Roasted soybeans are a very popular way of feeding soybeans, supplying both RUP and fat. They work well in most forage type rations with the greatest benefits being observed in heavy haycrop silage diets. They can be included in the ration up to 18% of the total ration dry matter. However, in many situations, when used with other concentrate ingredients, RUP and/or fat will limit the amount of beans that can be fed.
There are two main types of roasters used in the field - a drum roaster and high temperature air dryers where soybeans are conveyed over a perforated floor through which hot air is blown. With drum roasters soybeans are dropped into a rotating drum where air temperatures may range from 400°F to 600°F. Soybeans will remain about one minute in the hot air environment before exiting. If beans remain in the roaster longer than one minute, they can get scorched. The amount of damage to scorched beans typically is minimal.
Equipment that conveys soybeans across a perforated floor through which hot air is blown causes less scorching and may be more energy efficient than the drum roaster. This type of equipment usually is more expensive.
The main objective in the roasting process is to achieve even heating and allowing the beans to be steeped or held without cooling for additional time. Soybeans passed through a drum roaster can produce a fairly consistent product. The most commonly used method is open-flame roasting. This is where more variation occurs with respect to RUP levels.
Factors affecting RUP levels when using open-flame roasters are moisture content of the beans, how clean the beans are, and the environmental temperature. It is not unusual to see RUP range from 40 to 65% of the crude protein. This may explain some of the variable results observed in milk production response in both controlled research and field trials.
Researchers at Wisconsin have demonstrated the differences that occur in RUP and lysine availability when various heat-treatments are used (Table 2). It appears that optimum heat treatment for soybeans intended for lactating dairy cattle is to heat soybeans to 295°F and then steep them without cooling for an additional 30 minutes. The steeping temperature will always be less than the temperature of the soybeans exiting the roaster because the soybeans will be losing moisture by evaporation. This will cause the temperature of steeped beans to be 10 to 20 degrees cooler, depending on the moisture content of the beans.