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Extending the Grazing Season with Protein Supplementation

Consider grazing low-quality and low-cost forages this fall if limited protein can be supplemented

For much of the U.S., it was a long, hot, and dry summer. As a result, most pasture forages are low in quality, particularly protein. If you are lucky enough to have stockpiled forage, crop aftermath, or residual pasture – even if it is low in protein and appears to have little value – it should be considered for use. Dry, brown, and mature forages can actually be a tremendous low-cost resource to help extend the grazing season if protein can be strategically supplemented to cows.

In reality, the fall and early winter portions of the year are probably the most ideal times when protein supplements can be useful. Compared to the rest of the year, this is a cow’s time of lowest nutritional requirements since she is not lactating, not gaining weight, and pregnant with a very small fetus.

Comparing Requirements vs. Feedstuffs
The first step to strategically supplementing cows involves comparing a cow’s requirements with available feedstuffs. For example, according to the National Research Council (NRC), the minimum requirements for a 1,200 lb pregnant and dry beef cow in mid-gestation and not gaining weight are: 1.4 lbs of crude protein (CP) per day (or a diet consisting of 6.9% CP), and 10.1 lbs of total digestible nutrients (TDN) per day (or a diet consisting of 48.8% TDN). Further, the NRC estimates that this cow will consume 20.8 lbs of dry matter per day (this equates to 23.1 lbs/day on an as-fed basis if 90% dry matter hay is fed).

These requirements should then be compared to actual TDN and CP concentrations in forages being consumed, which should be analyzed regularly (see typical TDN and CP content of low-cost feedstuffs in Table 1).

Table 1. Typical total digestible nutrient (TDN) and crude protein (CP) content of selected grazed feedstuffs

Forage type

TDN (%)

CP (%)

Range (Fall)



Range (Winter)



Corn stalks/stover



Prairie hay



Cheat grass pasture (Fall)



Wheat stubble/straw



Barley stubble/straw



Based on average values reported by the NRC (1996), Western Beef Resource Committee’s Cattlemen’s Library, and University of Nevada Reno Extension.

Based on book values, half of the feedstuffs in Table 2 are inadequate in TDN and three-quarters are inadequate in CP. However, these feedstuffs are low cost and readily available. If there is a way for cows to be able to eat more of these low quality feedstuffs via strategic supplementation of protein, it is feasible to get enough energy from the feeds for a cow to maintain body weight and condition. Let’s consider a couple of aspects of ruminant physiology before deciding how to supplement low quality feedstuffs.

Rumen Microbes Need Nitrogen
Ruminants access protein and energy from forages via a relationship with billions of microbes that live in the rumen and convert forages into energy and protein. Microbes break down protein, amino acids, and nitrogen from forage particles and consume the nitrogen in order to build their own bodies. Microbes have short life-spans, and once they die and leave the rumen they are digested by the remaining compartments of the ruminant stomach. Ultimately, each microbe’s “body” provides a major source of protein for the animal.

To function optimally and “unlock” energy from forages, microbes require an adequate amount of nitrogen to be present in the rumen. When nitrogen is inadequate, microbes function at a depressed level and cause reductions in forage digestion, energy release, microbial protein synthesis, and feed intake by the animal. This causes the ruminant to survive on a reduced plane of nutrition – which can often lead to reduced body condition score and/or reproductive performance.

In this situation, intake can be as low as 1.5 to 1.75% of body weight due to less-than-ideal microbial activity, which is dramatically lower than the typical intakes of 2 to 3% of body weight seen with higher quality feeds. In essence, a cow consuming low quality forage without enough protein is unable to physically consume enough low quality forage to meet her body’s energy needs.

Protein Supplementation and Intake
Before deciding what to supplement, it is important to identify the first limiting nutrient. For most of the feedstuffs listed above, it is protein. When protein is supplemented to cows consuming only 1.5 to 1.75% of their body weight, microbial activity will increase and lead to: 1) more energy release, 2) more forage intake, and 3) enhanced animal performance. Therefore, when both protein and energy are inadequate, supplementation of just protein may actually solve both problems since enhanced microbial activity encourages more intake and more release of energy.

New Mexico State University researcher Clay Mathis provides this example to show the benefit of protein (nitrogen) supplementation on intake:

CP in total diet

Intake (% of body weight)

Cow body weight

Intake/day (DM basis)

No suppl.



1,200 lbs

19.2 lbs/day

With suppl.



1,200 lbs

27.6 lbs/day





+8.4 lbs/day

Adapted from Mathis (1999).

When total CP concentration of the diet increased from 5 to 7% via protein supplementation, microbial activity allowed for 8.4 lbs/day of additional intake (or a 44% increase!). This increased intake can boost total energy intake by 15% or more as a result of first correcting the protein deficiency, which can often also correct a TDN deficiency. Of cow/calf operations that extend their grazing season into fall and winter, protein is often the first supplementation priority, especially when CP is less than about 7%. However, it should be noted that this effect will only occur if there is enough total forage available in the pasture to allow for increased intake.

To achieve this effect, only a small amount (1 to 2 lbs) of a high CP supplement (at least 30% CP) needs to be fed. This would result in a cow consuming 0.3 to 0.6 lbs more CP per day from the supplement. Thus, for a 1,200 lb cow consuming 20.8 lbs of a 5% CP diet (20.8 x 0.05 = 1.04 lbs/day), this would result in a total daily CP intake of 1.34 to 1.64 lbs CP/day (1.04 lbs/day + 0.3 to 0.6 lbs/day). In addition to meeting the cow’s CP requirement of 1.4 lbs CP/day (based on NRC data above), the energy value of low quality feedstuffs can be increased by approximately 15% as well.

The Bottom Line
In some scenarios when protein and energy are inadequate and intake is reduced (particularly in fall and early winter when grazing can be extended), supplementing protein first can improve microbial activity, energy release, and feed intake if there is enough forage supply available. This can eventually improve the protein/energy status of a cow by overcoming both protein and energy inadequacies.

However, when forage is in short supply but protein concentration is adequate (as in a drought), producers may need to supplement energy. In cases where both protein and energy are lacking, and no amount of protein can overcome the energy deficit, both protein and energy may need to be supplemented.

Remember that even though forages may be very low in quality, rumen microbes still have an amazing ability to convert them into energy. However, it is vital that they have enough protein (nitrogen) to do their job.