Jeff Kaufman


Lactating cows at peak lactation can require up to 18% crude protein on a dry matter (DM) basis in their diet8. The protein portion of a diet accounts for the largest cost in a lactating cow’s ration. Comparing plant-based protein ingredients (soybean or canola meal) to energy (corn) and forage (hay and silage) ingredients, protein costs can be from 2 to 10 times greater per ton. Rumen undegradable protein (RUP) can be even more expensive (e.g. blood meal can reach over $1,000/ton) but can be critical for providing adequate essential amino acids (lysine and methionine) to support lactation requirements.

High-protein diets and their costs

In a high-protein diet (18% on a DM basis) [typically 10% rumen degradable protein (RDP) and 8% RUP], the protein proportion can represent up to 20-30% of the total feed cost. Data derived from a previous study5 indicates that the protein cost of a lactating cow’s diet can be about $1.75/cow/day, which is 23.8% of the total diet cost (Table 1). When milk prices are low, high-protein diets are not cost-effective for dairy farmers, as they fail to provide significantly greater amounts of milk. For example, a 14.9% protein diet (DM basis) had sustained milk and milk component yields compared to a 17.5% crude protein diet (a yield of 38.4 versus 39.0 kg/day of milk, 3.68 versus 3.60% of milk fat, and 3.30 versus 3.28% of milk protein)7. Similarly, a 15% protein diet with a high-quality amino acid profile sustained milk and milk protein yields compared to an 18% crude protein diet with a low-quality amino acid profile1. A major problem with providing a high-crude-protein diet is when cows are experiencing high environmental temperatures and humidity. In these circumstances, high-producing dairy cows become less efficient at using dietary protein for production, which worsens the cost-effectiveness of high dietary protein.

Table 1. Economic evaluation of diet with different protein levels
8% RUP 6% RUP
10% RDP 8% RDP 10% RDP 8% RDP
Milk Yield (lbs.) 82.7 76.9 69.4 76.1
Income ($/cow/day) @ $15/cwt 12.4 11.5 10.4 11.42
Total Feed Cost ($/cow/day) 7.35 6.94 6.86 6.70
Crude Protein Cost ($/cow/day) 1.75 1.48 1.46 1.21
Crude Protein Cost Over Total Cost (%) 23.8 21.3 21.3 18.1
Income Over Feed Cost ($/cow/day) 5.05 4.59 3.55 4.71
Milk Income Used for Feed Cost (%) 59.3 60.2 65.9 58.7
Source: Kaufman et al.5

 Consideration for heat-stressed cows

Environmental impact from dairy cows is a major issue facing the dairy industry. Heat stress increases the loss of nitrogen through urine and feces coming from protein fed to dairy cows4. Urinary nitrogen excretion increased by 41% when lactating cows were exposed to 28°C and 60% humidity3. Nitrogen excretion contributes to groundwater and air pollution. Fecal nitrogen in manure is more stable, whereas urine nitrogen is present as urea that easily pollutes water sources and the air2. Therefore, lowering the increased urinary excretion of nitrogen resulting from heat stress will be important for ensuring sustainable dairy farming and improving public perceptions. To optimize protein and nitrogen efficiency in dairy cows, strategies for protein nutrition are necessary for heat-stressed dairy cows.

Focusing on milk production from heat-stressed cows

Heat stress decreases the productive ability of lactating cows, which may result from reduced protein nutrition. Milk yields decreased by up to 28% or 9.6 kg/day when temperatures increased from 29 to 39°C11. Similarly, milk components such as milk protein decreased ranging from 4.8 to 9.6% when cows did not have mitigation from heat stress9,11. When dietary protein is excreted as nitrogen in the urine and feces, less nitrogen is utilized for milk production. For example, in the previously mentioned study, in which nitrogen excretion increased during heat stress3, milk protein content fell by 9%. The goal would be to optimize protein utilization in heat-stressed cows to improve lactation performance. In fact, my research focused on providing strategies to improve milk production and protein metabolism in heat-stressed dairy cows.

Lowering dietary protein to improve components

A study was conducted at the East Tennessee AgResearch and Education Center at the University of Tennessee to observe the effects of comparing a mixture of 2 levels of RDP (10% and 8% on DM basis) and 2 levels of RUP (8% and 6%) on milk performance and nitrogen-use efficiency5. The trial evaluated mid-lactation cows (126 days in milk) housed in a free-stall barn experiencing up to 29.2°C and 58% humidity. Those combinations of RDP and RUP equaled 4 treatments of 10% RDP:8% RUP (18% protein), 8% RDP:8% RUP (16% protein), 10% RDP:6% RUP (16% protein), and 8% RDP:6% RUP (14% protein). A diet balanced with 8% RDP and 6% RUP, as a 14% crude protein diet, showed the most beneficial results in regard to milk production and nitrogen use combined (Table 2). Energy-corrected milk yield production was similar (34.8 vs. 35.0 kg/day) in the 14% and the 18% protein diet. However, milk fat and protein content increased from 2.75 to 3.17% and from 2.90 to 2.98%, respectively, when dietary RDP and RUP were reduced (18 vs. 14% protein diet). This study had low milk fat levels mainly due to the impact of heat stress on cows. Increased milk components and sustained milk production may be attributed to an increment in nitrogen-use efficiency and a reduction in urinary nitrogen excretion. Nitrogen use efficiency increased from 31.6 to 37.8% and urinary nitrogen excretion decreased by 54% when both RDP and RUP were reduced. This data demonstrates that not only can milk production be sustained and components improved but that this will decrease feed costs during heat stress. Table 1 shows the income over feed costs (IOFC) ratio from this study, providing the lowered RDP and RUP diets. The 14% crude protein diet had the highest income over feed cost (IOFC), the lowest percentage of milk income going to feed costs, and a marginal difference in milk yield compared to the high crude protein diet. In summary, the 14% crude protein diet had a greater IOFC ratio, improved milk components, and decreased nitrogen excretion from urine.

Table 2.  Diet composition and performance of heat stressed cows
Ingredients (%) Diets
8% RUP 6% RUP
10% RDP 8% RDP 10% RDP 8% RDP
Corn Silage 45.0 45.0 45.0 45.0
Wheat Silage 1.50 1.50 1.50 1.50
Clover Hay 3.50 3.50 3.50 3.50
Concentrate 50.0 50.0 50.0 50.0
Composition (%)
Crude Protein 17.6 15.9 15.6 13.8
Rumen Degradable Protein 9.80 7.80 9.70 7.80
Rumen Undegradable Protein 7.80 8.10 5.90 6.00
ECM (kg/day) 35.0 33.8 32.0 34.8
True Protein (%) 2.90 2.88 3.17 2.98
Fat (%) 2.75 2.95 3.14 3.17
MUN (mg/dL) 11.7 7.98 9.17 5.46
Use Efficiency (%) 31.6 32.8 32.4 37.8
Urinary Excretion (g/day) 214 143 162 98.2
Source: Kaufman et al.5

Lowering dietary protein to improve utilization of amino acids

Heat-stressed cows have a negative energy balance that changes the animal’s normal state of maintenance, especially during lactation. Cows increase the breakdown of muscle tissue to provide protein and amino acids for energy purposes. For example, cows exposed to 28°C and 60% humidity showed a 96% increase in a blood marker for muscle breakdown (3-methylhistidine)4. As a result, fewer amino acids and less nitrogen are available for milk protein production. My research, explained in the previous section, confirmed that lowering RDP and RUP to a 14% crude protein diet increased the use of fat instead of protein for energy needs in heat-stressed cows6. This allows the body’s natural energy stores to be used for energy instead of protein and amino acids, which are used for productive reasons (muscle growth and milk production). This work showed that lowering RDP and RUP (18% to 14% protein diet) increased blood levels of fatty acids, which demonstrates a nutritional drive that supports increased energy needs by breaking down fat (Table 3). Likewise, more protein and amino acids were being used to support milk protein as demonstrated by the 27% increase in milk protein yield efficiency (the amount of absorbed amino acids from the diet making milk protein). This is evident from the existence of greater blood concentrations of essential amino acids used for milk protein synthesis (lysine and methionine) from low RDP and RUP diets compared to the 10% RDP and 8% RUP diet. For cows experiencing hot temperatures and high humidity, a lower crude protein diet with an equal or greater RDP and RUP ratio of 50:50 that equals between 14 and 16% crude protein should not only benefit milk component production and efficient use of dietary protein but also lead to a reduction in feed cost.

Table 3 Energy and protein metabolism of cows fed diets with different protein levels.
8% RUP 6% RUP
10% RDP 8% RDP 10% RDP 8% RDP
Energy Balance (Mcal/day) -2.08 -5.12 -2.23 -4.29
Milk Protein Yield Efficiency* (%) 45.3 44.2 50.8 57.3
Insulin (μU/mL) 22.8 19.8 19.7 12.0
Fatty Acids (μEq/L) 123 199 206 175
Essential Amino Acids (μM) 907 1,296 1,245 1,110
*Milk protein yield efficiency = milk protein yield / metabolizable protein supply.

Source: Kaufman et al.6


Provide sufficient dietary energy: Energy is needed to support the healthy microbial breakdown of RDP, especially for heat-stressed cows.

Quality and degradability of protein matters: Determining the digestibility and the amino acid profile present in the feed ingredients will dictate availability and use in the animal. A high-quality ingredient will make a big difference. In addition, sources of RDP can provide RUP in sufficient amounts (canola meal).

Optimize RDP use: This protein fraction must be provided to optimize the production of microbial protein from the rumen without oversupplying nitrogen that will be excreted as urea through urine and feces.

Evaluate and balance RUP: Knowing the amino acid profile of the feed ingredient is important so that specific amino acid requirements other than protein requirements can be met. Research has been summarized to suggest the ideal essential amino acid requirements10.

Balance with mixture of protein ingredients: To better meet amino acid requirements, mixing various protein ingredients will better supply limiting amino acids throughout the diet.

Stage of lactation, parity, and production: Lactation requirements from late to early lactation, multi- to primiparous, and low to high producer cows in heat stress may affect the need to increase protein closer to 16% crude protein.

Component focus during heat stress: Heat stress makes it difficult to increase milk yield without causing other health effects. However, increased components are manageable with proper dietary protein management.

Think about the future: Dietary protein is an easily managed nutrient in the diet that can largely reduce environmental pollution and help improve public perception.

About the author

Jeff Kaufman is a Graduate Research Assistant at the University of Tennessee pursuing a Ph.D. in Dairy Cow Nutrition. His research focuses on protein and amino acid metabolism, production efficiency, and heat stress. He has published four articles from previous research and is currently working on his Ph.D. dissertation titled “Nutritional Strategies to Improve Production and Nitrogen Efficiency in Lactating Dairy Cows”. He has presented most of his research at international meetings.


  1. Bach, A., G. B. Huntington, S. Calsamiglia, and M. D. Stern. 2000. Nitrogen metabolism of early lactation cows fed diets with two different levels of protein and different amino acid profiles. J. Dairy Sci. 83:2585-2595.
  2. Castillo, A. R., E. Kebreab, D. E. Beever, J. H. Barbi, J. D. Sutton, H. C. Kirby, and J. France. 2001. The effect of protein supplementation on nitrogen utilization in lactating dairy cows fed grass silage diets. J. Anim. Sci. 79:247-253.
  3. Kamiya, M., Y. Iwama, M. Tanaka, and S. Shioya. 2005. Effects of high ambient temperature and restricted feed intake on nitrogen utilization for milk production in lactating Holstein cows. Anim Sci J 76:217-223.
  4. Kamiya, M., Y. Kamiya, M. Tanaka, T. Oki, Y. Nishiba, and S. Shioya. 2006. Effects of high ambient temperature and restricted feed intake on urinary and plasma 3‐methylhistidine in lactating Holstein cows. Anim. Sci. J. 77:201-207.
  5. Kaufman, J. D., K. R. Kassube, and A. G. Ríus. 2017. Lowering rumen-degradable protein maintained energy-corrected milk yield and improved nitrogen-use efficiency in multiparous lactating dairy cows exposed to heat stress. J. Dairy Sci. 100:8132-8145.
  6. Kaufman, J. D., K. G. Pohler, J. T. Mulliniks, and A. G. Ríus. 2018. Lowering rumen-degradable and rumen-undegradable protein improved amino acid metabolism and energy utilization in lactating dairy cows exposed to heat stress. J. Dairy Sci. 101:386-395.
  7. Mutsvangwa, T., K. L. Davies, J. J. McKinnon, and D. A. Christensen. 2016. Effects of dietary crude protein and rumen-degradable protein concentrations on urea recycling, nitrogen balance, omasal nutrient flow, and milk production in dairy cows. J. Dairy Sci. 99:6298-6310.
  8. 2001. Nutrient Requirements of Dairy Cattle. 7th rev. ed. National Academy Press, Washington, DC.
  9. Rhoads, M. L., R. P. Rhoads, M. J. VanBaale, R. J. Collier, S. R. Sanders, W. J. Weber, B. A. Crooker, and L. H. Baumgard. 2009. Effects of heat stress and plane of nutrition on lactating Holstein cows: I. Production, metabolism, and aspects of circulating somatotropin. J. Dairy Sci. 92:1986-1997.
  10. Rulquin, H., G. Raggio, H. Lapierre, and S. Lemosquet. Relationship between intestinal supply of essential amino acids and their mammary metabolism in the lactating dairy cow. Energy and Protein Metabolism and Nutrition. EAAP Publ. No. 124, Wageningen Academic Publishers, Wageningen, the Netherlands (2007), pp. 587-588.
  11. Wheelock, J. B., R. P. Rhoads, M. J. VanBaale, S. R. Sanders, and L. H. Baumgard. 2010. Effects of heat stress on energetic metabolism in lactating Holstein cows. J. Dairy Sci. 93:644-655.

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