Irrigation
Corn requires the application of 3 to 3.5 acre feet of water. This is usually applied in 5 to 9 irrigations. It is important that moisture be available at the crown of the corn plant when the secondary root system is being established. This will occur from 3 to 4 weeks after planting. The tasseling-to-silking stage is also critical. In this short time, formation of the grain is initiated. If moisture is lacking during this time pollination is incomplete and poorly-filled ears result. An adequate irrigation at tasseling time is therefore essential. Yield and moisture content of the grain can be affected by lack of available water up to the hard-dough stage of maturity.
When corn is planted into good moisture, no irrigation is usually required for the first 3 weeks. The moisture requirements are met by the roots moving rapidly into moist soil. During this period, about 1 to l-l/2 inches of moisture are used. Growers should plan ahead in order to apply the first irrigation approximately 30 to 40 days after planting, unless sufficient rainfall has occurred to moisten the area at least to seed depth. Many growers now plant on beds or make small furrows at planting in order to facilitate this early irrigation. This is a very critical time in the development of the ear and moisture stress can cause abortion of the ear. This first irrigation can be delayed in some situations, such as high water table areas or if sufficient rainfall occurs. Sprinkler irrigation is an excellent method to apply this first irrigation. The vegetative period, from emergence to first tassel lasts for about 8 weeks for an early to medium-early variety. The moisture requirement is about 8 inches.
Pollination period covers the next 4 weeks to blister kernel stage; this period also requires about 8 inches of moisture. At Davis, the best irrigation system during this period was 8 weekly irrigations. The grain filling period covers the next 5 weeks to physiological maturity, the moisture requirement is 8 inches. At Davis this requirement was best met by starting this period with a full moisture profile and no applied irrigations. On shallow soils, subsequent irrigations would be necessary. County field studies indicated that most fields will need an irrigation during the grain filling period.
Irrigation Cut-Off
This is one of the most frequently asked questions by growers. The last irrigation should be timed to provide adequate moisture up to physiological maturity. Corn is mature when it has reached about 35% moisture. Some agronomists use the black layer formation to determine maturity. The newest system being used as a visual indicator of corn kernel maturity is the milk line. The milk line moves from the kernel tip to the kernel base. The black layer forms a few days after the milk line has reached the kernel base. When the milk line is half-way between the kernel tip and the kernel base, it contains about 40% moisture. The black layer begins to form shortly after dent occurs.
The black layer forms in the tip of each kernel. It forms in the kernels at the tip of the ear first and then spreads over the remainder. To find the black layer, scratch the surface off the base of the kernel on the flat side of the grain opposite the germ side. Scratch gently and a black layer can be seen beneath the surface near the point where the kernel was attached to the cob.