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Cover Crop Seeded into Sunflower (06/20/19)

Soil erosion is generally observed more with row crops than with close-growing small grains.

Soil erosion is generally observed more with row crops than with close-growing small grains. Sunflower stubble, if cut below one foot, is not very effective as a snow trap due to its limited surface cover. There has recently been an increased awareness among agronomists and producers of the importance of protecting soil from wind and water erosion. Cover crops have become increasingly important in farming systems and can boost soil fertility and reduce soil erosion. Top soil preservation and the maintenance or building of organic content in the soil are important management strategies to increase soil health. Some potential benefits of interseeding a cover crop mixture, including a legume, in sunflower, are dinitrogen fixation, soil erosion control, improved snow trapping, improvement of the soil structure and organic matter content, and fodder or green manure production the year after the legume establishment.

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Clearly, from the producer's perspective, the primary crop component (sunflower) should yield near its potential, when planted at its optimum plant density. It is important to give the primary crop (sunflower) a head start and plant the cover crop from the V4 to V8 growth stages. Planting too late in the season, from V8 to bloom, will not provide enough light for the cover crop establishment. A secondary planting window is near the end of the season, when the sunflower is starting to lose its lower leaves (approximately starting at R6 and beyond). In the fall, cover crop selection is limited to cool season or winter hardy plant species such as winter rye. In North Dakota, the rate of sunflower development is mainly influenced by temperature. Therefore, cumulative growing degree days are a valuable means to estimate growth stages (for instance the appropriate V-stages for interseeding).

The primary crop will receive full sunlight at the top of the canopy. However, the secondary crop will have long periods of dim light with short exposure to near full sunlight caused by holes in the canopy when upper leaves move. Cover crop leaves below the sunflower canopy will be exposed to differing levels of solar radiation, due to the change in sun's angle during the day and over the season affecting growth. Shading by sunflower influences the photosynthesis and dinitrogen fixing ability of the interseeded legume. For most cover crops, shaded conditions reduce growth.

In a study at two locations for two seasons, four legume cover crop species were interseeded into two sunflower hybrids. The legumes and seeding rate were: hairy vetch at 29 lb/acre, sweetclover at 9.5 lb/acre, alfalfa at 16 lb/acre and black lentil at 22 lb/acre. The cover crops were seeded the same day as the sunflower and at the V4 and V10 growth stages. Seeding the cover crop at the same time of sunflower resulted in a yield reduction of the sunflower compared to the control (without cover crop) and is not recommended. Sunflower yield with cover crops interseeded at the V4 or V8 was not reduced. In late October, cover crop biomass samples were taken and oven dried (Figure 1). On average, the biomass produced when the cover was seeded at the V4 sunflower growth stage was double compared with seeding at the V10 growth stage.

Hairy vetch had significantly more biomass compared with sweetclover, alfalfa, or black lentil when the cover crop was seeded at the V4 growth stage. However, hairy vetch is difficult to chemically kill and can become a weed in the following season.

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Lessons learned:

Black lentil can be seeded at the same time as sunflower or slightly later. Other legumes needed to be seeded after the V4 of sunflower in order not to negatively influence the sunflower yield.

Delaying planting of the cover crop beyond the V4 will reduce the amount of cover crop biomass produced. Use hairy vetch only if you have a plan to control the crop so it will not become a weed. Although only four cover crops were used in the trial, it is anticipated that various cover crops and cover crop mixtures may work in interseeding into sunflower.

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Hans Kandel

Extension Agronomist Broadleaf Crops

 

This site is supported in part by the Crop Protection and Pest Management Program [grant no. 2017-70006-27144/accession 1013592] from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the website author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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