Don’t plant corn too early into cool soils

This ‘tender’ crop won’t emerge well and you may be better off waiting for warmer conditions

Manitoba corn growers shouldn’t be in too much of a hurry to plant into cold soils in the spring if they want good, uniform stand establishment.

And they might want to consider seed treatments to help plants through germination and emergence, when they can be vulnerable to pests and disease.

Those were some of the key messages that Robert Nielsen of Purdue University had for agronomists and growers at the Manitoba Agronomists Conference in Winnipeg last December.

With trends towards more variable weather across the Prairies, crops are likely to experience more harsh growing conditions in the future, and that could present problems for more northerly corn growers. But with attention to some basic agronomy, they can continue to grow corn successfully in Manitoba, says Nielsen.

Successful corn stand establishment begins with excellent seed quality and genetic seedling vigour. Following that, corn seeds need adequate and uniform soil temperature, moisture and soil-to-seed contact, so a well-prepared seedbed free of crusting or compaction is essential, as is an adequate supply of nutrients.

“Conventional tillage often ties into the risk of having a hard surface crust after we plant that will impede emergence, or smearing of the sidewall if we plant on ground that’s too wet,” said Nielsen, who added some growers also use starter fertilizer as a way to help mitigate early-season stress.

“It’s not a 100 per cent response, but there are opportunities for starter fertilizer to help young plants, especially once they have emerged and are going through the stand establishment phase.”

Planting activities need to be error free, which means keeping the planter maintained and adjusted properly, and paying attention to planting speed.

Finally, establishing a good stand requires protecting seeds from soil-borne insects and diseases, which often comes down to seed treatment, although most, says Nielsen only offer protection for two to three weeks after planting. “If seed companies could come up with seed treatments that last five or six weeks to protect us from seedling blight and things like that, it would do wonders in helping uniformity of plant stand establishment and of plant growth early in the season,” added Nielsen.

Germination and emergence

When corn seeds are planted, they imbibe water within the first 24 to 36 hours after planting, which renews the cellular activity of the embryo (the process of germination). The appearance of the radicle root is the first visual evidence of seedling development.

This is usually around the time when 55 CHU (corn heat units) have accumulated. It’s also a critical time in terms of disease risk, because the radicle root ruptures the seed coat, providing a potential pathway for pathogens to attack the emerging seedling. So, time is of the essence when it comes to seedling germination and emergence, and the faster the corn plant goes through these stages, the more uniform the stand is likely to be.

Soil moisture is critical because corn seeds need to imbibe enough soil moisture to bring seed moisture content to about 30 per cent, and optimum soil moisture for germination is generally considered to be field capacity. Excessive soil moisture will cause inert seed and eventual death of the embryo, while excessive dry conditions will inhibit germination.

Responds to heat

The important thing to remember, said Nielsen, is that corn responds to heat, not the calendar. At around 86 CHU the mesocotyl and coleoptile (the protective covering over the first leaves of the seedling) begin to emerge, and at 102 CHU the rest of the seedling root system emerges.

The mesocotyl is the white stem-like tissue that connects the kernel and the base of the coleoptile (sometimes called the crown). It’s technically the first internode of the seedling, and it elongates to move the coleoptile (containing the leaves) towards the soil surface.

“The mesocotyl is a connective tissue that is important for transport of the energy and starch reserves from the embryo to sustain the growing plan,” said Nielsen, adding the deeper the seed is planted, the longer the mesocotyl will be.

Around 160 CHU is typically when the first true leaf emerges from the coleoptile; this is growth stage VE or emergence. The quicker that 160 soil-based CHU can accumulate, the fewer days it will take for emergence to occur and the less vulnerable the seedlings will be to stress.

Emergence versus stand establishment

It’s important to understand the distinction, said Nielsen, between emergence and stand establishment.

“Stand establishment is (when the plants) are knee-high, and what is going on between emergence and stand establishment is the primary initial development of the main root system of the plant, so even though it comes up fine, the next several weeks will determine whether you will have a uniform stand of knee-high corn or not,” said Nielsen.

“If it’s growing healthy, fast and vigorously, it’s more resilient to stress.”

Until the corn reaches knee height it is soil temperature that drives the pace of development because the growing point of the young plants remains below the soil until it is roughly to stage V5 or V6.

What does that mean in terms of days to emergence after planting? That varies, according to conditions. For emergence to occur seven days after planting, soil temperatures during that period must remain at no less than around 18 C.

Studies at Purdue University have shown that corn planted in late March to early April took 35 days to emerge on average, with average soil temperatures of around 11 to 12 C.

“The rule of thumb for planting corn is that soil temperature must be at least 10 C, but if that’s all you are averaging it takes a long time to emerge,” said Nielsen.

The later corn was planted into warmer soil temperatures, the faster it emerged. At an average of 12 C soil temperature, corn emerged in around 25 days and at average soil temperatures of 14 C, it came up in about eight days.

Research in Ontario in the early ’90s suggested that the earliest “safe” planting date for “tender” crops such as corn and soybeans was after three consecutive days with daily mean air temperatures around 12.8 C.

Variability

Variability in seedbed temperature can lead to variability in germination and emergence resulting from variability in soil colour and texture, seedbed moisture, seeding depth and disturbance of crop residue in no-till systems, and is especially important when soil temperatures are hovering around 10 C.

In addition to slowing the physiological development of the seedlings, low soil temperatures can physically damage the emerging plants. There are basically two types of chilling injury that can occur; Imbibitional chilling injury to embryo and chilling injury to the elongating mesocotyl.

“If soils are cold, or you have planted just ahead of a cold rain, and the soil has chilled down quickly in the first 24 to 36 hours after planting, seeds will swell when they imbibe water and if the seed tissue is cold, it’s not as elastic, so it swells and rips, cell contents leak out and it stops germination,” said Nielsen.

Chilling injury can also affect the external layers of the elongating mesocotyl, making it deform into a corkscrew shape and arresting development.

In conclusion, Nielsen said to manage the risk of cold soils, growers should choose hybrids with good seedling vigour and cold tolerance, avoid early planting in fields with poor drainage and/or high residue, ensure optimum seed quality and avoid extremely small seed lots (80,000 kernel bags weighing less than about 40 lbs.).

“A smaller seed has a smaller starch reserve and asking it to survive longer is riskier than a bigger seed,” said Nielsen.

They should also consider extra seed treatments if planting into cool conditions, even though their efficacy is limited, when planting into cool soil conditions, he added.

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