Abstract Maternal supplementation with rumen-protected methionine (RPM) has the potential to improve embryo development and calf performance, particularly when fed during early gestation. Previous data from our group indicated that male calves from dams supplemented with RPM during the periconceptional period were heavier at birth compared to controls, while females showed no differences, suggesting a sexually dimorphic response to RPM. We hypothesize that supplementing beef cows with RPM during the periconceptional period influences embryo development, resulting in larger and heavier male calves with no effect on females. In this study, lactating beef cows (58 ± 18 DPP) were randomized to receive 454 g/d of corn gluten supplemented with 15 g/d of RPM (RPM; n = 124) or not (CON; n = 126) from days −7 to 7 relative to the artificial insemination (day 0) using conventional semen from two bulls. Supplementation was delivered through an automated feeder that tracked intake and only pregnant cows with good intake remained in the experiment (CON; n = 27; RPM; n = 29). Blood was collected for pregnancy-associated glycoprotein (PAG) analysis. At birth, calves were weighed and tagged, and placentas were collected, measured, and sampled for further analysis (CON; n = 23; RPM; n = 25). Calves’ morphometric traits were recorded bi-monthly until weaning, and at three months males were castrated and testes weighed. Data was analyzed using MIXED procedure of SAS, with treatment, sex, and their interaction as fixed effects, and group as a random effect. Gestation length (CONₘale: 273. 78 ± 1. 64, CONfem: 274. 11± 1. 39; RPMₘale: 273. 65 ± 1. 40, RPMfem: 275. 35 ± 1. 48 days; P = 0. 73) and birthweight (CONₘale: 34. 27 ± 0. 95, CONfem: 32. 02 ± 0. 88; RPMₘale: 33. 94 ± 0. 85, RPMfem: 33. 17 ± 0. 87 kg; P = 0. 64) were not affected by treatment or by the treatment × sex interaction (P = 0. 40). Placental weight (CONₘale: 3. 18 ± 0. 34, CONfem: 2. 92 ± 0. 34; RPMₘale: 3. 43 ± 0. 32, RPMfem: 3. 68 ± 0. 36 kg) and length (CONₘale: 60. 68 ± 1. 53, CONfem: 60. 60 ± 1. 35; RPMₘale: 62. 99 ± 1. 36, RPMfem: 60. 96 ± 1. 38 cm) were also not affected by treatment, sex or their interaction (P ≥ 0. 05). Similarly, PAG concentrations at days 30 and 52 were not affected by treatment, sex or their interaction (P ≥ 0. 05). At 1, 3, and 5 months, no interaction or treatment effects were observed on calves’ heart girth, hip height and body length (P ≥ 0. 05). Testes weight was greater in RPM vs. CON males (CON: 13. 11 ± 1; RPM: 16. 57 ± 0. 86 g; P = 0. 01). These preliminary results suggest that RPM supplementation does not affect birthweight, gestation length, or placental traits but influences postnatal testicular development. Further analyses are warranted to determine long-term effects on calf growth and performance.
Infante et al. (2026) studied this question.