Co-author: Ishaya Gadzama
Introduction
In the quest for high milk production, understanding the role of protein in your dairy cow's diet is key. Just like us, cows need protein for growth, milk production, and overall health, but it's not just about the animals themselves. The microbes in their rumen (the first stomach) also need protein to thrive (Putri et al., 2021; Manoukian et al., 2021). Recent improvements in cattle genetics, monitoring, and nutrition have led to significant increases in meat and milk production globally (Gadzama and Ray, 2024; Gadzama and Madungwe, 2024). Protein is essential for milk production, growth, and general productivity of dairy cows (Gadzama, 2024). It's made up of amino acids, which are crucial for building tissues, hormones, and enzymes. Knowing the various sources of protein and how they're used in a cow's digestive system helps you create a balanced diet, ultimately improving production and saving money (VandeHaar and St-Pierre, 2006; Gadzama, 2024; Gadzama and Makombe, 2024).
The Ruminant Digestive System
Unlike humans, cows have a special four-compartment stomach. The first and largest compartment is called the rumen, and this is where most of the magic happens. The rumen is like a giant fermentation tank, housing billions of microbes, such as bacteria, protozoa, and fungi. These microbes play a vital role in breaking down feed, including protein, through fermentation.
Types of Protein in a Dairy Cow's Diet
The total crude protein in a ration consists of soluble protein, rumen degradable protein (RDP), and rumen undegradable protein (RUP). Dairy cow diets need these different types of protein for optimal protein use.
Rumen Degradable Protein
When a cow eats, some of the protein is broken down by the microbes in the rumen. This is known as RDP. The microbes use these broken-down proteins (amino acids) for their growth. Once these microbes have done their job, they move to the lower gut and become microbial proteins. This is a significant source of protein for the cow, often meeting around 60% of their needs. When there isn't enough energy for the microbes, the amino acids from RDP can be converted into ammonia, which the cow then detoxifies. This ammonia can be recycled back into the rumen or excreted in urine. The amount of RDP in feed varies depending on the feed material, ration composition, and feed intake. For example, distillers' grains are about 40% RDP, forages are mostly RDP, urea is 100% RDP, and corn silage is predominantly RDP. A sufficient amount of rapidly degradable protein positively impacts milk production. However, it is important to have a balanced ration. Excess ammonia is created when there's too much degradable protein in the rumen relative to energy (a positive RDP balance). This excess ammonia represents a loss of valuable protein and can negatively impact animal health and performance. The amount of RDP can be calculated by deducting the soluble protein and rumen undegradable protein from the total crude protein.
What this means for you
RDP can enhance the value of low-quality forage. A study suggests that RDP, especially when provided in a self-fed form, can help cows get more out of low-quality forages by increasing digestion (Manoukian et al., 2021). This could mean you need less high-quality feed to meet your herd's nutritional needs.
Rumen Undegradable Protein
Some proteins escape digestion in the rumen and pass into the “true stomach” (abomasum) and small intestine, where they are digested and absorbed. This is RUP, and it directly provides the cow with amino acids. High-producing dairy cows often need strategic supplementation with RUP to meet their high protein requirements. RUP sources include heat-treated soybean meal, blood meal, and fishmeal. Heat treatment can increase RUP, but excessive heat reduces digestibility. Overheating during feed processing can cause the Maillard reaction, which binds protein and sugar molecules, increasing RUP levels, but potentially decreasing digestibility in the small intestine. Mechanically processed soybean meal has a higher RUP content than chemically processed soybean meal because of the heat generated during processing (Kovarna and Vanderhoff, 2023).
Why are RDP and RUP important?
RDP feeds the microbes
The rumen is full of tiny microbes that help cows digest food. Rumen microbes are like a workforce inside the cow's stomach. They break down feed, making nutrients available for the cow. These microbes need ammonia from RDP to grow and reproduce. When RDP is broken down, it releases ammonia, which the microbes use to make protein. As microbes ferment feed, they produce volatile fatty acids (VFAs). VFAs are the main source of energy for cows. Putri et al. (2021) found that higher levels of RDP and energy increased total VFA production in vitro. It's not enough to have protein, energy, and RDP; they must be balanced. When they are balanced, the microbes work most effectively, which is referred to as synchronized nutrition.
RUP feeds the cow
Rumen undegradable protein provides cows with a direct source of digestible amino acids that rumen microbes cannot synthesize.
Non-Protein Nitrogen (NPN)
Ruminants have a unique ability to use NPN sources like urea to create protein. Microbes in the rumen convert the NPN to ammonia, which can then be turned into microbial protein, provided there is enough energy. However, too much NPN can be toxic, so it's essential to balance it with other protein sources and introduce it slowly under the guidance of a ruminant nutritionist.
Methods for Estimating RDP and RUP
In Situ Method
This widely used research approach involves placing feed samples inside porous bags within the rumen of cannulated animals and then measuring the amount of undigested crude protein (CP) over different time intervals. This method helps determine three CP fractions: A, B, and C. Fraction A is considered to be completely degraded in the rumen, fraction B is potentially degradable, and fraction C is undegradable. The in situ method is considered to best represent the actual digestive process in a live animal, but it is labor-intensive and requires animals with surgically implanted ports in the rumen, which makes it costly and not practical for on-farm use (Schwab et al., 2003).
In Vitro Enzymatic Methods
These methods utilize enzymes to simulate the protein degradation that occurs in the rumen, offering a less complex alternative to in situ methods. These methods are categorized as "ruminal in vitro methods" using mixed ruminal microorganisms and "nonruminal in vitro methods," employing cell-free enzymes. A challenge with the ruminal in vitro methods is the microbial uptake of released amino acids and ammonia, which can lead to underestimation of degradation. Nonruminal methods struggle to achieve degradation rates that accurately reflect those in the rumen (Schwab et al., 2003).
In Vitro Multi-Chemical Methods
The Cornell Net Carbohydrate and Protein System (CNCPS) uses multiple chemical analyses to divide crude protein into five fractions: A (non-protein nitrogen), B1 (rapidly degraded true protein), B2 (moderately degraded true protein and large peptides), B3 (slowly degraded true protein), and C (undegraded true protein). This method is more practical for routine lab analysis, using solvents to determine different fractions of protein (Schwab et al., 2003).
Comparison of Methods
Both the in situ and multi-chemical methods are used to predict RDP/RUP, and while there are differences in predicted RUP values for some feeds, most values are similar. In a study using 278 diets, the mean bias of prediction for non-ammonia non-microbial nitrogen (NANMN) was +1 g/d for the in situ method (as used in the NRC model) and −24 g/d for the CNCPS method (Schwab et al., 2003).
Factors Affecting Rumen Protein Degradation
The amount of degradable protein depends on several factors, including the degradation rate, ration composition, and feed intake.
Feed Characteristics
The type of feed and its physical and chemical properties can affect protein degradation. For example, the presence of tannins in legumes can reduce rumen pH and potentially affect protein degradation. Feed processing can also impact how quickly proteins are degraded in the rumen.
Protein Structure
The structure of the protein itself influences its susceptibility to degradation. Some proteins are more easily broken down than others, depending on their amino acid composition and molecular structure. Grass often contains a lot of rapidly degradable protein. Some proteins, like soybean meal, are highly degradable, while others, like fishmeal, are less so. Processing techniques can influence the rate of degradation of protein. Ensiling increases the degradation rate, while drying and adding acids decrease it.
Energy Availability
Microbes need energy to use protein efficiently, so having enough energy in the diet is important. Excess ammonia is created when there is too much degradable protein in the rumen relative to energy. This situation is referred to as a positive RDP balance. The excess ammonia represents a loss of valuable protein and can negatively impact animal health and performance. The in vitro study by Putri et al. (2021) showed that a 70% energy level was better than a 65% energy level for optimal nutrient utilization.
Rumen pH
The pH of the rumen environment affects microbial growth and enzyme activity. The microbes work best at a pH range between 5.5 and 7. If the rumen becomes too acidic (usually from too much grain), it can disrupt the growth of rumen microbes and can reduce protein degradation. While Putri et al. (2021) did not find significant pH changes due to increased protein (from 12% to 14% DM) and RDP (from 55% to 65% crude protein), maintaining a stable pH is crucial for optimal microbial function. This is because the microbes that degrade protein (proteolytic microbes) stop functioning correctly in acidic conditions.
Balancing Protein for Optimal Milk Production
To maximize milk production, it's important to remember that "we feed the microbes, and the microbes feed the cow". You should balance RDP for microbial growth and RUP to meet the cow's amino acid needs. This will help ensure efficient use of protein and high milk production. It’s best to try to synchronize the release of protein and energy in the rumen for efficient protein degradation and microbial protein synthesis, although this can be challenging in practice.
Different feedstuffs have varying proportions of RDP and RUP. For example, forages are predominantly RDP, while distiller grains are about 40% RDP and 60% RUP. Urea is 100% RDP. The proportion of RDP and RUP in feedstuffs varies and this influences how dairy cows utilize protein. Growing calves on forage-based diets often require RUP supplementation to meet their metabolizable protein needs because forages are high in RDP. Corn silage, a common feed, is mainly RDP, and its RUP content decreases with longer ensiling times. Diets based on corn silage might need both RDP and RUP supplementation to meet the protein needs of growing calves. Overfeeding RDP does not provide additional metabolizable protein once the microbial requirements are met (Kovarna and Vanderhoff, 2023). However, overfeeding RUP does provide more metabolizable protein as the dietary supply increases. Therefore, understanding the balance between RDP and RUP in feedstuffs is crucial for meeting cattle's protein requirements and optimizing their nutrition and growth.
Conclusion
The degradation of protein in the rumen is a complex process influenced by various factors, including the level and types of protein, the activity of rumen microbes and their enzymes, rumen pH, feed characteristics, protein structure, rate of passage, and the synchronization of energy and protein sources. Maintaining an optimal balance of these factors is important to ensure efficient protein utilization and overall ruminant productivity. Farmers should ensure their cows get a diet with the right amount of protein and energy for optimum productivity.
References
- Bach, A., Calsamiglia, S., & Stern, M. D. (2005). Nitrogen metabolism in the rumen. Journal of Dairy Science, 88(S), E9–E21. https://doi.org/10.3168/jds.S0022-0302(05)73133-7
- Gadzama, I. U., & Madungwe, C. (2024). Accurate enteric methane quantification on beef cattle: INRA Tier 3 vs. IPCC Tier 2 models. Wikifarmer. Retrieved from https://www.researchgate.net/publication/387038162_Accurate_Enteric_Methane_Quantification_on_Beef_Cattle_INRA_Tier_3_vs_IPCC_Tier_2_Models
- Gadzama, I. U. (2024). Whole cottonseed: High-protein, high-energy feed for ruminants. Wikifarmer. Retrieved from https://www.researchgate.net/publication/384489092_Whole_cottonseed_High-protein_high-energy_feed_for_ruminants
- Gadzama, I. U., & Ray, S. (2024). Precision livestock farming in pasture-based dairy systems: Monitoring grazing behavior. Wikifarmer. Retrieved from https://www.researchgate.net/publication/385053381_Precision_Livestock_Farming_in_Pasture-Based_Dairy_Systems_Monitoring_Grazing_Behavior
- Gadzama, I. U., & Makombe, W. S. (2024). Brewer’s grain as a sustainable feed supplement: Reducing methane emissions in goats and cattle. Wikifarmer. Retrieved from https://www.researchgate.net/publication/386572675_Brewer’s_Grain_as_a_Sustainable_Feed_Supplement_Reducing_Methane_Emissions_in_Goats_and_Cattle
- Hackmann, T. J., & Firkins, J. L. (2015). Maximizing efficiency of rumen microbial protein production. Frontiers in Microbiology, 6. https://doi.org/10.3389/fmicb.2015.00465
- Kovarna, M., & Vanderhoff, S. (2023). Rumen Degradable Protein Versus Rumen Undegradable Protein. SDSU Extension. https://extension.sdstate.edu/rumen-degradable-protein-versus-rumen-undegradable-protein
- Lee, C., Hristov, A. N., Cassidy, T. W., Heyler, K. S., Lapierre, H., Varga, G. A., de Veth, M. J., Patton, R. A., & Parys, C. (2012). Rumen-protected lysine, methionine, and histidine increase milk protein yield in dairy cows fed a metabolizable protein-deficient diet. Journal of Dairy Science, 95(10), 6042–6056. https://doi.org/10.3168/jds.2012-5581
- Liu, C., Li, D., Chen, W., Li, Y., Wu, H., Meng, Q., & Zhou, Z. (2019). Estimating ruminal crude protein degradation from beef cattle feedstuff. Scientific Reports, 9(1). https://doi.org/10.1038/s41598-019-47768-3
- Manoukian, M., DelCurto, T., Kluth, J., Carlisle, T., Davis, N., Nack, M., Wyffels, S., Scheaffer, A., & Van Emon, M. (2021). Impacts of rumen degradable or undegradable protein supplementation with or without salt on nutrient digestion, and VFA concentrations. Animals, 11(11), 3011. https://doi.org/10.3390/ani11113011
- Nocek, J. E., & Russell, J. B. (1988). Protein and Energy as an Integrated System. Relationship of Ruminal Protein and Carbohydrate Availability to Microbial Synthesis and Milk Production. Journal of Dairy Science, 71(8), 2070–2107. https://doi.org/10.3168/jds.S0022-0302(88)79782-9
- Paz, H. A., Klopfenstein, T. J., Hostetler, D., Fernando, S. C., Castillo-Lopez, E., & Kononoff, P. J. (2014). Ruminal degradation and intestinal digestibility of protein and amino acids in high-protein feedstuffs commonly used in dairy diets. Journal of Dairy Science, 97(10), 6485–6498. https://doi.org/10.3168/jds.2014-8108
- Putri, E. M., Zain, M., Warly, L., & Hermon, H. (2021). Effects of rumen-degradable-to-undegradable protein ratio in ruminant diet on in vitro digestibility, rumen fermentation, and microbial protein synthesis. Veterinary World, 14(3), 640–648. https://doi.org/10.14202/vetworld.2021.640-648
- Schwab, C. G., Tylutki, T. P., Ordway, R. S., Sheaffer, C., & Stern, M. D. (2003). Characterization of proteins in feeds. Journal of Dairy Science, 86(suppl_1), E88-E103. https://scholars.unh.edu/cgi/viewcontent.cgi?referer=&httpsredir=1&article=1228&context=nhaes
- VandeHaar, M. J., & St-Pierre, N. (2006). Major advances in nutrition: Relevance to the sustainability of the dairy industry. Journal of Dairy Science, 89(4), 1280-1291. https://doi.org/10.3168/jds.S0022-0302(06)72196-8