Crop Comments: Early Corn Planting’s Color Indicates Proper Timing for Next Planting

As I’m writing this column in late April’s pre-dawn, it appears that spring has not quite grabbed the climate reins from winter. Autmn-planted winter annuals are looking pretty good – crops like wheat, rye, triticale, barley and speltz. But perennial crops are taking their sweet time breaking dormancy.

Research agronomists at UMass Amherst explained that cool temperatures slow green development (vegetative growth) in spring crops by reducing metabolic activity, inhibiting water and nutrient uptake and slowing down the photosynthetic process. While some cool-season crops tolerate these temperatures, they still experience a reduction in growth rates compared to optimal warmer conditions.

These scientists explained why cool temperatures slow spring crop development. The main underlying causes are reduction of both photosynthesis and metabolism. Plants rely on enzymes to drive photosynthesis and convert light into energy. Low temperatures drastically slow down these enzymatic reactions, reducing the rate at which plants produce carbohydrates (sugars) for growth. Accompanying lowered temperatures we see lowered respiration; although reduced respiration can sometimes be efficient (due to lower energy loss), very low temperatures (barely above freezing) limit respiration to a point where plant growth becomes negligible.

Even if the air is mild, cold soil inhibits the ability of roots to absorb water and essential nutrients. Cold temperatures, interestingly, reduce water viscosity, making it harder for plants to transport water and nutrients within their tissues.

Cold temperatures (below 50º F) inhibit microbial activity in the soil, which reduces the release of nitrogen and other essential nutrients, leading to slower leaf development. Cool conditions slow down the rate of cell division; this means that tissue generation, new leaf formation and stem elongation occur at a much slower pace. Temperatures too cool for seeds can cause delayed or uneven germination, reducing the overall uniformity and initial vigor of the spring crop.

This last item I experienced first-hand about a dozen years ago when Daniel, a Mohawk Valley dairy farmer, had me examine an apparent corn planting failure. It was the second week of May that growing season, and the corn field in question appeared to have suffered about 95% early crop failure (having been planted 10 days earlier). On our hands and knees, we dug through the corn planter’s press-wheel marks, looking for any seedling residue. In my sampling, all I found was one seedcorn hull. The endosperm (the mostly starchy part of the kernel’s interior) was gone.

Next to that hull was a white, thread-like life form, a rather wiggly one (evidently not happy at being disturbed by a human hand). I determined that this critter most likely was a nematode of some sort. It – and possibly a million of its compatriots – had partaken of the sumptuous fare provided by the chilled, barely alive, stalled out corn seedlings.

Corn requires a 50º soil temperature to germinate. Planting into soil colder than this will cause the seed to sit dormant and lead to increasing vulnerability to diseases (like seed rot), insects and animal predators (like my presumed nematode acquaintance).

Additionally, cool temperatures slow green color development in corn seedlings primarily by inhibiting chlorophyll biosynthesis and causing the photo- destruction of existing chlorophyll. Because corn originates from tropical and subtropical regions, its chlorophyll synthesis mechanism is highly sensitive to cold, particularly during early seedling growth, leading to pale green or yellow leaves. In any plant, every unit of chlorophyll has a magnesium atom as its structural “cornerstone.” Magnesium-deficient corn plants display a green and white pin-striping pattern.

Honoring that 50º minimum soil temperature for corn, here’s what I recommend for corn planting timing: When the piece in question achieves the above sunrise soil temperature for three consecutive days, plant 5% 10% of your intended corn acreage (understanding that some fields may be colder than others). When those early-planted corn fields show distinctly formed rows (boasting the green color of a certain tractor brand), plant the rest of your corn.

According to Bayer crop scientists, cold soil temperatures stall root development, reducing the uptake of nitrogen and phosphorus. Nitrogen deficiency in particular leads to a yellowing of the older leaves (chlorosis). They say that even if nutrients are present in the soil, low temperatures reduce the microbial activity that converts organic matter into usable inorganic nutrients, leading to starvation-induced yellowing. Often accompanying cold spells, lack of sunlight limits photosynthesis, leading to “pale” or yellow seedlings.

Chinese scientists (Xiaoqiang Zhao et al.) stress that maize (corn) is a cold-sensitive crop, exhibiting “severe retardation of growth and development when exposed to cold snaps during and right after seedling emergence. Although different agronomic, physiological and molecular approaches have been tried to overcome the problems related to cold stress in recent years, the mechanisms causing cold resistance in maize are still unclear. Screening and breeding of varieties for cold resistance may be a sustainable option to boost maize production under low-temperature environments.”

Those (and other Chinese) researchers are laboring to develop corn varieties that are less frost-sensitive as well as more tolerant of less solar radiation. Their work, “Understanding and Comprehensive Evaluation of Cold Resistance in the Seedlings of Multiple Maize Genotypes,” is available at tinyurl.com/ yc4s5ckh.

I find it interesting that Nebraska agronomists have determined that wheat residue on a dryland field had the highest daily soil temperature on most days. On the other hand, corn residue in an irrigated field had the lowest soil temperature.

Meanwhile, later-planted corn stands, with good green color, most likely indicate that hot climate summer annuals (or HCSAs, like sorghum, sudangrass, their hybrids and millets) have enough warmth in soil and air to survive and thrive. The HCSAs are pretty insistent on 65º minimum soil temperatures.

If those later-planted corn seedlings are sporting a tractor brand’s “Ag & Turf Green,” soil temperatures can be presumed sufficiently cold-free for the HCSAs to start rapidly enough to dodge most weed pressure.

by Paris Reidhead