A frustrating scenario for fall calving herds is calves arriving smaller, weaker and less vigorous than expected.
“Each fall we receive questions about small, weak-born calves in fall-calving herds,” says Craig Payne, University of Missouri director of veterinary Extension. “There is rarely a simple explanation for why these problems occur, and several factors may be involved.”
Payne, along with Tim Evans, MU Extension state specialist in veterinary toxicology and animal health, and Jamie Courter, MU state beef genetics Extension specialist, say the calves are typically the result of a compounding “triple threat” — late-gestation heat stress, toxic tall fescue grazing and extreme genetic selection pressure for low birth weight.
Understanding how these environmental and genetic factors interact during the critical third trimester is the first step toward improving calf vigor, survival rates and weaning weights.
1. Late-Gestation Heat Stress Suppresses Fetal Development
Late summer heat is a primary cause of low birth weights in fall-calving herds. The biological impact peaks during the final 45 to 60 days before calving, a critical window when approximately 70% of fetal growth takes place.
The Mechanism: Impaired Blood Flow and Placental Function
When high temperatures persist, a cow’s physiological response redirects blood flow to the skin surface to dissipate body heat, starving internal reproductive tissues of circulation.
“Research has consistently shown that prolonged heat stress during late gestation reduces blood flow to the uterus and placenta,” Payne explains. “Over time, this can impair placental growth and function, reducing the transfer of oxygen and nutrients to the developing fetus. Because approximately 70% of fetal growth occurs during the last trimester, even modest reductions in oxygen and nutrient delivery during this period can affect fetal development and may contribute to lower birth weights and other changes associated with heat stress during late gestation.”Impacts on the Newborn Calf
Most research evaluating third-trimester heat stress originated in dairy cattle, but studies in Brangus and Angus-cross beef cows show matching results:
- Birth Weight Reductions: Calves born to heat-stressed dams weigh 9 to 15 lb. lighter at birth.
- Immunity Deficits: Calves exhibit lower blood oxygen at birth, reduced efficiency in absorbing antibodies from colostrum, and compromised immune function that can persist into the feedlot.
- Lagging Growth: Heat-stressed calves frequently lag in average daily gain through weaning.
While large dairy observational studies link extreme heat events to higher stillbirth rates, Payne notes current research in beef cattle provides limited evidence that heat stress alone is directly responsible for abortions or weak-born calves without other compounding factors.
2. Grazing Toxic Tall Fescue Restricts Uterine Nutrients
For beef producers in the fescue belt, grazing endophyte-infected tall fescue compounds the strain of summer heat.
“If your herd grazes toxic tall fescue, the picture may become even more complicated,” Evans warns.
The Role of Ergovaline and Vasoconstriction
Endophyte-infected tall fescue contains ergot alkaloids, primarily ergovaline, which cause severe constriction of blood vessels:
- Peripheral vs. Uterine Constriction: While cattlemen know ergovaline best for reducing blood circulation to the feet and tail switch (fescue foot), research confirms it also constricts uterine blood vessels.
- Compounded Oxygen Deprivation: This constriction further chokes off the delivery of essential nutrients and oxygen to the developing fetus during the late-gestation growth surge.
Cattle vs. Mare Sensitivity
While pregnant mares grazing toxic fescue suffer catastrophic reproductive outcomes — including agalactia (lack of milk), prolonged gestation and stillborn foals — cattle are comparatively more resilient. Rumen microbes partially degrade ergot alkaloids before systemic absorption. Nonetheless, when combined with high ambient heat, fescue toxicosis significantly heightens the risk of undersized, unthrifty calves.
3. Genetics: Calving Ease vs. Ultra-Low Birth Weight
Beyond environmental pressures, herd genetics heavily influence baseline calf vigor. A common issue during fall calving is selecting excessively low birth weight sires.
“Many producers select bulls for low birth weight, either by choosing a favorable birth weight expected progeny difference (EPD) or considering the bull’s own birth weight,” Courter explains. “But low birth weight is not the same as calving ease and should not be used as a substitute for selecting for calving ease.”
The genetic correlation between Calving Ease Direct (CED) and Birth Weight (BW) allows producers to select for unassisted deliveries without driving birth weights into dangerously low territory:
- Decoupled Traits: Producers can identify “curve-bender” bulls that offer strong calving ease while maintaining moderate, functional birth weights.
- The Compounding Danger: Selecting generation after generation solely for the lowest possible birth weight can drive calf size below what is necessary for neonatal vigor.
When an ultra-low birth weight calf is born with a shortened gestation during periods of high heat stress and fescue exposure, the result is a severely undersized calf that lacks the strength to stand and nurse quickly.
Key Takeaways for Fall-Calving Beef Producers
- Provide Ample Late-Summer Shade & Water: Mitigate third-trimester heat stress during the final 60 days of pregnancy using tree canopies, shaded paddocks, and cool water access.
- Dilute Toxic Fescue: Interseed legumes (like red and white clover), clip seedheads, or rotate heavily pregnant cows to non-toxic warm-season pastures.
- Select for CED, Not Just Low BW: Focus bull purchases on Calving Ease Direct (CED) EPDs rather than chasing extreme minimum birth weight numbers.
- Intervene Early with Colostrum: Have high-quality colostrum replacer on hand to assist light, heat-stressed calves that struggle to nurse in the first 2 to 4 hours of life.
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