Impacts of Nutrient Restriction on Heifer Pregnancy Success

Understanding the factors impacting reproductive failure within heifers is critical to implementing management strategies that will improve heifers’ chances of remaining within the beef herd and successfully producing calves. Heifers typically have greater pregnancy success if they achieve 65% of their mature body weight and sustain a body condition score of 6 by the beginning of the breeding season (Hall et al., 1995; Patterson et al., 1991). Method of heifer development may also impact heifer pregnancy success, with heifers that have prior grazing experience after weaning having increased conception rates to AI compared to heifers developed in a drylot (Perry et al., 2013). The timing of changes in nutrition in respect to breeding may also impact a heifer’s pregnancy success.

Recent Research
A study was conducted in the spring of 2017 at South Dakota State University (SDSU) to determine how changes in plane of maternal nutrition at the time of AI impact embryo development. Although this study was conducted in a drylot, changes in plane of nutrition occurred at the time of AI to simulate the change in nutrient intake drylot-developed heifers experience when turned out to grass at the beginning of the breeding season.

Nutritional Treatments
Sixty heifers were divided into two nutritional treatments for 36 days prior to AI and fed at either 64% of their maintenance energy requirements or 139% of their maintenance energy requirements. Heifers were synchronized to come into estrus, and at the time of AI, half of the heifers from the low treatment were moved to the high treatment diet while half of the high treatment heifers were moved to the low treatment diet. This resulted in four treatment groups: heifers on the low diet before and after AI (LL), heifers on the low treatment before AI and the high treatment after AI (LH), heifers on the high diet before and after AI (HH), and heifers on the high treatment before AI and the low treatment after AI (HL). Heifers were maintained within their respective treatments for six days after AI and were then flushed for embryo collection and uterine luminal fluid. Blood samples were collected from heifers daily from AI to embryo flush and analyzed for non-esterified fatty acids (NEFA), protein, and glucose to determine if these metabolites would reflect changes in heifers’ plane of nutrition in both peripheral concentrations (blood) and the uterus (uterine luminal fluid) during early embryogenesis.

Embryo Quality
Embryo quality was reduced from heifers restricted energy before and after AI. Peripheral concentrations of NEFA and protein were elevated in nutrient restricted heifers after AI (LL and HL), as these heifers were mobilizing body fat and tissue to meet the body’s demands for energy and amino acids; however, glucose and uterine luminal fluid metabolites did not reflect heifers’ nutritional status, indicating that although embryo development is sensitive to changes in maternal nutrition during the first six days of embryogenesis, these changes are not communicated to the embryo through NEFA, overall protein concentrations, or glucose.

Pregnancy Success
Typically, producers do not restrict intake to 64% of maintenance energy; however, heifers can experience a significant drop in nutrient intake when animals are moved from a drylot to a grazing situation, especially if these animals have limited grazing skills. The research team on this project anticipated that returning these restricted heifers to a balanced ration that met their nutrient needs for growth would result in similar pregnancy rates. Heifers were placed back on full feed and spring pasture after the experiment for 22 days in preparation for breeding. Heifers were synchronized to come into estrus, artificially inseminated, and then moved to pasture for the remainder of the breeding season and exposed to a clean-up bull. Pregnancy success to AI and overall breeding season success determined that heifers that had undergone nutrient restriction in the previous study (LL, LH, and HL) had a greater likelihood of being open after the breeding season (40%, 46.7%, and 40% respectively) compared to heifers that did not undergo nutrient restriction (HH: 20%). Although more animals are needed to support this data, it is evident that small antral follicular growth is sensitive to nutritional insults (Gutiérrez et al., 1997). As observed by pregnancy rates from heifers in the current study, even short-term nutrient restriction over six days (HL heifers) yields similar pregnancy success compared to heifers restricted energy for 42 days (LL heifers), indicating that negative impacts on follicular development may have the potential to impact future pregnancy success.

Follicular Development
Follicular development from preantral to preovulatory follicles requires approximately 42 days in beef cattle (Aerts and Bols, 2010); therefore, when managing beef nutrition prior to the breeding season, keep in mind that nutrient restriction during this time may have negative impacts on follicular development, which may in turn impact pregnancy success of those animals during the breeding season.

Source: Julie Walker, iGrow 

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