- Animals: bovine
- Animal Production: livestock breeding
Four studies concluded that developing heifers to less than 65% of mature weight prior to breeding utilizing dormant, standing forage or crop residue does not reduce pregnancy rates in virgin beef heifers. In addition, neither corn residue nor dormant winter range development post-weaning negatively impact subsequent calf production or second season rebreeding. Short-duration supplementation around breeding did not improve pregnancy rates in these experiments. However, further research is needed to explore the effect of breeding supplementation in more severely restricted animals.
Traditional heifer development programs involve placing weaned heifers into a feedlot or drylot situation and then providing them with a ration formulated so the heifers weigh 65% or more of projected mature weight at the time of breeding. This type of heifer development evolved because producers wished to achieve maximum pregnancy rates in the first breeding season, and dormant forages do not contain the nutrients necessary to support this level of performance. Drylot systems may attain maximum pregnancy rates, but not necessarily optimum performance in terms of profit or sustainability. The energy demands for this system are high because of the requirement for fuel to harvest the feed and then deliver it to the cattle. Cereal grains are often used as a major source of energy in the diet, detracting from the sustainability of this system due to growing demand for cereal grains for human food and ethanol production.
Recent research efforts in Nebraska are focused on developing a systems approach and mentality towards heifer development, where profit and sustainability are the indicators of success. Heifers have been developed to lighter than previously recommended target weights with no adverse affects on reproduction or production. A system for developing heifers on range while producing acceptable pregnancy rates, during a short breeding season has not yet been demonstrated. Ovum fertilization rates in heifers are routinely near or above 90% in controlled research; however, pregnancy rates to d 18 are 30-40% lower. The logical target for improving pregnancy rates in heifers is to improve embryo survivability and increase the percentage of pregnant heifers that remain pregnant.
Overcoming embryo loss in heifers with targeted, short-duration energy supplementation should achieve our goal of demonstrating acceptable pregnancy rates in heifers developed on range.
In order to realize progress toward improving sustainability and reducing risk for ranchers developing heifers, it is essential that a low-cost energy source be available for supplementation. Dried distillers grains (DDG) are a by-product of ethanol production that provide approximately 120% the energy value of corn for cattle on forage diets. Therefore, DDG are a logical and sustainable option for short-term supplementation of heifers developed on range.
Replacement heifer development is critical for the future reproductive success and profitability of the cowherd. Heifer development should be achieved at low cost in order to manage financial risk and prevent over-investment in a non-productive female. Previous studies (Patterson et al., 1992) indicated that puberty occurs at a genetically predetermined size, and only when heifers reach their target weight can high pregnancy rates be obtained. Recommended guidelines generally have been 60 to 65% of mature WEIGHT in beef heifers (Patterson et al., 1992). This development program requires substantial resources because an accelerated rate of gain is needed to reach the target weight. Development programs that allow heifers to conceive early as yearlings at the lowest cost possible are needed.
Funston and Deutscher (2004) proved that heifers developed to only 53% of mature weight could achieve similar pregnancy rates compared to heifers developed to 58% of mature weight. Further research indicates that heifers developed to 50% of mature weight attained similar pregnancy rates as those developed to 55% of mature weight, but the lighter heifers were allowed a 33% longer breeding season (NE Beef Report, 2005). These data were generated using drylot heifer development regimes, rather than developing heifers on range. Demonstration of a system for developing heifers on native range with targeted supplementation to achieve high pregnancy rates is needed to motivate producers to develop heifers in a more cost-effective way.
Embryonic mortality refers to losses that take place from fertilization until the period of differentiation, at approximately 42 days of gestation (Committee on Reproductive Nomenclature, 1972). Embryo loss is the major cause of reproductive failure in cattle and represents significant economic loss to the industry (Dunne et al., 2000). Numerous studies report a fertilization rate of 80-90% in beef heifers (Henricks et al., 1971; Diskin and Sreenan, 1980; and Roche et al., 1981). However, these studies also report that at 42 days following insemination, embryo survival rate was only 60% of initial. Furthermore, Roche et al (1981) demonstrated that embryo loss occurs primarily in the first 16 days following breeding. Dunne et al (2000) found that embryo survival rates decreased to day 14 with no further reduction. These studies clearly demonstrate that embryo loss occurs primarily in the first 14-16 days following insemination.
Research in suckled beef cows, which may represent a similar nutritional demand as growth in heifers, indicates a positive influence of concentrate supplementation three weeks prior to and three weeks following breeding (Khireddine et al., 1998). In this study, all cows were restricted to 70% of maintenance requirement from calving until three weeks prior to breeding. Subsequently, cows were either maintained on the restricted diet or placed on a supplemented diet. On day 21, the supplemented group had a greater pregnancy rate (100% vs. 20%) compared to the nonsupplemented group. This indicates a positive effect of supplemental nutrition on embryo survival.
Supplementation may be more beneficial in heifers that are nutritionally challenged or on a low plane of nutrition (Funston, 2004). Supplementation with a high-energy feedstuff containing a modest level of fat, such as DDG, may be beneficial. Ciccoli et al. (2005) found heifers that were supplemented in a drylot with corn and soybean meal or corn and distiller’s grain reached puberty at a younger age than control counterparts grazing pasture and supplemented only with soybean meal. However, data demonstrating the effects of short-term supplementation with distillers' dried gram (DDG) as a component of a low-cost heifer development system is needed.
The primary expected short-term outcomes of this research are to demonstrate the feasibility and sustainability of a low-cost heifer development strategy to beef producers in the north central region. This short-term outcome includes not only demonstrating that heifers can be developed successfully on range, but also that targeted energy supplementation from one week before breeding until 18 after should improve pregnancy rates. For ranchers, this should result in more heifers pregnant in a shorter time, with reduced cost compared to feedlot heifer development.
Our projected intermediate outcomes include ranchers adopting this strategy for developing heifers on range, reduced heifer development costs, diminishing use of fossil fuels and cereal grains, and lower capital investment in heifer development enterprises. Additionally, we expect to increase awareness of embryonic loss in beef heifers and educate producers about strategies to control early pregnancy loss in developing heifers.
Long term, these results may effect how beef females are managed prior to the breeding season. The periods immediately before and after breeding are novel in a heifer’s development and represents new nutritional demands. Implementation of a system that promotes a targeted, cost effective supplementation program could improve pregnancy rate and reduce economic risk for livestock producers. This short-term supplementation system could be incorporated on ranches with widely varying forage resources.