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Warm Season Summer Cover Crops can Produce Good Forage During the Summer Slump

Summer cover crops can be an important tool in a grower’s toolkit for protecting soil during the warm summer months following an early summer harvested crop, or in fields where summer crops didn’t get planted in the spring. Summer cover crops, if managed properly, can increase infiltration of heavy summer rains, protect soil from runoff, contribute to soil organic matter, and importantly, compete against weeds and produce valuable forage, especially during the late “summer slump” period where forage production falls off in late July-September. “Warm-season” crop species are favored for summer cover crops due to their drought- and heat-hardiness. Sorghum-sudangrass, a cross between the high biomass-producing crop sorghum, and sudangrass, a drought-hardy hybrid grass with high forage quality and regrowth, is already commonly grown in the Upper Midwest. However, sorghum-sudangrass requires high nitrogen for growth and biomass production, depleting soils of nitrogen and necessitating extra fertilizer or manure applications. This high-N-use characteristic can create problems for organic growers, especially those trying to rotate to corn. Furthermore, inserting sorghum-sudangrass into typical corn-soybean or corn-soybean-small grain rotations in the Midwest does little to add diversity to the agricultural landscape and provide additional ecosystem services. To meet nitrogen input, biodiversity, and rotation diversification goals, legumes and mixes need to be identified that meet the goals of weed suppression, ecosystem services provisioning, and high-quality forage production, all while keeping costs low.

This summer, MFAI researchers evaluated four warm-season cover crops for their abilities to contribute to these goals. They compared monocultures of two grasses, sorghum-sudangrass and teff (a warm-season, fine-stemmed grass from Ethiopia); and two legumes, sunn hemp and cowpea; as well as bicultures of grasses and legumes, and a four-way mix diverse polyculture mix of all four species. All monocultures and mixes were cut at 60 days after planting to determine forage yields from a first cutting.

Sorghum-sudangrass monoculture and the 4-way diverse polyculture mix suppressed weeds the most, followed by sunn hemp bicultures with both sorghum-sudangrass and teff grass, which had approximately 10% greater weed pressure than the most suppressive treatments. Sorghum-sudangrass mixed with sunn hemp produced very similar forage yields at the first cutting (3.5 to 4 tons dry matter per acre) as the sorghum-sudangrass monoculture, with the sunn hemp reaching 5+ feet tall. The sorghum-sudangrass benefited from the nitrogen fixed by the leguminous sunn hemp, as sorghum-sudangrass in the mix was visually greener than when in monoculture. In terms of seed costs, adding sunn hemp into a sorghum-sudangrass stand cost an extra $15 per acre, although the cost per ton forage for both cover crop treatments was similar. Soil sampling in the fall will reveal if any nitrogen benefit could be observed from having sunn hemp in the mix, and will represent an additional benefit from investing in the sunn hemp seed earlier in the year. Further, forage quality tests (pending) will determine if having sunn hemp in the mix increased the feed value of the forage, representing extra value compared to sorghum-sudangrass monoculture.

We will continue to collect data throughout the season to monitor regrowth and the potential for meaningful regrowth to allow a second cutting of summer cover crops in the fall, as well as soil fertility and health benefits a grower could enjoy from investing in summer cover crops rather than summer fallow. Stay tuned!