Incorporating soybeans and their byproducts into dairy-cattle rations is a fairly common practice. Soybeans are an excellent source of essential amino acids. They complement most forages but do have some limitations. Depending on how they’re processed they can provide good-quality protein, energy, fat and fiber.

If they’ve been properly heat-treated soybeans can provide additional rumen-undegradable protein and fat. Soybeans that haven’t been heated provide a source of rumen-degradable protein.

At a time of declining soybean prices now is a great time to consider feeding homegrown soybeans. If roasting soybeans is economical in a dairy farmer’s area it can be cheaper to feed homegrown soybeans to partially replace a cow’s protein needs.

On a dry-matter basis heat-treated soybeans can range from 33 percent to 44 percent crude protein and 15 percent to 22 percent fat. Moisture content averages 12 percent. An average rumen-undegradable protein value as a percent of crude protein is 50 percent for properly heated soybeans.

The two most common methods of heat treatment are roasting and extrusion. Both have advantages and disadvantages.

Roasted soybeans are popular because they supply both rumen-undegradable protein and fat. They work well in most forage-type rations. They provide the greatest benefits as part of heavy hay-silage diets. They can be included in a ration by as much as 18 percent of total dry matter. But in many situations and when used with other concentrate ingredients, rumen-undegradable protein and-or fat will limit the amount of soybeans that can be fed. We most commonly see roasted-soybean inclusions of 3 pounds to 4 pounds in lactating-cow rations.

Drum roasters and high-temperature air dryers are the two most commonly used roasters. With drum roasters soybeans are placed into a rotating drum with air temperatures ranging from 400 degrees to 600 degrees Fahrenheit. The soybeans remain in the hot air for about a minute. If they remain longer than that they can become scorched. But the amount of damage incurred when soybeans are scorched is typically minimal.

In high-temperature air dryers soybeans are conveyed by a perforated floor through which hot air is blown. Scorching is less common with this process. It also may be more energy efficient. But equipment associated with such dryers is usually more expensive.

The main objective in the roasting process is to provide uniform heat among the soybeans and allow them to be steeped or held without cooling for an additional amount of time. Soybeans passed through a drum roaster can produce a consistent product. The most commonly used method is open-flame roasting where more variation occurs with respect to rumen-undegradable protein levels.

Factors that can affect rumen-undegradable protein levels with open-flame roasters are bean-moisture content, cleanliness and temperature. It’s not unusual to see rumen-undegradable protein range from 40 percent to 65 percent of crude protein. That might explain some of the variations in milk-production response in both controlled and field research studies.

Research has demonstrated how rumen-undegradable protein and lysine availability can vary with different heat treatments. It appears the optimal heat treatment for soybeans fed to lactating cows involves heating soybeans to 295 degrees Fahrenheit. The soybeans are then steeped without cooling for 30 more minutes. Steeping temperature will always be less than the temperature of the soybeans exiting the roaster. That’s because soybeans will lose moisture through evaporation. The temperature of the steeped soybeans will be 10 degrees to 20 degrees cooler depending on soybean moisture.

Monitor heat treatment

Improper roasting procedures can lead to variations in how cows respond when fed roasted soybeans. If soybeans are roasted with too little heat the amount of rumen-undegradable protein supplied in the ration could be greatly reduced. Using too much heat could result in a Maillard reaction. That makes protein unavailable in the small intestine. Improper roasting procedures also can diminish the amount of lysine available post-ruminally. Therefore it’s necessary to implement quality-control measures to ensure a good-quality product.

Forage laboratories offer tests to determine whether heat treatments have been adequate or excessive. A common procedure is the urease-activity test where results are expressed as an increased unit of pH. Values of 0.05 to 0.30 are reasonable evidence that soybeans have been properly cooked. If soybeans are used in a total-mixed ration or a high-moisture grain mix containing urea, a range of 0.05 to 0.10 is preferred.

The protein-dispersibility-index test also is popular. The solubility of a feed ingredient decreases as heat-exposure time and temperature increase. The procedure can determine how much heat soybeans were exposed to during roasting. The ideal protein-dispersibility value for optimally heated soybeans ranges from 9 to 11. Soybeans with a value greater than 14 are considered underheated.

A major disadvantage of the protein-dispersibility index is that its sensitivity decreases as optimum heat treatment is approached. For example as the index moves from 14 to 9, there may be an increase in rumen-undegradable protein and post-ruminal available lysine. Even a small change could affect the value of heated soybeans.

Another way to analyze heat treatment is the Ross rumen-undegradable protein analysis, which can be performed at various forage laboratories. It can help determine accurate percentages of digestible rumen-undegradable protein and total rumen-undegradable protein. It also can further identify whether under- or overheating occurred during roasting.

Research indicates that particle size influences the protein degradability characteristics of roasted soybeans. The particle size of roasted soybeans can affect how a cow utilizes protein. The primary concern is that small-particle protein is more likely to degrade faster in the rumen than large-particle protein.

Several studies indicate that to retain the rumen-undegradable protein value of feed, optimal particle sizes of roasted soybeans are whole-half and half-quarter. In total-mixed rations the whole-half particle size should result in little to no separation. In grain mixtures or supplements a half-quarter particle size may work better. If the goal of feeding roasted soybeans is to supply rumen-undegradable protein then grinding and pelleting aren’t recommended. Visit hubbardfeeds.com for more information.

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Jon Pretz is a dairy nutritionist for Hubbard Feeds.