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Feed the cow and her microbesFeed the cow and her microbes

SDSU animal scientist drills down into details of supplementing protein for cattle eating low-quality forages.

September 9, 2016

4 Min Read

Protein tends to be a buzzword in human diets, particularly among those who are limiting carbs and wanting a nutritional boost.

But protein sources should also be top-of-mind among producers as a supplement to their cattle when animals are grazing low-quality forages such as dormant grass or crop aftermath.

Without protein to help stimulate rumen microbes for fiber digestion, the process can slow down — and a cow’s intake can decrease threefold, says Ken Olson, South Dakota State University Extension beef specialist.

Ruminant basics
Ruminants are able to digest forage fiber because of their four-compartment stomach, and specifically because of the microbes in the reticulum and rumen.


Olson explains that the microbes provide enzymes that allow for fermentation of fiber, which is broken down into energy. “The energy feeds the microbes to continue supporting their digestive process, and the volatile fatty acids created from rumen fermentation are absorbed and used as an energy source by the animal,” Olson says. “Producers should remember they are feeding for [these] two — the cow and the microbes.”

Picture the fermentation vat (rumen) like a water tank, he says. “Cows have receptors to stop feed or forage intake when the ‘tank’ is full. They then need to digest the feed in the tank through chewing of the cud to get particles smaller and microbial action. The only way feed leaves the rumen is if it fits through a small hole into the omasum. When the rumen empties, the cow will eat again.”

However, when the forage being consumed is high in fiber and low in protein from low-quality forages, it often slows this digestion process down.

“There are two bad things going on with low-quality forages — the cow eats a lot fewer pounds, and there are fewer nutrients in what she eats," he says.

This is where a supplement may be beneficial to the cow to help overcome those two limitations.

Protein vs. energy
“Ruminants can digest the fiber as an energy source,” Olson says. “There’s energy out there. What’s needed as a supplement on low-quality forage is protein.”

Protein supplements provide nitrogen for rumen microbe growth. This, in turn, promotes improved fiber digestion, which increases the rates of digestion and passage through the four-compartment stomach. When this digestion process is working efficiently, it ultimately supports increased intake of low-quality forage by the animal.

Protein supplementation can increase the intake and nutritional value of low-quality forages, whereas grain-based energy supplements (like corn) that are high in starch and low in protein have a negative effect on forage intake and digestibility. And, high-starch energy feed supplements can shift the fiber-fermenting bacteria in the rumen to starch-fermenting bacteria — exacerbating the decrease in microbes capable of digesting fiber.

“Microbial growth is not stimulated when starch is fed to cattle on low-quality forages, and there is a negative associative effect on digestion. Forage intake is actually decreased, which also limits energy intake,” he says.

Research shows forage utilization by cattle has almost universally shown negative effects from low-protein supplements and positive effects from high-protein supplements.

Supplement strategies
In determining when to supplement protein, Olson suggests forages offering less than 7% crude protein should be the threshold. Previous research suggests 7% crude protein is the minimum in cattle diets to maintain rumen microbial function so forage fiber can be digested.

Test forages for nutritional content so producers know what nutrients are available to their cows — and what supplements may be needed. Not all dormant forage or crop aftermath is the same, and some will have higher crude protein content.

Protein supplements to consider, according to Olson, might include soybean and cottonseed meal, which have crude protein contents of 49% and 46% respectively. Corn is only 9% crude protein. Distillers grain, wheat middlings and corn gluten feed are also good protein supplement options. They have slightly lower crude protein content and can cost less

Feedstuffs should be compared on an equal crude protein basis, which means adjusting for differences in feed price, crude protein content and dry-matter content. Cost per unit of crude protein can be calculated by dividing the price of the feedstuff by the dry matter and crude protein contents in decimal form (e.g., soybean meal CP $/ton = $500 ÷ 0.89 ÷ 0.49 = $114.

Be sure to consider the cost of delivery. Trucking dried distillers grain is typically less expensive than trucking wet distillers grain. However, “if you live within about 60 miles of an ethanol plant, wet may pencil out,” he says.

Supplementing thin cows?
In situations where supplemental energy may be needed, such as for thin cows that need to gain weight or young cows that are still growing, Olson advises feeding a fiber-based energy supplement instead of a starch-based supplement such as grain. Soybean hulls, sugarbeet pulp or wheat middlings are high in digestible fiber and have little to no starch.

Fiber-based energy supplements will not have a negative effect on microbes in the rumen and will not create competition between starch and fiber for preferential digestion.

Gordon writes from Whitewood, S.D.

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