RDP & RUP Balance & Feed the Microbes and the Milk

Rumen-degradable protein (RDP) is the share of dietary crude protein that the rumen microbes break down and use, together with fermentable energy, to grow and make microbial protein. Rumen-undegradable protein (RUP), or bypass protein, escapes the rumen intact and is digested in the small intestine, adding directly to the cow's metabolizable protein. A ration needs both in balance: enough RDP to grow microbes, enough RUP to top up the protein reaching the cow.

Crude protein is just total nitrogen × 6.25 — it says nothing about where that protein ends up. Two rations with the same crude protein can differ wildly: one may flood the rumen with RDP it can't use (wasted as ammonia, urea and high MUN) while still short of bypass protein for milk. The RDP/RUP split is what actually limits milk and decides how much nitrogen you waste, which is exactly what this tool exposes.

The RDP requirement is set by how much microbial protein the diet's energy can support. The tool uses the NRC 2001 relationship: microbial crude protein (MCP) ≈ 0.13 g per g of TDN (130 g MCP per kg TDN), and the RDP needed ≈ MCP ÷ 0.90. So a ration supplying 10 kg of TDN supports about 1,300 g of MCP and needs roughly 1,444 g of RDP. RDP beyond that is fermented to ammonia and lost.

Metabolizable protein (MP) = MP from microbes + MP from RUP. Microbial MP = realised MCP × 0.64 (true-protein fraction) × 0.80 (intestinal digestibility); RUP MP = RUP × 0.80. The microbial part is capped by the RDP actually supplied, because you can't grow more microbes than the RDP allows. The tool sums both flows and compares them to the cow's MP requirement.

MP requirement = maintenance + milk. Maintenance MP ≈ 3.8 × body weight^0.75 (NRC 2001 scurf, endogenous and metabolic faecal protein); milk MP = milk kg × milk-protein % × 10 ÷ 0.67, where 0.67 is the efficiency of converting MP into milk protein. For a 650 kg cow giving 35 kg of 3.1% protein milk, that is roughly 490 g maintenance plus 1,620 g for milk.

The limiting fraction is whichever of RDP-for-microbes or metabolizable protein is short, because that is what is actually capping performance. If RDP is the limit, the microbes can't grow to their energy potential, so even adding bypass protein won't help until you feed the rumen. If MP is the limit, you need more bypass protein. If both are in band, the ration is balanced and you should hold it.

Excess RDP beyond what the microbes can use is fermented to ammonia, absorbed, converted to urea in the liver and excreted — showing up as high milk urea nitrogen (MUN). It is wasted nitrogen, wasted feed money and an extra metabolic load on the cow. The fix is to cut the most rapidly degraded protein (urea, high-RDP legumes) and, if milk needs more protein, shift toward a bypass source instead.

Bypass-rich feeds include treated (heat- or chemically-protected) soybean meal, blood meal, fish meal, corn gluten meal and distillers grains — their RDP fraction is low, so most of their protein survives the rumen. By contrast urea, fresh legumes and grass silage are almost entirely RDP. The feed table on this page lists the RDP fraction of each feed so you can pick the right lever to rebalance.

Add a quickly-degraded protein or more fermentable forage so the rumen microbes have the nitrogen they need to grow. A small amount of urea, soybean meal or canola meal, or a higher-quality forage, raises RDP and lifts microbial protein. Because microbial protein is high-quality and cheap to make, fixing an RDP deficit is often the most cost-effective way to raise the metabolizable protein reaching the cow.

When MP is short but RDP is adequate, add a high-RUP (bypass) source — treated soybean meal, blood meal, fish meal, corn gluten meal or distillers grains — so undegraded protein reaches the intestine without overloading rumen ammonia. Adding more RDP at that point would only raise MUN, not milk; the gap is in bypass protein, so that is what to feed.

The RDP/RUP/MP framework applies to all ruminants — dairy and beef cattle, buffalo, sheep and goats — because they all rely on rumen microbes. The feed fractions are the same; only the body weight, milk yield and milk protein you enter change. Enter the animal's weight and production and the tool computes the same balance. For non-lactating animals, set milk yield to zero to size maintenance alone.

Microbial crude protein is the protein in the rumen microbes themselves, grown from RDP and fermentable energy. It is the cheapest, highest-quality protein a ruminant gets — well-balanced in amino acids — and on many rations it supplies more than half the cow's metabolizable protein. Maximising MCP by matching RDP to the diet's energy is the foundation of efficient ruminant protein feeding, which is why the tool computes it explicitly.

No — it is a solid planning estimate that exposes the RDP/RUP balance crude protein hides, using the NRC 2001 system and representative feed fractions. Actual degradability varies with processing, passage rate and forage quality, so confirm a marginal ration with a laboratory feed analysis and a full ration-evaluation model before making large feed changes. Use this tool to spot the imbalance and the likely fix quickly.