Late Season Corn Irrigation and Termination

Crop Progress – This season’s corn crop, particularly that planted in  early March, is extraordinarily early.  In fact, climatic records show this season’s growing degree day (GDD50) accumulation at Stoneville for a March 10 corn planting date are about 15 days ahead of normal.  Correspondingly, one of our cooperator’s Corn Verification fields planted in early March in the Lake Washington area had the milk-line progressed about one-quarter of the way down the kernel on June 20, so we expect it should reach physiological maturity about July 5.  So since dry weather has returned, we will likely need supplemental irrigation or additional rainfall to finish out this crop.  In fact, forgoing management practices which may mitigate severe stress or pest issues near the dent stage can easily cut corn yield from 15 to 50 bushels per acre or profit $90 to $300 per acre.  The bottom line is you don’t want to give up on the crop yet, if you can influence the crop outcome.

Corn water use varies considerably depending on growth stage. Thus, there is great opportunity to adapt your irrigation schedule to optimize profitability.

Likely one of the most critical late-season management inputs in a droughty season is irrigation needs.  Thankfully, corn moisture requirement steadily drops from a peak of 1.5-1.75 inches per week prior to the dough stage (four weeks post tassel) to an inch or less per week after dent and even lower as the grain approaches physiological maturity.  This reduced crop moisture demand may allow you to scale back irrigation application, as the crop approaches maturity.  If you use our predominant furrow-irrigation systems, you can lengthen intervals between irrigation cycles to reduce expense and prevent unnecessary saturation, which is harmful to the crop.  For example, two well-timed irrigation events (shortly after dent and at 50% milk-line) should generally provide plenty of moisture for the corn crop during the last 20 days.  Pivot irrigation volume may also be trimmed to adjust for reduced corn water use during latest growth stages. The key to proper irrigation timing is simply checking soil moisture using a probe, auger, shovel or other tools, rather than simply doing what you did last week.  The crop moisture demand certainly changes substantially from week to week as shown in the graphic above.  Weather, soil moisture and crop conditions can vary widely from field to field, week to week, and year to year, so I highly encourage you to closely monitor soil moisture level and schedule accordingly.

Corn Irrigation Termination – You certainly do not want to terminate irrigation so that the crop stresses before corn physiological maturity (black layer) occurs (for more information see http://wp.me/p1jA50-iV. When late June and July rainfall fails to meet crop demand, water deficit or premature irrigation termination will accelerate maturity, prohibiting kernels from reaching their full potential size and weight.  Although kernels outwardly appear mature and corn water use begins declining at the dent stage, this is far too early to terminate irrigation. Potential kernel weight is only about 75% complete at the dent stage.  Thus, termination of irrigation at the dent stage can reduce grain yields as much as 15-20% (30-40 bu/a) when hot, dry conditions persist.   Early irrigation termination will also likely reduce stalk strength and promote lodging, because plants will cannibalize energy from vegetative organs to fill kernels when they are stressed.

This cross-section of an ear of corn shows the milk-line advanced half-way down the kernels. It is about 10 days from physiological maturity and needs sufficient moisture to fill kernels during this time.

Kernels mature from the outside-in when hard starch begins forming at the crown.  The crown will turn hard and become the bright shiny golden yellow color of mature kernels.  This starch and weight accumulation will steadily progress towards the base of the kernel (where it attaches to the cob) taking about 20 days to complete.  The most reliable method for you to monitor kernel maturity for irrigation scheduling purposes is to observe this progression of the milk-line (or hard starch layer) between dent stage and black-layer.  The milk-line is more relevant than the black-layer, because it indicates maturation progress, before the black layer is evident. The milk-line is the borderline between the bright, golden yellow color of the hard seed coat outside the starch, compared to the milky, dull yellow color of the soft seed coat adjacent the dough layer.  To observe the milk line, break a corn ear in half and observe the cross-section of the top half of the ear (the side of kernels opposite the embryo).  If you have difficulty seeing this color disparity between layers, you can easily find it by simply poking your fingernail into the soft, doughy seed, starting at the kernel base and repeating this procedure progressively toward the tip, until you feel the hard starch.