Late Season Corn Irrigation and Termination

Crop Progress – Climatic records show this season’s growing degree day (GDD50) accumulation at Stoneville for a March 20 corn planting date are about one and half days ahead of normal.  However, this is behind schedule compared to last year and in 2007 (when we produced the record high yielding crop).  Correspondingly, several of our cooperator’s Corn Verification fields which were planted from March 19 – 23 near or slightly south of Highway 82 are now approaching dent stage and should reach physiological maturity about July 22-27.  So although we have had extreme drought in much of the southern regions of the state, the irrigated corn crop still has a considerable way to go before the crop is made.  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 relative weekly water demand changes considerably throughout the growing season. Therefore, you should adapt your irrigation schedule accordingly 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, compared to the past several weeks.  If you use our predominant furrow-irrigation systems, you can lengthen intervals between irrigation cycles to reduce expense and prevent unnecessary saturation, which may be 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 below.  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 https://www.mississippi-crops.com/2011/06/24/identifying-corn-reproductive-growth-stages-and-management-implications/), considering our current drought.  When normal July weather persists and 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 photo shows a cross-section of an ear of corn with the milk-line advanced half-way down the kernels. This corn is about 10 days from physiological maturity and needs sufficient moisture to fill seed weight during this period.

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 find it by pressing 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.