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”. The Equation of Reproduction includes the following 4 areas:
- Percentage of animals detected in standing estrus and inseminated
- Inseminator efficiency
- Fertility level of the semen
- Fertility level of the herd.
Each of the preceding areas was discussed in this series.
The concluding segment (Part IV) of the “Managing for Reproductive Success” and final factor of the “Equation of Reproduction” discussion involves “Fertility of the Herd”. Fertility of the herd may be the most difficult factor to evaluate. Accurate detection of estrus, inseminator efficiency and fertility of the semen (Parts I, II, and III) 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.
Herd fertility includes cycling/puberty status, compliance with protocols, embryonic mortality, body condition score (nutrition level) and disease control. 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%-65% 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. However, regimented use of them is essential for satisfactory results. Timing of pre-breeding vaccinations (well in advance of insemination), when injections or feeding need to occur, access to facilities, additional labor, and when insemination will occur must be planned well in advance of protocol use.
Fertilization rates are usually between 89% and 100% when semen is present at the time of ovulation. However, early embryonic mortality causes that percentage to moderate to about 60% to 70%. Several management decisions can impact the percent of embryos lost to early embryonic mortality. When should cows/heifers be transported after insemination? Research conducted at the USDA research center in Miles City, MT 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. Changes in nutritional status can also have a tremendous influence on embryonic survival. Work done at Oklahoma State University has reported severe changes in intake of energy and protein can result in heifer stopping normal estrous cycles. Furthermore, work done at South Dakota State University moving heifers that were developed on all winter in a feedlot to pasture immediately after AI can increase early embryonic losses.
Body condition score (BCS) and disease are two additional causes of marginal fertility rates in cow herds. Research recommendations suggest that cows be in a minimum BCS of “5” and heifers “6” at calving time for them to cycle and re-breed on an annual basis. This allows sufficient body reserves for lactation and to initiate normal estrous cycles after calving. However, if adequate nutrition is not available after calving, body condition can be lost and delay the return to normal estrous cycles. Proper pre-breeding vaccination programs along with well-managed internal and external parasite applications limit disease occurrences in the herd and promote herd fertility. Special care should be taken with virgin heifers. Several studies have reported negative impacts on pregnancy success by vaccinating naïve heifers (heifers that have never been vaccinated before) with a modified live vaccine (MLV) for BVD or IBR around time of breeding. 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 management decisions the entire year and not just prior to the breeding season that will generate reproductive results that meet producer expectations. 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.
Source: Jim Krantz with contributions from George Perry.
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