Nutrient Needs and Impact on Reproduction
Focus on feeding programs that meet the nutrient requirements for the young cow. These requirements are well defined in the 1996 Nutrient Requirements of Beef Cattle. Once the requirements are determined, it is simply a matter of fitting the proper quantities of feedstuffs available to supply the proper levels of protein, energy, vitamins, and minerals.
Feed and manage the young cow so that she is in adequate body condition at calving. Cow body condition at calving is the most important factor in determining how quickly a cow will breed back after calving. Some producers feel that if cows are thin at calving, then "flushing" (feeding a high level of energy) the cow after calving will compensate for poor condition and result in acceptable breed back. This management practice works in limited situations. In most cases, extra energy after calving will stimulate milk production and not result in replenishment of energy reserves that seems to be necessary to signal the reproductive axis to gear-up and assure acceptable reproductive performance. Trying to play catch-up after calving is almost always expensive because extra energy is needed in the diet.
Research conducted at Oklahoma State University was designed to either intentionally feed a group of first-calf heifers to achieve a body condition score less than 5 (thin) by calving or a comparable group of heifers were fed to achieve a condition score of 5 and above by calving. After calving, half of each group was fed to either maintain the same level of condition while the other half of the heifers were fed to gain condition after calving. Heifers that calved thin (below condition score 5) and maintained in thin condition after calving had a 36% pregnancy rate the following fall. Those that calved in thin and then fed to gain condition had a 66% fall pregnancy rate, while those that were calved and maintained in good condition had an 91% fall pregnancy rate.
The nutritional (both protein and energy) needs of the young cow during her first lactation are high. Because the young cow has a small rumen compared to mature cows, the quality of the diet needs to be high to meet her nutritional requirements. Feeding medium quality hays without supplementation is not an option. Young, lactating females managed on these diets will loose weight and body condition. Young females that loose weight and body condition before the start of their second breeding season will have reduced rebreeding performance.
In another experiment conducted at Oklahoma State University illustrates the importance of not trying to starve calving difficulty out of young females. Some feel first-calf heifers should be restricted in feed prior to calving to decrease the size of the newborn calf. Even though feeding heifers to achieve good body condition (condition score 5.5) by calving may increase calf size slightly, research consistently shows that calving difficulty or dystocia is not increased. What is often encountered in this feeding management practice is a weak heifer at calving trying to give birth to a slightly smaller calf. Instead of starving calving difficulty out of the 1st-calf-cow, her ability to become pregnant with her second calf is greatly compromised. As suggested in the research, 89% of the 1st-calf-cows became pregnant with their second calf when fed to achieve a BCS of 6 at her first calving as compared to 78% and 65% for those heifers fed to achieve a BCS of 5 or 4, respectively, at their first calving.
There are management practices that can induce cows to cycle and may be used in situations where cows, especially young cows, are not exhibiting estrous cycles before the start of the breeding season and there are concerns about breed back. Considerable research has shown that exposure to sterile bulls aids in inducing cows to show estrous 14 to 21 days earlier than females not exposed to bulls.
Another practice that aids in initiating cycles in females that are anestrous and has a very positive effect on reproductive performance is weaning the calf before the breeding season starts. This is usually not a practical approach and should only be considered under extreme circumstances.
Finally, separating the calf from the cow for a short period of time (48 hours, removal of the suckling stimulus) is a practice that can be used for females that are on the verge of cycling and need a jump-start to begin cycles. This management practice has been used routinely in some herds without any harmful effects on the calf or cow's future milk potential. Data suggest a decreased in the number of days from calving to first estrus and in some cases improved fertility approximately 5%. If you use this practice, you must be set up to keep cows and calves separated and comfortable as possible during this time.
Dr. Rick Rasby, Professor of Animal Science
Animal Science, University of Nebraska–Lincoln, Lincoln, NE