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Protein requirements of growing calves explained

by Wyoming Livestock Roundup

In a recent BeefWatch webinar, hosted by the University of Nebraska-Lincoln (UNL) Extension, Dr. Mary Drewnoski discusses the importance of understanding the protein needs of growing calves. Drewnoski, a beef systems specialist with UNL Extension, shares protein, as it relates to growing calves, depends greatly on the goals of the producer and provides information for producers to make feed decisions in order to meet nutritional needs of their calves. 

In an example, Drewnoski explains a study in which 500-pound steer calves were fed three different supplements – corn, corn plus five percent urea and dried distillers’ grain solubles (DDGS). Total digestible nutrients (TDN) were similar for all three supplements, and crude protein (CP) was similar for the corn plus urea supplement and the DDGS, while the corn supplement alone contained about one-third less CP than the other two. 

Study results show calves consuming the corn supplement gained approximately 0.31 pounds per day, calves on the corn plus urea supplement gained around 0.53 pounds per day, and calves receiving DDGS as a supplement gained roughly 1.32 pounds per day. 

The big question, Drewnoski asks, is what makes DDGS a better supplement than corn, or corn plus urea, for these calves? 

Ruminally degradable protein

Drewnoski starts the discussion with background information regarding ruminant nutrition, including information on ruminally degradable protein (RDP), ruminally undegradable protein (RUP) and how protein consumed by calves becomes available for digestion either by rumen microbes or the animal itself. 

“RDP can be degraded and digested in the rumen, and becomes available for digestion by rumen microbes,” shares Drewnoski. “Those microbes will use the protein to grow and multiply. Then those microbes that grow and multiply eventually get washed out of the rumen to become a source of protein to the animal.” 

However, Drewnoski explains, this is only the case if there is enough energy available in the system for microbes to utilize RDP. 

“If there isn’t enough energy in the system, the microbes will use the carbon from the RDP as energy and release the nitrogen as ammonia,” she explains. “In the case of RDP, we only get as much microbial CP as energy will allow.” 

Adding a high-energy supplement, such as corn, into an energy deficient system has the ability to increase the utilization of RDP so the corn and released ammonia may work together to multiply microbes, according to Drewnoski. 

“It is all about the microbes when we think about RDP,” notes Drewnoski. “The protein source to the animal is solely microbes.” 

Ruminally undegradable protein

On the other hand, RUP is a protein source which provides protein directly to the animal, bypassing use by rumen microbes. 

“RUP washes through the rumen and becomes a source of amino acids or protein for the animal itself,” says Drewnoski. “If the animal can utilize the protein, it will be used to grow and make muscle, immune cells, enzymes and anything else the proteins would be needed for.”  

However, Drewnoski notes, overfeeding RUP relative to the amount of available energy, can lead to the animal starving for energy, much like the microbes starved for energy by RDP. 

“The animals will break the protein down and use the carbon as an energy source to grow and make more muscle, or use it as energy to maintain their body or store fat,” she explains. “They also, of course, break off the nitrogen and release it as ammonia, which their body then has to detoxify.” 

Drewnoski explains the high RUP content in DDGS is what helped calves in the study gain better than calves that only received corn or corn plus urea. 

“RUP is a protein source the animal can use. It doesn’t go through the microbes and become a secondary protein source,” she shares. “If we overfeed RUP, it can become energy. Part of the reason why DDGS is a high-energy feed is because the bypassed protein is high in energy, in terms of availability.” 

Protein availability

Protein available to the ruminant animal is microbial protein, which comes as microbes washed out of the rumen and RUP, says Drewnoski. With the exception of urea, we can typically expect any feedstuff to have both RDP and RUP. 

“The thing to understand is microbial CP production is dependent on the balance and amount of energy and protein available in the rumen,” she explains. 

For example, by feeding a dairy-quality alfalfa hay to calves, there might not be enough energy supplied to the calves to make use of the protein available. Adding corn to the ration will help balance the energy available in the rumen and make the protein from the alfalfa available, which in turn increases gain on the calves. 

“A majority of the protein in alfalfa hay is RDP – around 70 to 90 percent – so it will really feed the microbes, but there is not much there to bypass the rumen and feed the animal without utilizing those microbes as the protein source,” says Drewnoski. 

“In forage-based diets it is really hard to get enough microbial crude protein to sustain high rates of gain, especially for light weight calves,” she continues. “The smaller the calf, the higher the relative protein requirement.” 

She adds a 400-pound calf will have a much higher protein requirement than a 700-pound calf. Often, the balance of energy to protein as calves grow can translate into getting away with feeding less protein as calves get bigger. 

Drewnoski recommends feeding to meet the protein requirements of lighter weight calves to avoid making smaller calves smaller and bigger calves bigger. She notes this strategy might not improve performance of larger calves, but it makes a large difference when it comes to smaller calves. 

Forage and cost analysis

Increasing the gain of smaller calves could be accomplished by adding as little as one pound of alfalfa a day, says Drewnoski. However, she notes the nutritional value of alfalfa hay is variable. 

“If producers are utilizing forage or hay as the base of the diet, I recommend testing it to know the protein content,” she notes. “Forage sources can be extremely important in growing calves.” 

Drewnoski notes producers often assume alfalfa is less expensive than DDGS. 

“In a ration for a 500-pound calf with a corn silage base, alfalfa can be supplemented at around 47 cents per head per day, feeding 5.4 pounds per day at $150 per ton,” she explains. “DDGS can be supplemented for around 34 cents per head per day, at 2.8 pounds per head per day at $220 per ton.” 

“Looking at the cost per pound of CP, alfalfa comes in at 39 cents per pound and DDGS at 41 cents per pound,” she continues. “But, on a per head per day comparison, DDGS is more cost effective.” 

Drewnoski says figuring growing calf rations is one of the best times for producers to get in touch with their local beef educators and specialists. She recommends knowing the feed options available for use, having a forage analysis if using forage as a supplement of diet base and knowing what the goals for the herd are before contacting a specialist so they can work to come up with the best option for each situation. 

Averi Hales is the editor of the Wyoming Livestock Roundup. Send comments on this article to

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