Fertility of the herd may be the most difficult factor to evaluate, Perry says.
"Accurate detection of estrus, inseminator efficiency and fertility of the semen (Parts l, 2, and 3) of this discussion are all vital to the success of any breeding program. However, even when these three elements are well managed, if the cow herd fertility level is compromised, pregnancy rates may not meet cattlemen’s expectations," Perry said.
When Perry discusses herd fertility he is referring to a herd’s cycling/puberty status, compliance with protocols, embryonic mortality, body condition score (nutrition level) and disease control.
Perry says non-cycling cows at breeding time may result from a number of factors including dystocia, calving late, inadequate nutrition levels (pre and post calving), cow age or excessive milk production in relationship to the feed resources available or severe weather conditions. In addition, heifers not developed properly and failing to reach 55 percent to 65 percent of their mature weight by breeding time may not cycle or conceive if they do.
Synchronization protocols that utilize a progestin can help cows/heifers that have not initiated normal estrous cycles if they are almost ready to begin having normal estrous cycles.
"These protocols are the result of time-consuming research and are a valuable tool when incorporated accurately into breeding programs in conjunction with good herd management," Perry said. "However, regimented use of them is essential for satisfactory results."
When implementing protocols, Perry says advanced planning is important.
"Timing of prebreeding vaccinations needs to be well in advance of insemination," he said. "Cattle producers need to plan when injections or feeding need to occur; plan access to facilities and line-up additional labor. When insemination will occur must be planned well in advance of protocol use."
Embryonic mortality
Fertilization rates are usually between 89 percent and 100 percent when semen is present at the time of ovulation. However, Perry says early embryonic mortality causes that percentage to drop to about 60 percent to 70 percent.
"Several management decisions can impact the percent of embryos lost to early embryonic mortality," Perry said.
One factor he says is the timing of transporting cows and heifers after insemination.
"Research conducted at the USDA research center in Miles City, Mont., reported transporting cows/heifers from day 5 and 42 after insemination is a very sensitive time for the embryo and can be a major factor in embryo mortality," he said.
Another factor is changes in nutritional status.
"This can also have a tremendous influence on embryonic survival," Perry said.
He points to research conducted at Oklahoma State University showed that sever changes in intake of energy and protein can result in heifer stopping normal estrous cycles.
"Furthermore, work done at South Dakota State University showed that moving heifers, who developed all winter in a feedlot, to pasture immediately after AI can increase early embryonic losses," Perry said.
Body condition score & disease
Body condition score (BCS) and disease are two additional causes of marginal fertility rates says Perry.
"Research recommendations suggest that cows be in a minimum BCS of 5 and heifers 6 at calving time in order for them to cycle and re-breed on an annual basis," he said. "This allows sufficient body reserves for lactation and to initiate normal estrous cycles after calving."
However, Perry notes, if adequate nutrition is not available after calving, body condition can be lost and may delay the return to normal estrous cycles.
Overall health of the herd can impact herd fertility says Perry.
"Cattle producers need to implement a proper pre-breeding vaccination program along with a well-managed, internal and external parasite application program. This will help limit disease occurrences in the herd and promote herd fertility," Perry said.
He adds that special care should be taken with virgin heifers.
"Several studies have reported negative impacts on pregnancy success by vaccinating heifers that have never been vaccinated before with a modified live vaccine (MLV) for BVD or IBR around time of breeding," he said. "Therefore, general recommendations for vaccination of replacement heifers include; before and at weaning, with both heifers and cows receiving a booster vaccine at least 30 days before breeding. If it is absolutely necessary to give a modified live vaccine less than 30 days prior to breeding, the vaccine should be administered as soon as possible and only to animals that were vaccinated both before and at weaning. Animals that have not previously been vaccinated (naïve animals) should not be vaccinated near the time of breeding."
The "Equation of Reproduction," which has been discussed in this four-part series, highlights management practices that are essential to any successful beef breeding program. 
When we are "Managing for Reproductive Success," it involves cattle producers making management decisions throughout the entire year – not just prior to the breeding season. By doing this, producers can expect to generate successful reproductive results.
"As we increase the reproductive efficiency within a herd, we can increase our management decisions on genetic improvement and other factors to increase the profitability of your herd," Perry said.

For more information related to detecting standing estrus contact, Jim Krantz, SDSU Extension Cow/Calf Field Specialist at [email protected] or 605-995-7381 or Dr. George Perry, SDSU Extension Beef Reproductive Management Specialist at [email protected] or 605-688-5456. To listen to a recent iGrow Radio Network interview on this topic with Dr. George Perry, and to review all four articles in this four-part series released by SDSU Extension visit iGrow.org.

Part 4 of a four-part series

Fertility is influenced by many factors, and one of the best methods to look at factors that influence fertility is with the ‘Equation of Reproduction,’ says George Perry, SDSU Extension Beef Reproductive Management Specialist.
Perry explains that the ‘Equation of Reproduction’ includes the following four areas:
Percentage of animals detected in standing estrus and inseminated;
Inseminator efficiency;
Fertility level of the semen and;
Fertility level of the herd.

Each of the areas will be discussed in the four-part series on managing for reproductive success by SDSU Extension.