Interseeding alfalfa (Medicago sativa L.) into maize (Zea mays L.) silage is a forage production system in which corn and alfalfa are planted together to simultaneously produce a corn silage crop and establish alfalfa for subsequent forage production. The interseeding system has been shown to increase overall forage production compared with a conventional corn silage-alfalfa rotation, and a farm budget analysis suggested interseeding could increase farm profitability.
While economic factors often drive farmer practices, innovations in agricultural systems are needed to reduce negative environmental impacts of agriculture. Nitrate pollution of groundwater from intensive dairy production is a major environmental concern in many humid agricultural regions. Corn silage production often involves high rates of nitrogen (N) fertilizer application and a fallow period of 6+ mo after harvest, thereby creating an extended window of high nitrate leaching potential.
Interseeded cover crops can reduce soil nitrate pools and leaching potential by increasing living cover and uptake of available soil N, but the soil nitrate benefits of interseeded alfalfa have not been quantified.
Effects of interseeded alfalfa on soil nitrate during the corn growing season were limited, as the amount of available soil N was not clearly larger in the corn monoculture than the interseeded corn/alfalfa system at a given N rate.
This result was surprising because interseeding of alfalfa slightly decreased the dry matter yield and N uptake of corn and simultaneously increased the N rate at which corn silage yield was maximized, suggesting competition for N between alfalfa and corn in the interseeded system.
A possible explanation was that light or chemical cues from alfalfa altered the growth of corn in a manner analogous to that observed with weeds, thus reducing corn N uptake and offsetting alfalfa N uptake. Additional research toward alleviating yield loss of corn interseeded with alfalfa is needed.
Interseeded alfalfa reduced overwinter and spring NO3– leaching risk, illustrated by the deep soil samples. A large reduction in soil NO3– occurred between the late fall and spring in the typical rotation, with the highest N rate at the AARS location, most likely due to loss via leaching below the sampling depth or denitrification. The large amount of fall NO3– in this treatment indicates excessive N fertilization, which is supported by a previous analysis that showed corn yield was maximized at ∼116 kg N ha–1.
Importantly, the 224N rate applied to interseeded corn/alfalfa did not result in similar fall soil NO3–, likely due to a combination of greater N demand during corn production (corn yield was maximized at 199 kg N ha–1) and alfalfa N uptake following silage harvest. These results largely support earlier findings that showed reductions in soil NO3– over winter and early spring from corn/interseeded legume systems.
While the size of the soil nitrate pool can indicate nitrate leaching risk, direct quantification of nitrate leaching requires measurement of water movement through the soil, which were unable to in this study; such direct quantification of nitrate leaching will be important to further advance understanding of interseeding on leaching dynamics. Additionally, while the interseeding effects were relatively consistent in this study, the results need to be validated under different soil types and weather conditions.
The stratification of NO3– under interseeded alfalfa in the spring suggests that significant soil N uptake and redistribution occurred. Nitrogen in alfalfa herbage may have been deposited at the soil surface following winter senescence. Such upward nutrient movement by plants, termed “nutrient pumping,” has been observed for many cations and phosphorus (P) in both natural and agricultural systems.
The consequence of this nutrient pumping effect would be to counterbalance the downward movement of soil N, thus reducing NO3– leaching potential. In contrast, the typical rotation treatments that had been fallow since corn silage harvest showed evidence of downward movement of NO3– in the spring sampling.
Overall, results show a reduction in pool size and downward movement of soil NO3– that indicates an environmental benefit of interseeded alfalfa. Additionally, living groundcover provided by interseeded alfalfa during and after corn production reduced runoff losses of soil by 49 to 87%, N by 37 to 74%, and P by 37 to 81% relative to a conventional corn silage–alfalfa rotation.
As a perennial crop, alfalfa provides important environmental benefits for multiple years, including reduced NO3– leaching and soil erosion and improved soil quality relative to annual row crops such as corn. However, forage production has shifted from alfalfa to corn silage in recent decades, likely reducing this benefit. The yield and economic advantages of interseeding may encourage greater production of alfalfa, helping to reverse this trend and increase alfalfa acreage, thereby potentially improving the environmental impacts of forage production.
Source: Osterholz, W., Ruark, M. D., Renz, M., & Grabber, J. H. (2021). Benefits of alfalfa interseeding include reduced residual soil nitrate following corn production. Agricultural & Environmental Letters, 6(3), e20053.