Embryo Transfer Viable Option During Summer Heat Stress
Source: Dr. Jillian Bohlen, Assistant Professor Ms. Lark Widener, Dairy Graduate Student, University of Georgia
Many only think of embryo transfer (ET) as it relates to expanding the genetic elite. For those in the south, with daunting months of elevated temperatures, this advanced reproductive technique (ART) may provide an additional benefit. Collecting cows in cool months for transfer in hot may help bypass the debilitating stress of summer, when high humidity and ambient temperature wreak havoc on a producer’s reproductive program. Proper cooling mechanisms can help offset some of the deleterious effects but most would agree that getting cows pregnant over summer is a considerable challenge. Pregnancies achieved over summer would help avoid long days in milk and reduce days of low income over feed costs.
Dairy cows are amongst a larger cohort of species that suffer reduced reproductive success during times of heat stress. The lactating dairy cow is unique in that heat stress occurs at a much lower ambient temperature than for most other species. This is due in large part to the vast amounts of internal, metabolic heat she creates when converting large volumes of feed to milk. Knowing when cows are heat stressed is critical; however can be challenging because stress levels are dependent on both temperature and humidity. Most are familiar with the temperature‐ humidity‐index (THI) for lactating dairy cows and now there is an “app for that.” Purina Animal Nutrition, LLC offers an app called Purina Cool CowTM that can help you track when animals in your herd are under various levels of heat stress.
Embryo transfer has the capability of providing more pregnancies over artificial insemination (AI) during the hot summer months where elevated THI numbers have cows feeling the stress. To begin to understand why, it is important to understand how reproduction is negatively impacted by heat stress. Heat stress hinders a number of critical control points for reproduction in hot summer months. These include reduced estrous expression (heats), changes in follicular development, reduced oocyte competency, decreased fertilization rates, and reduced embryonic quality. It is unlikely that these factors work independently to hamper reproduction but instead that the combination of events results in pregnancy failure.
The first three factors mentioned (heats, follicular development, oocyte competency) all precede AI while the fourth (fertilization rate) occurs just hours after. If utilizing ET, these first four factors would have occurred in the donor cow, which was collected in cooler months and thus would not be impacted by the high THI environment the recipient cow is currently enduring. Interestingly, the effect of heat stress on the remaining factor (embryo quality) may also be circumvented with the use of ET. This piece of the puzzle is a more complicated to explain.
Early embryonic death (EED) is one of the leading reasons for reduction in pregnancy rates over the summer months. Early embryonic death is best defined as those embryonic losses that occur prior to maternal recognition of pregnancy, which occurs at approximately 14‐17 days after fertilization. For the producer, these 2 deaths are unrecognizable and pregnancy failure often misclassified because there is no disruption in cyclicity. Evidence of why heat stressed embryos undergo higher rates of EED is found abundantly in the literature. The basic concept is that heat stressed embryos exhibit retardation of development with a reduced developmental stage and alterations of morphological characteristics. Whether the embryo is developing in a heat stressed dam or exposed to elevated temperatures in culture, it is apparent that heat has a direct impact on embryonic development and its competency to establish pregnancy. The inability to establish pregnancy may be the direct result of embryonic death or the inability of the developmentally delayed embryo to signal for recognition by the dam. The complete cause of EED as a result of heat stress is likely multifactorial.
Numerous trials have directly demonstrated the impact of heat stress on early embryonic development and pregnancy rates. In these research trials, animals were kept at thermoneutral or “comfortable” conditions through breeding. After breeding, half of the animals had their environment changed to hyperthermic or “heat stressed” for approximately seven days with the use of environmental chambers. In both heifers and cows, pregnancy rates were greatly reduced in those animals that moved to hyperthermic conditions post insemination. Studies where hyperthermic conditions were induced later in embryonic development (after day 7) showed less impact on pregnancy rates. This latter piece of research evidence is where the promise of ET lies.
As it happens, ET is capable of at least partially overcoming this final insult that heat stress plays on reproduction because as the embryo develops, its resistance to heat stress actually increases. Thereby, using ET would allow a more developed and more heat resistant embryo to be transferred into the heat stressed cow.
Further work to demonstrate the promise of using ET to combat developmental issues of the embryo caused by heat stress is provided by crossover trials. In these trials, standard embryo flush and recovery procedures were followed with embryo collection at approximately 6.5‐7 days of age. Recipients, either in thermoneutral or heat stressed conditions were then assigned embryos from either thermoneutral or heat stressed donors.
Embryos collected from a thermoneutral donor animals maintained a higher rate of successful pregnancies than those from a heat stressed donors regardless of the recipient’s thermal environment. Additionally, heat stressed recipients of embryos from thermonetural donors had higher pregnancy rates than heat stressed animals bred with conventional AI. As a final piece, in vitro produced embryos transferred into heat stressed animals resulted in improved pregnancy rates when compared with AI during bouts of heat stress.
As a leaving note, ET is an alternative way to explore improving reproductive performance of cows during the battle of heat stress. As with AI, success with ET will rise when coupled with exceptional heat abatement practices. The embryos should be recovered from cows during periods without heat stress and stored for use in the warmer months. ET may also be important in the months coming out of heat stress as the effects of heat stress can stay with cows after the days of summer have passed.
Evidence suggests that there is an impact on the developing follicular pool that may remain evident for approximately 2 months or 3‐4 cycles after final heat insult. Only after this point might a producer reclaim the reproductive rates that he finds in the depth of winter. In addition to ET, there are a number of other proven strategies to help offset the effects of heat stress on a reproductive program.
These include forcing accessory corpora lutea (CLs), providing supplemental progesterone after AI, and the use of rBST. Using ET does require a higher investment and preplanning than traditional AI but offers the promise of paying back with interest in pregnancies during summer months.
