Cover crops are planted to increase health and fertility of soils and to benefit the surrounding environment. In systems producing either beef or dairy cattle, cover crops are also grazed or harvested for feeding.

In dairy farming systems, the use of annual crops for forage is typically oriented to winter annual grasses, although interest in using more diverse mixtures has increased over recent years. As part of the Conservation Innovation Grant program, and in collaboration with the USDA – Natural Resources Conservation Service at Virginia Tech, we evaluated the yield and the nutritional quality of diverse mixtures of winter crops for grazing or silage use.

The project was performed at three experiment research stations from Virginia Tech, which were located in Blacksburg, Blackstone and Orange, Virginia.

For this project, the grasses barley, rye, ryegrass, triticale and wheat were planted in monoculture, which means they were planted alone without any other species. In addition to this, the same five grasses were planted in combination with one of two legume species, namely crimson clover or hairy vetch.

These cover crops were planted during the fall of 2014 while the forage biomass was harvested during spring of 2015. After harvesting, one fraction of the samples was processed to determine the biomass yield and the nutritional composition of the harvested forage. All chemical analyses were performed in the Dairy Nutrition Laboratory within the department of dairy science at Virginia Tech.

The forage dry matter yield tended to increase when grasses were grown in mixture with crimson clover. Dry matter yields were 1.1 and 1.3 tons per acre for grasses in monoculture and grasses combined with crimson clover, respectively.

Different to crimson clover, planting grasses in mixtures with hairy vetch did not affect dry matter yield of the forage, which was on average 1.1 tons per acre.

Even though the effects of mixing grasses with legumes on forage yield were not substantial, several changes were observed in the nutritional quality of the forages. As would be expected, mixing grasses with legumes increased the concentration of crude protein of the harvested forage.

On average across all sites, the concentration of crude protein was 13 percent for grasses in monoculture, 15.5 percent for mixtures including crimson clover, and 17.3 percent for mixtures including hairy vetch.

The implications of these changes on crude protein concentrations depend on the system of production. If beef cattle were to graze winter crops, then a 4 percent unit increase in crude protein concentration can be considered substantial.

However, for dairy cattle eating TMRs with high inclusions of concentrates, the increase in crude protein may have a smaller impact, although it could still help decrease some of the feeding costs attributed to protein concentrates.

Keep in mind, protein concentrates are typically one of the most expensive components of diets for dairy cows. From an environmental perspective, similar or greater yields of forage with greater concentrations of crude protein indicate that cropping grasses with legumes increased the extraction or retention of nitrogen from the soil and atmosphere by more than 30 percent.

In regards to carbohydrates, the concentration of neutral detergent fiber was 49.7 percent for grasses grown in monoculture and 46.5 percent for grasses grown in mixtures with crimson clover or hairy vetch. Typically, grasses contain greater concentrations of fiber than legumes, and this was the case for our study.

These data indicate that, although marginally, growing grasses in mixtures with legumes may increase the energy concentration of cover crops for forage.

Also related to carbohydrates, the concentration of sugars decreased when grasses were grown in mixtures with hairy vetch. Specifically, grasses in monoculture contained 14.3 percent sugars, while grasses grown in mixture with hairy vetch contained 10.5 percent sugars.

When grasses were grown in combination with crimson clover, the concentration of sugars was 13.2 percent, which was closer to the concentration of sugars contained in grasses grown in monoculture. A lower concentration of sugars does not necessarily mean the nutritional quality decreased.

When talking about percentages, something goes up when something else goes down. In this sense, the increased concentration of protein was translated in less fiber and less sugar concentrations.

Finally, we also analyzed the in vitro dry matter and fiber digestibility of the forages. The in vitro digestibility of dry matter was lowest for barley and greatest for ryegrass (83.9 and 90.6 percent, respectively). Similarly, the in vitro digestibility of fiber was also lowest for barley and greatest for ryegrass (72.2 and 78.1 percent, respectively).

These observations put ryegrass in the top quality among grasses grown alone. In regards to the mixtures, the addition of legumes to the winter crop tended to slightly decrease the in vitro fiber digestibility (70.9 and 73.1 percent for barley and ryegrass, respectively).

A 1.1 percent unit decrease in fiber digestibility has marginal nutritional implications. However, a 5 percent unit decrease in fiber digestibility may have a greater impact depending on the diet consumed.

The marginal impact of growing grasses in mixture with legumes on digestibility makes sense. In general terms, fiber digestibility of legumes is typically lower than fiber digestibility of grasses. In addition, fiber concentration is typically lower in legumes than in grasses.

In this study, even though the digestibility of the fiber was reduced with the addition of legumes, the concentration of highly digestible non-fibrous components (i.e., cell contents) was increased in barley, rye, triticale and wheat. Because the non-fibrous components of these forages are completely and uniformly digestible, the nutritional composition was actually enhanced by adding legumes.

In conclusion, growing grasses in mixtures with legumes increased the concentration of crude protein and decreased the concentrations of fiber and sugars in the resultant forage. As crude protein is an expensive component of diets, adding legumes to mixtures could lower feeding costs in dairy farming systems.

Also, higher concentrations of crude protein indicate that grasses in mixture with legumes increase the extraction of nitrogen from the soil and atmosphere, adding environmental sustainability to the system. Finally, as fiber has lower digestion rates than non-fiber components, growing grasses in mixture with legumes may result in higher concentrations of energy in the forage.  

PHOTO: When grasses and legumes, such as this wheat and crimson clover, are grown together, forage dry matter yield and crude protein concentration increased. Photo by David W. McIntosh, University of Tennessee.

Gonzalo Ferreira is with the Department of Dairy Science at Virginia Tech. Email Gonzalo Ferreira.