Microclimite of Crop Canopy

Data was collected in corn, soybean, and corn-alfalfa at Prosper, Fargo, and Forman, ND, and Ames, IA in 2016 and 2017 to characterize light conditions under the crop canopies. Soil water content in the top 15-cm, and especially the surface of the soil determines establishment success and survival of interseeded cover crops under the canopies of established summer cover crops. These results indicated that available soil water during the three weeks following interseeding is critical for the cover crop survival.

Photosynthetically active radiation (PAR) under both the corn and soybean leaf canopies is the main driver of interseeded cover crop growth.  Averaged across both North Dakota sites, at full leaf canopy, soybean intercepted 85% of the PAR both years, leaving only 15% of PAR available for cover crops growth under the leaf canopy (Fig. 1). Seeding dates for this experiment were 25-26 July for R4 stage and 6 August for R6 stage in 2016. In 2017, seeding dates were 21 July for R4 and 21 August for R6. Cover crops interseeded at R4 stage had less than 15% PAR available for growth. The limiting light received made cover crops elongated and weak.  By the time the second seeding was conducted the intercepted PAR and soybean Leaf area index started to decrease, indicating soybean started to drop their leaves.  In less than two weeks, intercepted PAR dropped from 80% to less than 30%. This rapid increase in incident solar radiation to the cover crops is detrimental to the seedlings weakened by the lack of light.  This combined with lack of rain in the same period will kill most seedlings.  As indicated, below in experiments in Objective 2, cover crops interseeded after R6 in soybean in general have better survival and growth. In corn, light environment under the canopy is different from that of soybean.  In corn, interseeding dates were at V8 and at R4 on 7 July and 15 August, respectively. More than 90% of PAR was intercepted until after harvest (Fig. 2).