Rainfall has been widespread this past week, providing much needed moisture in many areas of North Dakota. Although water is needed in only small quantities in the chemical process of photosynthesis from which all plant growth occurs, it is essential and required in large quantities for transpiration. Transpiration is the name given to the movement of water from the soil to the roots, through the plant’s vascular system and out the stomata of the leaves. This process provides cells with the water needed remain turgid and for chemical processes, brings nutrients from the soils, cools the plant and allows the stomata to open and thus permit the entrance of CO2 needed for photosynthesis into the leaves. When water demands from transpiration exceed the availability of water from the soil, the stomata close and leaves begin to curl or rolled. Leaf rolling is first visible only during the heat of the day when transpiration demands are greatest. Yield is impacted when leaves are rolled because CO2 is limited within the cells where photosynthesis is taking place. When water stress become more prolonged and severe, leaves turn yellow and die back, usually the lowest leaves first. Unlike leaves that are rolled, leaves that senesce will not be able to contribute to future plant growth if and when drought stress is alleviated.                                                                                                                                   

Most data on yield loss due to drought are based on the number of days that the leaves are rolled and the plant stage of development. Most data suggest that there is little yield loss when leaves show stress during early vegetative stages, though we have all seen corn crops that were completely decimated by severe stress well before tasseling. At the V12 stage, yield losses can be two to three percent of the total yield potential of the crop per day. This increases to nearly seven percent per day at silking (see Lauer 2003 for a complete table) before it tapers off during later grain filling. Stress during pollination through early grain filling stages impacts yield more than at any other stage due to the potential for poor fertilization and kernel abortion during this period.

For growers that have fields that have been stressed prior to recent rain events, the question is, to what extent will my corn crop recover? If the stress has been sufficient to reduce leaf numbers and permanently damage the leaf area of the plant, we might be able to use the leaf defoliation table printed in this issue of the Crop and Pest Report to help estimate a loss (this assumes a somewhat normal fertilization process). For less severe stress and assuming adequate rainfall for the rest of the season, for fields where the leaves were rolled for part or all of the day for several days and tasseling and silking proceeds normally, yield losses will probably be in the 5 to 15% range and you should plan future management accordingly. The key to a reasonable yield from previously stressed fields will be how well fertilization proceeds. It is not that uncommon for severe pre-tasseling stress to induce asynchrony in silking and pollen shed. This means that the tassel emerges and sheds most of its pollen prior to the emergence of the silks, severely reducing kernel numbers. For previously stressed fields, I would certainly recommend keeping an eye on the fertilization process in the coming days as that will be key to any future yield potential development. Soon after the silks turn brown, consider checking representative ears for kernel numbers. With good kernel counts and healthy leaves above the ears, the potential for high yield is still possible.