Introduction           

Feeding rumen-protected bypass methionine (RP-Met) in pre-fresh and lactating cow rations is common in the dairy industry. Methionine is considered one of the first two limiting amino acids in modern diets, along with lysine. There is considerable literature on the effects of feeding RP-Met on milk and component production, especially that of milk protein. Many factors impact what effect RP-Met will have on lactating cows, including nutrient makeup of the diet, production level, and management. Rumen-protected amino acid products are typically expensive ingredients and nutrition consultants and dairy managers should consider what response they would need to achieve in order to have a positive return on investment (ROI). Feeding RP-Met to transition and early/mid lactation cows may prove to be a positive investment regardless of milk and component pricing. This article will focus on the benefits of feeding RP-Met to transition cows as well as evaluating these products for lactating cows in general.

Feeding RP-Met to Transition Cows:

The onset of lactation brings large demands for protein and energy, and management and nutrition strategies focusing on transition cows are well-researched topics. Rumen-protected methionine and rumen-protected choline (RP-Chol) products that are often fed to transition cows. These nutrients are involved in different biological pathways but both have a variety of benefits. The major reason for feeding RP-Chol in transition diets is because it is a precursor for phosphatidylcholine, which is needed for the export of fat from the liver, whereas methionine is biologically useful to cows throughout lactation. Both of these products potentially have their places in transition cow programs. This article will focus on RP-Met.

A study by Zhou et al. (2016) looked at the effects of feeding RP-Met to transition cows. Cows were fed either 0g (control) or 13g RP-Met pre-fresh (-21 days) and 0g or 18g RP-Met in the post-fresh (30 days) diets (Table 1). Supplementation of RP-Met increased DMI in both pre- and post-fresh cows compared to the control group. The RP-Met treatment also increased milk yield 6.2 lbs., milk protein concentration and yield by 0.24% and 0.37 lbs., and butterfat concentration and yield by 0.14% and 0.33 lbs. respectively. Overall, energy-corrected milk (ECM) was increased 9.48 lbs. in RP-Met supplemented cows compared to the control group. Toledo et al. (2021) ran a similar study feeding additional RP-Met to both pre- and post-fresh cows at two separate universities and found that there was an overall effect of methionine to increase butterfat concentration by 0.10% and protein concentration and yield by 0.12% and 0.11 lbs. respectively, but had no impact on other production measures or DMI.

The transition period is a critical time for dairy cow health. Although RP-Chol can be a great tool for transition cows, many farms do not have a separate pen for their fresh cows, and RP-Choline does not likely have much biological or economic return after the transition period. Feeding RP-Met can have a significant impact on milk and component production for cows in many stages of lactation. For every pound of milk at peak, cows may increase production by 250 lbs. over the course of their lactation and feeding RP-Met can increase milk and component production for fresh and high-producing cows.

Overall Results Feeding RP-Met

As previously stated, RP-Met is commonly fed to cows of all lactation stages. A meta-analysis of feeding different sources of RP-Met was published by Zanton et al. (2014). Briefly, the impact on DMI and milk production when feeding RP-Met was variable, with some studies reporting slight increases and others small decreases. Authors showed an average of 0.07-0.08% milk protein concentration increase when feeding RP-Met, and milk fat concentration had a numerical increase as well. Increases in milk protein and fat yield showed a lot of variation but overall had positive numerical impact. The data on health events and somatic cell count observed while supplementing RP-Met are inconsistent.

Evaluating RP-Met supplementation is often dependent on many factors including stage of lactation, production level, and nutrient makeup of the diet. Energy-deficient diets may be limited in responses to RP-Met supplementation. Finally, there is a growing body of research showing the importance of amino acids other than methionine and lysine. Research regarding the importance of histidine, leucine, and isoleucine is ongoing. 

Overall Conclusions:

Feeding RP-Met can be a great tool to increase milk protein concentration and yield in cows of all stages of lactation for farms of all sizes. Supplementing RP-Met to transition cows can improve milk and component yield for cows in the first month of lactation. Given the large increase in production from fresh cows, supplementing RP-Met to these groups may be warranted regardless of milk and component prices. Nutrition consultants should rely on ROI calculations when supplementing RP-Met in non-transition lactating cow diets by evaluating the milk and component response. Proper amino acid balance and adequate energy levels have an impact on the response that RP-Met delivers. Finally, there are many RP-Met products on the market, and farms and consultants should be aware of costs, availability, and quality of the products they are selecting. 

Table 1. Overall intake and production responses when supplementing cows with rumen-protected bypass methionine in the pre- and post-fresh periods.

• *Adaptd from Zhou et al. (2016)
• ¹Treatments were either 0g of RP-Met in both the pre- and post-fresh rations (Control) or 13g and 18g RP-Met in pre- and post-fresh rations, respectively.
• ²P-values of the overall response of the inclusion of RP-Met
• ³Energy-corrected milk (ECM) is calculated by ECM = (0.327 x milk yield) + (12.95 x fat yield) + 7.65 x protein yield)
• ⁴ECM / DMI is used as a proxy for feed efficiency

References:

• Toledo, M.Z., M.L. Stangaferro, R.S. Gennari, R. V. Barletta, M.M. Perez, R. Wijma, E.M. Sitko, G. Granados, M. Masello, M.E. Van Amburgh, D. Luchini, J.O. Giordano, R.D. Shaver, and M.C. Wiltbank. 2021. Effects of feeding rumen-protected methionine pre- and postpartum in multiparous Holstein cows: Lactation performance and plasma amino acid concentrations. J. Dairy Sci. 104:7583–7603. doi:10.3168/jds.2020-19021.
• Zanton, G.I., G.R. Bowman, M. Vázquez-Añón, and L.M. Rode. 2014. Meta-analysis of lactation performance in dairy cows receiving supplemental dietary methionine sources or postruminal infusion of methionine. J. Dairy Sci. 97:7085–7101. doi:10.3168/jds.2014-8220.
• Zhou, Z., O. Bulgari, E. Trevisi, M.A. Ballou, F.C. Cardoso, and D.N. Luchini. 2016. Rumen-protected methionine compared with rumen-protected choline improves immunometabolic status in dairy cows during the peripartal period. J. Dairy Sci. 99:1–14. doi:10.3168/jds.2016-10986.

Defining the nutritional and functional differences between skim milk powder and whey protein concentrate and exploring the effects of these milk protein sources on nutrient digestibility and performance in calves.

Calf milk replacer (CMR) is fed to a majority of the dairy calves in the United States in place of whole milk because it is often more economical, provides convenience and consistency, and lowers risk of disease transmission from unpasteurized milk. Many CMRs are made exclusively from dairy ingredients due to the calf’s innate ability to efficiently digest, absorb, and utilize the nutrients that naturally exist in ingredients of dairy origin.

Research into products to replace whole milk began in the mid-20th century and primarily utilized casein, skim milk, and whey as protein sources. As reviewed in Kertz et al. (2017), the prevailing thought of the time was that quality of protein within a milk replacer was directly related to the ability of the milk replacer to form a clot in the abomasum, and that poor-quality CMR would not form clots in the abomasum, resulting in diarrhea and reduced calf performance. Given that CMRs formulated with dried skim milk powder (SMP) readily clot in the abomasum due to the presence of casein and CMRs formulated with whey protein concentrate (WPC) do not clot in the abomasum due to the absence of casein, one might assume that CMR formulated with SMP would result in improved health and digestibility in calves. A review by Logenbach and Heinrichs (1998) dispels this myth and states that factors other than clotting are responsible for observed differences in calf performance.

This paper defines the nutritional and functional differences between SMP and WPC and explores the effects of these milk protein sources on nutrient digestibility and performance in calves.

A PRIMER ON MILK PROTEINS

While both WPC and SMP originate from whole milk, they are derived through very different forms of milk processing. Dried WPC is a co-product of the cheesemaking process – liquid whey is separated from curds during the cheesemaking process and subjected to ultrafiltration and drying, resulting in various whey protein concentrates containing 34-80% crude protein. Dried SMP, in contrast, results from the separation of cream from milk for butter manufacturing – the resulting skim milk is dried to produce a 34% crude protein ingredient. A key difference among these ingredients is that SMP contains all proteins found in milk, whereas WPC does not contain casein because casein is utilized in the cheesemaking process. As a result, the amino acid profiles of whey protein and skim milk protein differ slightly.

Historically, skim milk was a prime candidate for inclusion in CMR – it was readily available and inexpensive compared to whole milk. More recently, there has been an emergence of skim milk protein use in human and sports nutrition, which has driven the cost of SMP higher and precluded use of much SMP in modern CMR in the United States. In contrast, whey was long considered a waste product with little to no value. As soon as technology allowed for efficient collection and use of whey proteins, milk replacer manufacturers were quick to adopt WPCs in CMR, and WPC is now one of the most commonly used ingredients in CMR due to its excellent nutritional value, high digestibility, and relatively low cost. Heinrichs et al. (1995) surveyed milk replacer use in the United States and reported that approximately 90% of CMRs sampled did not clot in the presence of rennet, indicating little to no casein-containing ingredient inclusion. Another 8% of CMRs sampled formed only a soft clot, indicating 5% of less of the protein in the CMR being derived from casein-containing ingredients. Only 2% of sampled CMRs formed firm clots in the presence of rennet, suggesting very little usage of casein-based ingredients such as SMP in most CMRs.

RESEARCH ON NUTRIENT DIGESTIBILITY AND CALF PERFORMANCE

Several studies have evaluated nutrient digestibility and calf performance when feeding CMR containing WPC and/or SMP. Terosky et al. (1997) fed calves CMRs containing 0, 33, 66, or 100% of protein from either WPC or SMP through 8 weeks of age and measured calf performance and nutrient digestibility (Table 1). The authors reported no difference in bodyweight gain, average daily gain (ADG), dry matter intake (DMI) or feed efficiency with increasing WPC inclusion. Apparent digestibility also did not differ across treatments. There was also no difference in number of scour days per calf between treatments.

_{Table 1. Performance and nutrient digestibility in calves fed CMR containing 0,33,66 or 100% of protein as WPC or SMP, Adapted from Terosky et al., 1997.}

A study conducted by Lammers et al. (1998) fed diets similar to those above in two trials until 6 weeks of age. Results are shown in Table 2. In the first trial, calves were fed CMR only, and in the second trial, calves were fed CMR plus ad libitum calf starter. When calves were fed CMR only, ADG and feed efficiency were improved with increasing WPC inclusion, and there was no difference in DMI or scour days per calf among treatments. When calves were fed CMR and calf starter, there were no differences in DMI, ADG, feed efficiency, or scour days per calf among treatments.

_{Table 2. Growth and performance in calves fed CMR containing 0, 33, 67 or 100% of protein as WPC or SMP, Adapted from Lammers et al., 1998.}

A more recent study by Marsh and Boyd (2011) reported no difference in weaning weight, 12 week weight, coat bloom score, ADG, DMI, or feed efficiency when Holstein bull calves were fed CMR containing protein from either SMP or WPC. Finally, Petit et al. (1988) sought to determine if clotting ability of CMR affected nutrient digestibility. Calves were fed either a control CMR containing SMP, or the control CMR with added oxalate, an anti-clotting factor that does not denature milk proteins. As shown in Table 3, there was no difference in dry matter, protein, or fat digestibility when calves were fed identical CMR that clotted or did not clot, disproving the notion that CMR quality was linked to ability of CMR to form a clot in the abomasum.

_{Table 3. Dry matter protein, and fat digestibility in calves fed either clotting or non-clotting CMR containing SMP}

Taken as a whole, these data suggest that WPC provides similar digestibility and calf performance to SMP, and that ability of protein to clot in the abomasum is not indicative of nutrient digestibility.

CONCLUSION

While casein-containing ingredients such as skim milk were first on the scene in CMR formulation due to low cost and high availability, they have all but been replaced with more cost-effective whey based protein sources such as WPC. Despite differences in ability of these protein sources to form clots in the abomasum, substituting WPC for SMP has little to no impact on calf performance – nutrient digestibility, intake, calf growth, and feed efficiency are maintained. Whey-based protein sources such as WPC are a cost-effective, ideal ingredient for inclusion in CMR.