resumption of the oocyte in vitro and that 2) in vivo hyperthermia alters intrafollicular components which others have shown to potentiate ovulation and promote meiotic resumption, it is biologically plausible that an acute elevation in body temperature at or around time of insemination is functionally important to maximize pregnancy outcomes.
Keywords: Bos indicus; Bos taurus; estrus; fixed-time artificial insemination; hyperthermia; pregnancy.
Reproductive efficiency remains a major challenge for beef producers with 35% to 55% of females failing to become pregnant after a single insemination. While basis for failure is multi-factorial, heightened estrus activity matters for pregnancy outcomes, even when synchronizing ovulation for fixed time artificial insemination. Body temperature increases of 1.5 °C+ are common during estrus. We hypothesize that higher estrous-associated temperatures (HEAT) at/near insemination are functionally important to maximize pregnancy outcomes. Elevated temperatures equivalent to what is observed in females exhibiting HEAT have been shown to induce oocyte meiotic resumption. An acute episode of hyperthermia after the LH surge alters intrafollicular components known to potentiate ovulation and affect the oocyte. Effort to examine the relationship of rectal temperature at fixed time artificial insemination with pregnancy outcomes in a breeding season with expected pregnancy rates >50% represents a next step in the continuum of hypothesis testing. The positive relationship of rectal temperature at insemination with pregnancy outcomes that was discovered adds to foundational knowledge. Because the degree of hyperthermia is related to highest pregnancy outcomes, a case is made for HEAT to be biologically and functionally important to maximize pregnancy outcomes in cattle.
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