Calf Sex and Breed Don’t Matter: Parity Holds the Key to Tropical Dairy Yields

On a government-run livestock station in the heart of Trinidad, a six-year audit of dairy records has delivered a result that challenges one of the more persistent beliefs in cattle science: that the sex of a calf shapes how much milk its mother produces. Researchers who mined 35,743 daily milk records from 155 lactations at the Centeno Livestock Station found that whether a cow gave birth to a bull or a heifer made no statistically significant difference to her milk yield or the length of her lactation. Breed fared no better. What did matter, decisively, was parity — the number of times a cow had calved — and, above all, how much milk she produced in the first ninety days after calving.

The study, published in the journal Discover Agriculture by Erica Auguste of Trinidad and Tobago’s Ministry of Agriculture and Fisheries and colleagues Martin Hughes and Sivakumar Karuppusamy at the University of the West Indies, set out to answer a question with real economic stakes. Dairy farming across the tropics is dominated by smallholders, and their productivity is chronically low. In Trinidad and Tobago, national milk production collapsed from roughly 9.4 million kilograms in 2001 to about 1.6 million kilograms in 2024. Jamaica saw annual yields fall by around 57 percent over a comparable period. Understanding which biological factors actually constrain output on these farms is therefore not an academic exercise but a matter of livelihood and food security.

The Centeno Livestock Station occupies about 502 acres of flat to gently sloping land in El Carmen, St. Helena, in the county of Caroni. Its soils belong to the Cunupia series — heavy clays with impeded drainage and a pH between 4.5 and 5.4 — and the climate is classically tropical, with average annual rainfall near 1,877 millimetres and temperatures ranging from 22.7 to 31.3 degrees Celsius. A wet season runs from July to December and a dry season from January to June, during which pasture quality and quantity typically decline. The dairy herd of roughly 60 cows grazes rotationally on Brachiaria arrecta and unimproved native grasses, is milked twice daily with an automated system, and receives about 2.0 to 2.5 kilograms of an 18 percent crude protein concentrate at each milking.

Two breeds dominate the herd: the Centeno Crossbred, a Bos taurus × Bos indicus animal developed in the late 1970s by crossing Holstein-Friesians with Zebu-type breeds to combine high yield with heat tolerance and resistance to tick fever, and the Jersey. The researchers deliberately excluded a small group of graded Holstein-Friesians because so few were lactating and in poor condition. Breeding is carried out by artificial insemination, and the cows range from three to twelve years old. It was this herd, managed under consistent conditions, that provided the retrospective dataset spanning 2014 to 2019.

The team analysed the records using analysis of variance in SPSS version 27, treating calf sex, breed, parity and the breed-by-parity interaction as fixed effects, with Bonferroni-adjusted comparisons and Tukey’s test to separate means. Data normality was confirmed with the Ryan-Joiner test, and lactations shorter than 140 days in milk were excluded from total-yield calculations. Early lactation was defined as the first 90 days in milk. The statistical verdict was unambiguous: calf sex, breed and the breed-by-parity interaction had no significant effect on milk yield or lactation length.

Parity told a different story. Cows in their fourth through sixth parities produced 849 ± 66 kilograms of milk in early lactation, and those beyond their sixth parity produced 1,005 ± 116 kilograms — both figures significantly higher than the 673 ± 41 kilograms recorded for cows in their first to third parities. Daily milk yield followed the same gradient, at 5.6 ± 0.33 kilograms for the youngest parity group compared with 8.4 ± 1.0 kilograms for cows beyond their sixth lactation. The pattern makes physiological sense: young cows are still growing, so nutrients and energy must be split between body development, gestation and milk synthesis, often pushing them into a deeper negative energy balance. Mature cows, with larger mammary glands and more developed secretory tissue, can channel a greater share of their intake into milk.

Perhaps the most striking finding came from regression analysis. Early lactation milk yield accounted for 73 percent of the variability in total lactation yield — a coefficient of determination of 0.73. In other words, a cow’s output in her first three months of milking is a powerful predictor of her entire lactation. This aligns with a well-established body of literature showing that early lactation is a physiologically critical window in which milk yield peaks while energy demands often exceed dry matter intake. It also suggests that phase-specific management — concentrating feed resources and attention on freshly calved cows — could pay dividends even on smallholder, pasture-based tropical farms, where such approaches are rarely practised.

The null result on calf sex is notable because the literature on sex-biased milk synthesis is genuinely contested. Some studies, including a well-known analysis of Holsteins, reported that cows carrying female fetuses produced around 143 kilograms more milk than those carrying males, possibly because longer gestations for male calves shorten the subsequent lactation. Others found the opposite, attributing higher yields after male births to larger fetal size and elevated placental estrogen, which stimulates prolactin release — though bigger calves also raise the risk of difficult calvings that depress yield. The Centeno findings suggest that such sex effects, where they exist, are contingent on factors like calf size, dam health, parity and production system, and that within a uniform herd under consistent management, they may simply wash out.

The equivalence of the Jersey and Centeno Crossbred cows is equally consequential. Despite their different genetic backgrounds, the two breeds performed comparably under identical conditions, with the crossbreds posting numerically higher early lactation and daily yields while Jerseys showed numerically higher total yield and longer lactations. Since neither difference reached significance, the results hint that both breeds carry similar genetically linked performance characteristics when mature body size and management are held constant. For farmers choosing breeding strategies, that is reassuring: the locally adapted crossbred, bred for heat tolerance and disease resistance, gives up little in productive potential to the imported Jersey.

The sobering counterpoint is that absolute productivity at the station remains very low. Jersey cows elsewhere in subtropical climates yield around 11 kilograms of milk per day, and 20 to 25 kilograms in temperate environments — between 27 and 68 percent more than the CLS herd. Worryingly, low productivity in Centeno Crossbreds was flagged four decades ago, and the new data suggest performance may have declined further, pointing to persistent inadequate nutrition, unimproved pastures, dry-season feed gaps, costly concentrate supplementation and possible genetic depletion. The authors argue that improving pasture quality and quantity, supplemental feeding, reproductive efficiency and heat-stress management — measures that meta-analyses suggest can lift smallholder tropical milk yields by as much as 29 percent — must target cows in fourth and later parities and, above all, cows in early lactation, because that is where milk production is maximized and where the whole lactation is won or lost.

Subject of Research: Effects of calf sex, breed and parity on milk yield and lactation length in a tropical dairy herd

Article Title: Effects of calf sex, breed and parity on the lactation performance of dairy cows in Trinidad and Tobago

Article References: Auguste, E., Hughes, M., & Karuppusamy, S. (2026). Effects of calf sex, breed and parity on the lactation performance of dairy cows in Trinidad and Tobago. Discover Agriculture, 4(1), Article 295. https://doi.org/10.1007/s44279-026-00779-x

Image Credits: AI Generated

DOI: 10.1007/s44279-026-00779-x

Keywords: dairy cattle, milk yield, parity, calf sex, lactation length, early lactation, Centeno Crossbred, Jersey, smallholder farming, Trinidad and Tobago, tropical agriculture, animal breeding