There are many things that can affect pregnancy rate in a herd, and number of open cows at the end of the breeding season, including nutrition, calving difficulty, disease, level of expertise in doing an A.I. program, and bull fertility if breeding by natural service.
Some of the problems with calving difficulty (which can lead to injury, bruising or infection in the cow--all of which can delay her recovery and inhibit her ability to rebreed) can be avoided these days with use of EPD's and selective genetics. The rancher has more information now when selecting bulls and replacement heifers, on inheritable factors that affect calving, such as birthweight.
Some of the disease problems in a herd that can affect rebreeding and pregnancy rate can be resolved or avoided with good bull management, eliminating some of the major contagious diseases such as trichomoniasis, with regular bull testing.
AI Programs -- The rebreeding success rate in an A.I. program will depend a lot upon the people doing it. Robert A. Bellows (USDA-ARS Livestock and Ranch Research Laboratory, at Miles City, Montana) says, "You have to have good operator ability and expertise, for a good reproductive rate. We've also found (as have the scientists at Clay Center, Nebraska) that three seconds of clitoral massage after insemination, in cows, definitely will increase the conception rate. It doesn't seem to work in heifers, for some reason, but it does in cows."
Another factor affecting A.I. success is how cows are handled after breeding. "Colorado studies show that if you move cattle after they've been synchronized and bred, it is very crucial WHEN you move them. They should be moved soon after breeding. If you wait 10 days, or 30 days later, there's more chance of losing the pregnancy -- the conception rates go down. It puts stress on the cow at the wrong time, when the message is going from the embryo to the cow that she's pregnant (10 days post breeding) so she won't come into heat again. And about the 30 day range is another critical time, when the embryo is implanting in the uterus. So don't move those cows 10 days or 30 days after breeding," advises Bellows.
Bull Fertility -- It's also important to check bulls, for a good rebreeding rate in the cow herd, if cows are bred by natural service. Making sure bulls are fertile, healthy, with good libido, is always a good idea, as is feeding them properly. Bulls should always be in reasonably good flesh--not thin, and not too fat. Most producers tend to run more bulls than they need. Breeding soundness exams, checking for abnormalities and fertility, can help assure that bulls can do the job. This is especially important when using just one bull in a group. But even with several bulls in a group of cows, you can't be sure they will be bred and settled.
Marie S. Bulgin (University of Idaho, Caine Veterinary Teaching and Research Center, at Caldwell, Idaho) says "sometimes you can have an infertile dominant bull, and though there are other bulls with the cows, the dominant bull keeps them from breeding. Bulls should always be checked before the breeding season. Just because they were checked last year and found to be fine does not mean they'll be ok this year."
She also points out that bulls can pass BVD to the cows during breeding. "You can have bulls tested for BVD. Some people don't think about this as a possibility because they have a closed herd and have not had BVD before." But if they are buying bulls, they could inadvertantly introduce BVD unless the bulls are tested.
Proper nutrition for bulls is crucial (not underfed nor overfed). Bellows did a study using the NRC (National Research Council) requirements for feed, and compared nutrient requirements of a 1,700 pound bull with those of a 1,200 pound cow giving 10 pounds of milk daily. "That bull isn't too big, and the cow isn't a good milker," he says, "but it gives an idea of their respective requirements. You always hear about how high the lactating cow's requirements are, but the bull's dry matter requirement is greater than hers, and his metabolic energy requirements are higher. Their total protein needs are almost equal, but he needs more TDN (total digestible nutrients). Their calcium and phosphorus requirements are almost equal, but he needs more Vitamin A. So the thing that becomes critical is that he needs to be well fed for a couple of months before the breeding season, since the spermatogenic cycle is eight weeks in duration; in other words, the sperm formed today will be ready for ejaculation eight weeks from now." You have to plan ahead when feeding bulls for breeding.
"When we look at the number of cows being bred by natural service -- more than 90 percent of the nation's cow herd -- that means we're looking at that many cows depending on a fertile bull, for rebreeding success. Nutrition can affect not only semen quality and sperm production, but also his general health and breeding ability, his energy level, etc." Thin bulls may not be able to do their job, and overly fat bulls may also be impaired. They wear out more quickly and can't stand the pace of a hectic breeding season, and may have fertility problems due to extra fat deposits in the scrotum that keep the testicles too well insulated and too warm for optimum fertility, says Bellows.
There are also some unpredictable factors that can enter the picture. Dennis Maxwell (McNay Research Farm, Iowa State University) says, "Last year we had a lot of herds here in Iowa hit by a very hot spell during the middle of the breeding season. This had an adverse effect on bull fertility, and showed up significantly at preg-checking time. Hot weather can affect a bull's fertility, and also his aggressiveness and desire to breed."
Late spring calving, which is generally an ideal time because the cows are on grass (the most economical and nutritious feed, at a time of year when their nutrient requirements are highest) can have this drawback; breeding in July and August can sometimes be detrimental to bull fertility. Maxwell says that studies in Nebraska "are looking at early summer calving (June), matching up with peak grass production, and also putting the breeding season later (September), when hopefully the worst of summer heat is past." Each producer must choose the best calving-breeding season for his own situation, taking into consideration available feed resources and climate--to most effectively and economically get an optimum pregnancy rate. Weather at calving is often a major factor in a rancher's decision, but climate during breeding season should be one of those considerations also.