Propionate, Not Extra Protein, Drives Reproductive Gains in Young Range Cows

A three-year study of 253 young beef cows grazing native Sandhills rangeland in Nebraska has found that the metabolic route by which dietary glucogenic precursors reach the cow matters far more than the total amount supplied. Researchers at Oregon State University and the University of Nebraska-Lincoln reported that postpartum supplementation with a direct ruminal propionate source raised pregnancy rates to 91 percent, compared with 75 to 80 percent for cows receiving equivalent or greater glucogenic potential from rumen undegradable protein. The work, published in Frontiers in Animal Science, challenges the assumption that simply increasing the estimated glucogenic potential of a supplement will translate into better reproductive performance in young, lactating range cows.

The experiment took place at the University of Nebraska Gudmundsen Sandhills Laboratory near Whitman, Nebraska, using Red Angus/Simmental composite cows from a March-calving herd. All animals were two- and three-year-olds, a life stage in which lactating cows grazing dormant or low-quality native range face a pronounced mismatch between nutrient supply and the demands of milk production and rebreeding. Cows were stratified by body weight and age at calving and randomly assigned to one of four supplement treatments, each individually delivered daily through a Super SmartFeed system for an average of 98 days postpartum, ending at the close of the breeding season.

The four treatments were engineered to deliver increasing levels of glucogenic potential, an estimate of how much glucose the cow can derive from the supplement, through two distinct metabolic pathways. The control group received 910 grams per day of dried distillers grains with solubles, providing an estimated 67 grams of glucogenic potential. A second group received 908 grams of the same distillers grains plus 40 grams of propionate salt, raising estimated glucogenic potential to 97 grams. The third and fourth groups replaced part of the distillers grains with porcine blood meal, a rich source of rumen undegradable protein whose glucogenic amino acids were counted at 40 percent, yielding estimated glucogenic potentials of 97 and 127 grams per day respectively. Notably, the propionate salt and the lower blood meal treatment supplied identical total glucogenic potential, allowing the researchers to isolate the effect of precursor source from the effect of quantity.

The physiological rationale rests on ruminant glucose metabolism. Propionate, produced mainly in the rumen, is the primary gluconeogenic precursor in cattle, and low-quality forage diets ferment toward greater acetate production and a higher acetate-to-propionate ratio. That imbalance has previously been associated with altered energy metabolism and reduced reproductive performance in young beef cows. Glucose is a critical substrate for reproduction: it fuels the central nervous system, and inadequate availability of utilizable glucose reduces hypothalamic release of gonadotropin-releasing hormone. During early lactation, range beef cows often become insulin resistant, which may further limit glucose entry into hypothalamic cells and blunt the reproductive axis precisely when breeding decisions are being made.

Against that backdrop, the simplest outcomes were largely unremarkable. Cow body weight and body condition score showed no influence of supplementation strategy at any measurement point across the entire study, with P values of at least 0.66 for weight and 0.31 for condition score. The one nuance appeared in weight change from calving to breeding, which tended to differ among treatments at P = 0.10: cows receiving either blood meal supplement lost less weight, about 6.6 and 6.1 kilograms, than cows fed distillers grains alone, which lost 14.0 kilograms, while propionate salt cows were intermediate at 10.8 kilograms. Calf birth weight and pre-breeding calf weight were likewise unaffected by their dams’ supplementation.

The picture changed sharply for weaning weight and milk production, both of which showed significant supplementation-by-cow-age interactions. Calves from two-year-old dams showed no response to any treatment. But among three-year-old cows, those receiving the propionate salt supplement weaned calves at 192 kilograms, significantly heavier than calves from three-year-old dams fed distillers grains alone at 184 kilograms or the lower blood meal treatment at 178 kilograms. Milk production, measured around day 60 postpartum with a modified weigh-suckle-weigh method on a subset of 96 cows, followed the same pattern: three-year-old propionate salt cows produced 5.52 kilograms in 24 hours, significantly more than the 3.77 to 4.64 kilograms recorded for their counterparts on the other three supplements, while two-year-old cows again showed no treatment effect.

Milk composition told a subtler story. Protein, fat, lactose, and solids-not-fat percentages were unaffected by supplementation strategy. Milk urea nitrogen, however, tended to differ at P = 0.09, with propionate salt cows recording the lowest concentration at 16.33 milligrams per deciliter compared with 17.94 and 18.25 for the distillers grains and high blood meal groups. Because milk urea nitrogen reflects the ratio of protein to energy intake and the efficiency of protein metabolism, the authors interpret this reduction as a hint that the added propionate spared metabolizable protein from deamination, improving nitrogen utilization rather than simply adding more protein to the diet.

The headline result was reproductive. Overall pregnancy rate differed among treatments at P = 0.05, with cows receiving the propionate salt supplement achieving 91 percent pregnancy compared with 75 percent for distillers grains, 75 percent for the lower blood meal treatment, and 80 percent for the higher blood meal treatment, with no statistically significant differences among the latter three. Critically, increasing rumen undegradable protein supply, which raised estimated glucogenic potential from 67 to as much as 127 grams per day, did not improve reproduction at all. The two treatments with identical estimated glucogenic potential of 97 grams diverged dramatically: the propionate route delivered a 16 percentage point pregnancy advantage, while the amino acid route delivered none.

An economic analysis reinforced the biological findings. Using a two-year partial budget model for hypothetical 100-cow herds, with calf prices averaged over ten years of Nebraska auction data, the researchers found that supplement costs ranged from $20.58 per cow for distillers grains to $45.67 for the high blood meal treatment over the 98-day feeding period. In the supplement year, net revenue for the propionate salt herd increased by 1.67 percent relative to the control herd, while the blood meal herds declined slightly. Because the 91 percent pregnancy rate left 88 cows in the propionate salt herd versus 72 in the control herd entering year two, the second-year net revenue advantage ballooned to 24.30 percent for the propionate salt strategy. The analysis excluded income from cull cows and the cost of developing replacement heifers for open cows.

The authors conclude that the biological response to increasing glucogenic potential depends not only on the quantity of estimated glucogenic substrate supplied but on the metabolic pathway through which it arrives. A direct source of propionate produced reproductive and lactation responses in three-year-old cows that additional rumen undegradable protein, whether from distillers grains or porcine blood meal, could not match, even at higher estimated glucogenic potential. For nutritional management of young range beef cows, the study suggests that programs should consider both adequate metabolizable protein supply and direct post-ruminal glucogenic sources, and the researchers note that further work is warranted to elucidate the mechanisms underlying the reproductive response to the propionate salt supplementation.

Subject of Research: Effects of glucogenic precursor source in postpartum supplements on reproduction, milk production, and calf performance in young range beef cows

Article Title: Glucogenic precursor source impacts reproductive performance in young range cows

Article References: Lang, W. S., Musgrave, J. A., McCarthy, K. L., & Mulliniks, J. T. (2026). Glucogenic precursor source impacts reproductive performance in young range cows. Frontiers in Animal Science, 7, Article 1933326. https://doi.org/10.3389/fanim.2026.1933326

Image Credits: AI Generated

DOI: 10.3389/fanim.2026.1933326

Keywords: beef cattle, glucogenic potential, propionate, rumen undegradable protein, postpartum supplementation, pregnancy rate, milk production, range cows, calf weaning weight, reproductive performance, nutrient partitioning, partial budget