Why do some cows breed back more successfully than other cows in the same herd? Why do herd conception rates vary from year to year? Body condition and nutrition can offer some explanation. Research has demonstrated that cows in poor body condition and/or under nutrient stress will have lower conception rates and take longer to rebreed compared to cows in good body condition and provided with a ration meeting nutrient requirements. But, are there other factors that can affect re-breeding and conception rates? I want to summarize a presentation from Dr. James K. West, DVM and director of the embryo transfer unit veterinary service at IowaStateUniversity, in which he lists both non-infectious and infectious causes for reduced conception rates in beef herd.
Some non-infectious factors that can reduce conception rates include: lethal genes, polyspermy, endocrine disorders, elevated blood urea nitrogen, heat stress and handling/transportation stress. Lethal genes and polyspermy (fertilization of the egg with more than one sperm, which is a lethal condition) both refer to genetic factors that do not allow a viable pregnancy to occur. Endocrine disorders result in short or irregular heat cycles or a condition where not enough progesterone is produced to sustain a pregnancy. With these 3 factors the producer has little control and culling is probably the best option for the producer.
Elevated blood urea nitrogen, heat stress and handling/transportation stress are other factors that can cause reduced conception rates. Producers may be able to minimize the detrimental effects of these factors through management practices. Elevated blood urea nitrogen can occur on lush pastures and is related to crude protein consumption. Crude protein content of pasture grass can be increased through nitrogen fertilization. High crude protein levels in the diet increase the blood urea nitrogen content which can bring about alterations in uterine secretions and a decrease in uterine pH that results in lower conception rates. Avoid nitrogen fertilization of pastures immediately before grazing lush pastures during the breeding season. Heat stress occurs when there are prolonged periods of temperatures over 90 degrees F combined with humidity of 40% or greater. Breeding during these conditions can result in early embryonic death, especially in the first 6 days after egg fertilization. Finally, reducing handling stress can be accomplished by working cattle in the cool part of the day, providing working facilities that are efficient, and working cattle in a calm and un-hurried manner.
Infectious diseases that cause reproductive problems include: Infectious Bovine Rhinotrachetis (IBR), Leptospirosis, and Bovine Viral Diarrhea (BVD). It is beyond the scope of this article to go into details about each of these diseases, but each can cause reduced conception rates, and, in many instances, abortions. Vaccines are available, often in combinations to provide protection against these diseases. Unfortunately, in the case of Leptospirosis and BVD, vaccines are not always effective.
High conception rates and full-term pregnancies are necessary to maintain a profitable beef herd. By recognizing non-infectious and infectious factors that can reduce conception rates and pregnancy, producers can adopt management practices that will provide cows the best opportunity for successful re-breeding.
Source: Rory Lewandowski, Extension Educator, Ag/NR AthensCounty