Breeding season nutrition for yearling replacement heifers
Developing replacement heifers is costly, but tremendously important. First, replacements are obviously important as culled cows leave the herd if herd size is to be maintained. Additionally, these replacement females are the daughters of the latest bulls and thus should be a major source of future genetic improvement in the cow herd. If not done properly, these young females will not become productive members of the cow herd. This increases the annual costs of production for each cow. Realize that a cow has to pay the cost of replacing herself over her lifetime. If she has to pay for several false starts before a heifer successfully makes it into the herd to replace her, then the costs go up.
Thus, our management needs to ensure that the heifer calf is developed properly up to and beyond the first breeding season with the goal of having her pregnant early in that first breeding season. Past research has shown that heifers that get pregnant early in their first breeding season will have greater lifetime productivity than heifers that get pregnant later. First, calves born earlier will be older and heavier at weaning. Second, the heifer that gets pregnant earlier the first time is more likely to return to estrus early so she is more likely to remain an early calving cow throughout her life. Thus, more pounds wean across several calves. Additionally, if something ever does delay her pregnancy in subsequent years, it is better to get pregnant late rather than already have been late and not get pregnant at all.
There are several phases a heifer calf has to successfully make it through to become a productive cow. Once weaned and chosen to be kept as a replacement, she has to be grown adequately to reach puberty and be fertile by the beginning of her first breeding season. Our age-old recommendation is that heifers should reach 65 percent of mature body weight by the time they are yearlings to ensure a high percentage of them are pubertal and will get pregnant. Recent research has indicated that the target can be lower than this in modern heifers, probably because genetics have improved over the last few decades since the 65 percent rule was developed.
Assuming the heifer has been developed adequately to be pubertal and fertile as a yearling, then she needs to get pregnant and stay pregnant. Recent research with ultrasound has indicated that many more heifers conceive than we realized in the past, but that early embryonic mortality means that the pregnancy is not sustained. This research indicates that from six to 40 percent of pregnancies are lost between days 25 to 90 after breeding. When this happens, she has to be bred again on a subsequent cycle, if she gets bred at all. The research also indicates that nutrition (and particularly changes in nutrition) at the time of breeding has a large impact on retention of pregnancy.
Unfortunately, the beginning of the breeding season in the spring is a time of dramatic changes in nutrition for grazing livestock. This is particularly true for heifers that were developed in drylot from weaning until turn out to green grass at about the same time as breeding starts. If AI is used, it is very typical to AI the heifers at the end of the drylot period and then turn them immediately out to grass. This is an abrupt transition from consuming harvested feed to grazing. A variety of issues come into play. First, the nutritional characteristics of grazed forage are different from harvested feed and changing rapidly during spring green up. Sudden nutritional changes can negatively affect reproductive performance. Both excess and deficient nutrition can be detrimental. Excess protein, which is common in early spring grass, has been shown to change the uterine environment and reduce embryo survival. Malnutrition will cause heifers to stop cycling. Second, these heifers have to re-develop their grazing skills after spending the winter eating harvested feeds. Research shows that ruminants learn grazing skills from their mothers and other adults as suckling calves and that these skills diminish if they are not grazing. Nutrient intake drops dramatically during the first weeks after turnout because of these poor grazing skills. Developing heifers on rangeland through the winter after weaning has been shown to reduce the drop in nutrient intake in the spring because they don’t need to adjust to grazing from drylot. Preliminary research suggests that turning heifers developed in drylot out a month or so before the breeding season starts may help them to adjust to grazing and get through the reduced nutrition period before breeding starts. Other preliminary research indicates that supplementation for about 45 days after turn out to pasture may help overcome this problem by providing nutrients while the heifers are building their ability to graze adequately to meet their requirements.
The bottom line is that proper management of replacement heifers is important in all phases past weaning. If effort is less than needed in any step, it may negate good management in all other steps. Ultimately, successful management has its rewards when we see young cows that are highly productive. F
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