Livestock producers throughout the U.S. and Canada are turning their breeding stock out to graze summer pastures. Early summer generally provides the highest-quality forage of the production cycle. Producers frequently assume most nutrient requirements are met, yet many nutritional and metabolic details are overlooked.
In a May 2024 article in Progressive Cattle, Iowa State University’s Erika Lundy-Woolfolk highlighted several stresses challenging production livestock: vaccination, handling, transportation, heat and nutrition. These stresses compound on one another from late gestation through maintenance, parturition, lactation and on to spring/early summer grazing. Transitions for cows are the norm, all while maintaining focus on a 365-day production cycle. Gestation is 283 days, leaving 82 days remaining for rebreeding. When producers prepare their livestock to strike a balance for these stresses, optimum productivity is achieved; without balance, productivity is compromised.
Pastured beef cow-calf systems thrive on the profound adaptability of beef cows, which allows them to balance many variables. Even so, vaccination, handling, transportation and heat stress all can lead to a decrease in dietary intake, resulting in early embryonic loss. Having an inadequate quantity or quality of nutrients available has a negative impact on reproductive outcomes. The period for potential early embryonic loss is from fertilization through day 42 of gestation. A broader time frame known as the periconceptual period (60 days prior to insemination to 60 days after insemination) compared to the 42-day time frame previously mentioned has recently been presented by researchers. The periconceptual period encompasses calf suckling, uterine involution, early lactation, postpartum anestrus and the initiation of postpartum estrus due to the current calf being at the cow’s side. The periconceptual period also includes the events that pertain to next year’s calf: ovulation, insemination, conception in the oviduct, conceptus cleavage, embryonic growth and development, transition from morula to blastocyst, hatching, transport to the uterus, blastocyst elongation and initial placentation. Any metabolic disruption during the progression of the events listed results in infertility.
Producers must coordinate the timing of management decisions such as vaccination and transportation to make the least disruptive choice possible. For example, at the early embryonic stage (three to seven days after conception), the next calf crop is an eight-cell conceptus or embryo known as the morula. Its progression through the reproductive tract is at the oviductal-uterine junction. The next step in the reproductive tract is the uterus, where secretions known as histotroph provide the needed nutrients and signals for differentiation/organogenesis and growth. Histotroph have been called uterine milk and is a complex mixture of growth factors, hormones, enzymes, transport proteins, ions, fats, glucose and amino acids. This is the nutrient source for the growing embryo before it implants in the uterine wall by day 20 to 25 post-fertilization. Nutritional or metabolic disruptions for the cow at this important period of embryonic development results in the loss of that embryo.
Researchers in Texas have found that fertilization rates in beef cows are generally good, yet a failure rate of 28% is noted in the first seven days – and most of those before day 4. By 30 days post-fertilization, about 48% of the inseminated cows are diagnosed as not being pregnant. These first 25 days of gestation are critical as the embryo is nourished by histotroph. Then by 30 days, the embryo has implanted in the uterine wall and the physiologic signals that are recognized by the maternal system as “pregnancy” are identified by the cow. This is the point in gestation considered the transition from the embryonic stage to the fetal stage, and the placenta begins to provide nutrients to support the growing fetus.
As reproductive cows accommodate a variety of stresses throughout a 365-day production cycle, small decreases in dietary intake are reflected in the endocrine system. Insulin and growth hormone (GH) are master coordinators of nutrient prioritization, determining, for example, whether dietary protein goes to produce milk or build muscle. Growth hormone conducts its actions through another molecule known as insulin-like growth factor-1 (IGF-1). GH and IGF-1 are the “growth signals” for the cow’s body, and insulin is the “storehouse keeper,” signaling the storage of energy. The metabolic or endocrine signals rise and fall in concentration as total dietary intake or specific components increase or decrease, respectively.
Insulin, GH and IGF-1 are the endocrine signals within the cow, supporting a growing conceptus or embryo signaling the growth of uterine tissues and contents. Studies demonstrate that cows with good or greater levels of insulin/GH/IGF-1 have much better confirmed pregnancy rates than those with decreased levels during the periconceptual period, which encompasses the early embryonic stages.
The quantity of dietary intake significantly affects animal metabolism through the signals of the endocrine system. Livestock producers can manage herds to enhance insulin/GH/IGF-1 levels beyond the increase in dietary intake by supplementing a specific type of protein. When cows are turned out to spring/early summer pastures, protein is usually 10% to 12%. Yet, 90% or more of protein in early pastures is rumen-degradable protein (RDP) rather than rumen-undegradable protein (RUP). RUP is important to achieve a consistent and significant increase of insulin/GH/IGF-1 levels in cows consuming 0.025% to 0.05% of bodyweight of a RUP source. As a supplement, dried distillers grains plus solubles (DDGS) are a good source of RUP that is very palatable and valuable during the early breeding season to optimize the reproductive performance of cows grazing good forages.
Increasing the insulin/GH/IGF-1 levels helps to maintain the conceptus, whether at the embryonic or fetal stage, and increases the number of embryos that go full term. Establishing pregnancies as early as possible in the breeding season diminishes one of the larger variable costs for livestock producers: cow replacement. In addition, using a RUP supplement source helps early season conceptuses grow through the periconceptual period to established pregnancies, providing the producer with a greater proportion of first-service calves in the subsequent calf crop and a greater number of calves close in age. Managing stress in light of the reproductive stages of the periconceptual period and using strategic supplementation allow for more pregnancies to be established early in the breeding season.
