Reproductive efficiency is one of the most important drivers of profitability on a dairy farm. Despite advances in genetics, veterinary medicine, and reproductive technologies, many dairy producers continue to struggle with conception rates, pregnancy losses, and delayed return to fertility after calving.

Among all amino acids, methionine has emerged as a key nutrient influencing not only milk production but also reproductive performance, embryonic survival, and even the future growth of calves. Recent research suggests that optimising methionine supply can improve fertility outcomes at multiple stages of the reproductive cycle, from postpartum recovery to fetal development.

Methionine, a limiting amino acid for high producing cows

Methionine is considered as the first limiting amino acid in dairy rations of high producing cows because milk contains relatively high methionine concentration while common feed ingredients contain comparatively low levels.

Beyond protein synthesis, methionine serves as a precursor for S-adenosylmethionine (SAM), the body's principal methyl donor. This molecule supports liver function, antioxidant defences, immune responses, and DNA methylation, processes that are increasingly recognised as important regulators of reproduction.

Table 1. Methionine content of milk, microbial protein and common feed ingredients (methionine, % of crude protein)

Milk¹: 2.7
Rumen bacteria²: 2.3
Corn silage³: 1.6
Grass silage³: 1.5
Barley³: 1.6
Corn grain³: 2.1
Wheat³: 1.6
Canola meal³: 1.9
Soybean meal³: 1.3

¹ NRC (2001), ² Sok et al. (2017), ³ AMINODat 6.0

Methionine as a reliable tool to prepare cows for a successful fertilisation

Postpartum uterine functions

The first nutritional factor affecting reproduction is an excessive body reserve mobilisation (negative energy balance; NEB) after calving, which results from a nutrient intake of the cow that is lower than its requirements. The excessive fat mobilisation from lipolysis of adipose tissue increases the blood concentration of non-esterified fatty acids (NEFA), which will be further converted to ketone bodies (as β-hydroxy butyrate; BHB) in the liver to partially fulfil the sudden energy requirements of the animal. However, ketone bodies have deleterious effects on the overall metabolism of the cow as they may lead to inflammatory issues (e.g. mastitis, metritis). In addition, elevated NEFA may also down-regulate immune functions. All these metabolic changes are particularly problematic as the postpartum uterus in dairy cows is susceptible to infections caused by multiple bacterial pathogens, and it is known that uterine infections delay the return to cyclicity.

Additional metabolisable methionine in the diet can reduce NEB and support the immune system during this challenging time. In a study conducted at the University of Illinois, two groups of 30 multiparous Holstein cows were fed a control diet alone or supplemented with 0.1% Mepron® (rumen-protected methionine (RPM); Evonik Operations GmbH) during the transition period. Reduction of NEFA and BHB levels in the plasma of Mepron® supplemented cows (Figure 1) were observed which indicated a lower NEB.

Figure 1. Effect of Mepron® on blood NEFA and BHB concentration in transition dairy cows (Batistel et al., 2018).

In addition, neutrophils activity was enhanced with Mepron® supplementation, which shows that additional RPM may prevent uterine infections and help the cows to improve postpartum uterine functions (Batistel et al., 2018).

Follicular development and ovulation

The development of an ovulatory follicle can take approximately 100 days. Nutritional status during this period influences follicular growth, oocyte quality, and the likelihood of successful conception.

One challenge is that excessive dietary protein can elevate blood urea levels, which may impair oocyte development and reduce fertility. Balancing diets with limiting amino acids such as methionine allows producers to reduce dietary crude protein while maintaining productivity and improving nitrogen efficiency. In a study by Alonso et al. (2008), crossbred beef heifers were fed a basal diet with or without 10 g/d of Mepron® for 45 days.

Results showed that Mepron® supplementation increased the proportion of large ovarian follicles (Figure 2) and improved ovulation rates (86.7 vs 62.5%) in heifers due to additional metabolisable methionine supply. Commercial farm observations also showed fewer days to first service among Mepron® supplemented cows, indicating a faster return to breeding readiness (Figure 3).

Figure 2. Effect of Mepron® on follicle development in heifers (Alonso et al., 2008).

Figure 3. Effect of Mepron® on days to first service in two commercial dairy farms (internal data).

How can methionine help to maintain the embryo during the early days?

Maintaining the embryo during the days following insemination is the most challenging aspect of reproduction success. In Holstein-Friesian dairy cows, early embryonic losses (until day 30) are estimated at around 27% whereas late pregnancy losses (from day 30 to day 45) are around 13% (Albaaj et al., 2023). This fetal loss is related to the late implantation of the embryo in cows.

Indeed, during the early post-fertilisation period, the embryo remains free-floating and depends entirely on the uterine environment for survival.

Impact on embryonic losses

Supplementing RPM appears to support embryo survival with higher lipid content (potentially providing a valuable energy reserve during early development), increased embryo size and larger amniotic vesicles in cows (Acosta et al., 2016; Toledo et al., 2017). It strengthens the pre-implantation embryo, reducing embryonic losses. In a recent large-scale study, supplementing RPM from day 28 to day 61, after insemination, significantly reduced pregnancy losses (19.6% vs 6.1%; P = 0.03) in multiparous cows (Toledo et al., 2017). Perhaps the most intriguing effect is the influence of methionine on embryonic DNA methylation. Methionine-derived methyl groups can alter gene expression, favouring specific metabolic pathways involved in embryo development and immunity, without changing the DNA sequence itself.

Benefits extend to the next generation

The effects of methionine do not stop with pregnancy maintenance. The RPM also contributes to fetal development, which further impacts the performance and metabolism of the pre-weaning calves. In a study conducted in the US, 2 groups of 30 Holstein cows were fed a basal diet alone or supplemented with 0.9 g Mepron® per kg DM four weeks before expected parturition. Upon birth, all calves followed a common feeding management programme until weaning (56 d). Calves from Mepron® fed cows were heavier at weaning due to higher average daily gain (+50 g/d) of calves (Batistel et al., 2019; Figure 4).

Figure 4. Effect of maternal Mepron® supplementation on body weight of the offspring before weaning.

Summary

In summary, supplementing Mepron® to cows can improve pregnancy rate and support fetal development (Figure 5).

Figure 5. Summary of the effects of rumen-protected methionine supplementation on reproductive performance

After calving: RPM can stimulate immune functions and help the cows to resolve postpartum uterine functions.
Follicular development and ovulation: RPM can favour the establishment of ovarian activity and follicular dynamics.
From fertilisation to fetal stage: RPM strengthens the preimplantation embryo.
Fetal growth to calving: RPM impacts the establishment of the metabolic functions of the fetus to favour better health and growth of the calves.

Additional metabolisable methionine can stimulate immune functions to help cows improve postpartum uterine functions. It can favour the re-establishment of ovarian activity and follicular dynamics after calving. Methionine can also strengthen the preimplantation embryo, to prevent embryonic losses. Finally, it impacts the establishment of the metabolic functions of the fetus to favour better health and growth of the pre-weaning calves.

Full list of references is available on request. First published in the September 2026 issue of Livestock & Feed Business.

Evonik: Boosting Reproductive Performance in Dairy Cows with Rumen-Protected Methionine