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Increase fertility, heifer pregnancies during heat stress

Todd Bilby Published on 12 May 2011

Reproductive failure is the number one reason for involuntary culling, which contributes to the economic importance of getting cows pregnant in a timely manner. Fertility results from a combination of many factors, including increase in milk production per cow, management and environmental factors such as heat stress.

Heat stress not only reduces milk production, but further reduces fertility. The total annual economic impact of heat stress on the American dairy industry has been estimated at $1.5 billion, with an estimated economic loss of $132 million to the Texas dairy industry alone.



One reason for the drastic decline in fertility during summer is the early-growing embryo’s (two to three days of life) susceptibility to the negative effects of heat stress. However, the use of embryo transfer of a seven-day-old embryo potentially could bypass those negative effects.

Recently, our group conducted a study to determine whether transfer of fresh or frozen embryos could improve fertility during summer in lactating dairy cows compared to artificial insemination (A.I.). All embryos were produced in vitro using sex-sorted semen and cultured for seven days until transferred.

A total of 722 lactating dairy cows were enrolled last summer at two commercial dairies in central Texas. Cows were randomly assigned to one of three treatments. Conventional A.I. (n = 227) was used as the control compared to embryo transfer of either frozen (n = 279) or fresh (n = 216) embryos seven days after a synchronized estrus.

All cows were submitted to the farms’ estrous synchronization protocol. The control group was bred either at timed-A.I. or A.I. following detected estrus. The other two groups had embryos transferred seven days after the synchronization protocol was completed.

At initial pregnancy diagnosis (40 ± 7 d), the percent of cows pregnant was greater for fresh embryos (42.1 percent) versus both frozen embryos (29.3 percent) and A.I. (18.3 percent). Also, the percent of cows pregnant was greater for frozen embryos (29.3 percent) than A.I. (18.3 percent).


At second pregnancy diagnosis (97 ± 7 d), the percent of cows pregnant remained greater for fresh (36.4 percent) and frozen (25.7 percent) compared to A.I. (17.0 percent). Again, the percent pregnant was greater in the fresh (36.4 percent) than frozen (25.7 percent) treatment group.

The amount of embryo loss did not differ between treatments. Transfer of both frozen and fresh embryos, produced in vitro using sex-sorted semen, improved fertility and number of heifer pregnancies in lactating dairy cows during summer. In addition, fresh embryos restored fertility to levels normally seen during cool months.

In conclusion, the use of in vitro-produced embryos with sex-sorted semen can bypass the deleterious effects of heat stress on fertility while increasing the number of heifer pregnancies. This could be a viable option for producers to adopt to maintain fertility during summer months. PD

References omitted due to space but are available upon request to .

—Excerpts from Texas Dairy Matters newsletter Vol. 4, Summer 2010

Todd Bilby is an extension dairy specialist with Texas A&M University. Email Todd Bilby.