Getting cows to rebreed - especially right after calving or at the conclusion of a long winter - may be the biggest, most important challenge facing an operation each year.

Getting the already cycling cattle on track for breeding is the easy part. Difficulties arise when trying to get anestrous cows (cows not ready to breed following calving) back on track. While there is plenty of information regarding estrous synchronization for cycling cows and heifers, little has been done to help bring cows out of anestrus.

There is hope, however. New research from Kansas State University (KSU) is helping producers improve the success of their artificial insemination (AI) programs through new estrous synchronization techniques. These programs help induce ovulation in both cycling and non-cycling cows, and take some of the guesswork out of insemination timing.

Body condition score, age of cattle and days postpartum all play a role in cow cyclicity. Cows that are more likely to cycle at the beginning of the breeding season are well managed nutritionally. Cows should have abody condition score of at least 5 at calving.

"Body condition at calving time is most critical to cycling activity after calving," says KSU's Jeffrey Stevenson. He says 70% of beef cows with a body condition score of 5.5 are cycling at the beginning of the breeding season. "Greater body condition scores at the onset of the breeding season are associated with greater rates of cycling activity," says Stevenson.

Stevenson also notes that two-year-olds and cattle less than 60 days postpartum are at risk for failing to cycle by the beginning of the breeding season.

"Even though replacement heifers are bred to calve up to three weeks earlier than cows," says Stevenson, "as two-year-olds, their cycling rate is still less than that of older cows. Cycling activity peaks in cattle about 70 days postpartum, so fewer than 60 days postpartum is an unpredictable time for breeding."

Stevenson lists four, basic breeding programs to induce estrus and increase pregnancy rates in noncycling, suckled cows.

* Ovsynch. Devised for use in a timed AI breeding program, the protocol uses GnRH and prostaglandins to stimulate and induce ovulation. At day 0, a GnRH injection is given to stimulate endogenous lutenizing hormone (LH). The LH release causes ovulation of a follicle and subsequent formation of a corpus luteum on the ovary.

After ovulation of a follicle, the ovary begins to mature another dominant follicle. At day 7, a prostaglandin injection is given to lyse or kill the corpus luteum.

Approximately 48 hours later, the dominant follicle will have reached maturity, so another GnRH injection is given to cause ovulation. Timed insemination at 16 hours after the second GnRH injection produces the best pregnancy rates.

In noncycling cows, studies show that about 40% of cows given Synchro-Mate-B became pregnant, while closer to 50% became pregnant on the Ovsynch program. Compared to 38% for Synchro-Mate-B, the Ovsynch protocol's pregnancy rate for cycling cows was 59%.

* Cosynch. Another timed-insemination program, Cosynch follows the same protocol as Ovsynch, except timed AI was done at the time the final GnRH injection is given.

"The logic behind this protocol was to reduce the number of trips through the working facility to three," says Stevenson.

While there are no differences in pregnancy rates between Cosynch and Ovsynch, removing the calf 48 hours after the prostaglandin injection boosts pregnancy rates.

"Calf removal produced pregnancy rates that were nine percentage points greater than rates after those same protocols without calf removal," says Stevenson.

* Select Synch. A modified Ovsynch program, Select Synch calls for heat detection rather than timed AI. At day 0, a GnRH injection is given to stimulate ovulation and create a new follicle growth. At day 7, the prostaglandin is injected to lyse the corpus luteum.

The follicle is allowed to ovulate naturally, so no second GnRH injection is needed. From this point on, heat detection is used to determine estrus, with insemination occurring the standard 12 hours after detection.

The majority of cycling cows come into estrus 36-71 hours after the prostaglandin injection with Select Synch, and noncycling cows tend to come into heat very early. Because of this, Stevenson recommends that when using Select Synch on cows, start heat detection one day before the prostaglandin injection. If any cows are observed in estrus prior to the prostaglandin shot, they should be inseminated (12 hours after detection) and not given the prostaglandin injection.

Select Synch and Cosynch protocols have similar effects on pregnancy rates in noncycling cows. On the other hand, 70% of cycling cows on the Select Synch protocol became pregnant, while about 43% became pregnant on the Cosynch program. Thus, the Select Synch program seems to work best on cycling cows.

* Modified Select Synch Program. To try and increase the number of noncycling cows becoming pregnant, Stevenson modified the Select Synch program. While the injection protocol stayed the same, a pre-estrus exposure to progestin was added.

"The progestin is included to provide an additional stimulus to jump-start the noncycling cow," says Stevenson. So, at the time the GnRH injection is given, cows were implanted with norgestomet (usually a Synchro-Mate-B implant). The implant is removed at the same time the prostaglandin injection is given. Heat detection followed by insemination rounded out the protocol.

Stevenson found that Select Synch + NORG, as it is called, was effective in jump starting noncycling cows. In noncycling cows, the NORG implant prevented early heats and produced a peak in estrus activity between 36 and 48 hours after prostaglandin.

In noncycling cows, the Select Synch + NORG protocol produced a 71% pregnancy rate when compared to Select Synch alone and norgestomet alone. As Stevenson points out, "the addition of a progestin consistently improves chances for conception to the AI at the beginning of the breeding season."

All in all, giving suckled cows GnRH 7 days before prostaglandin helps with the problem of anestrus. Stevenson states, "Addition of a progestin at the time of GnRH injection further improves the response, whereas fixed-time inseminations that follow a second GnRH injection after prostaglandin may reduce fertility unless the treatment also includes a progestin."