By Mike Stamm, Canola Breeder, and Ignacio Ciampitti, Cropping Systems Specialist

Until beneficial rains fell in mid- to late-September, canola planting in Kansas was delayed due to inadequate soil moisture conditions. Thus, canola plants may be smaller than normal going into the winter acclimation period. How could this, and other factors, affect the winter survival of canola?

Effect of Canola Size
on Winter Survival

Canola overwinters — and is the most tolerant to cold temperatures — in the rosette growth stage (Figure 1). At this stage, the crown develops at the soil surface with larger, older leaves at the base and smaller, newer leaves at the center. The stem thickens but its length remains unchanged.

For optimum winter survival, a winter canola plant needs 5 to 8 true leaves, 6 to 12 inches of fall growth, a root collar diameter of ¼ to ½ inch, and an extensive root system. Hardened winter canola can withstand temperatures below 0 degrees Fahrenheit for short periods of time.

On the other hand, canola that has too much top growth (typically 20 inches or more) can succumb to winterkill for a number of reasons, including overuse of available soil water and nutrients, stem elongation above the soil surface, and physical damage to the unprotected crown as winter temperatures arrive.

Causes of Excessive
Fall Stem Elongation

Stem elongation in the fall — not to be confused with bolting, i.e. stem elongation with visible flowering structures -- may occur because:

• The crop was planted too early (Figure 2)
   
• The crop was seeded at higher-than-optimal plant populations (Figure 3)
   
• Excessive soil fertility is present (particularly nitrogen)
   
• An unusually warm fall persists
   
• Selection of a poorly adapted cultivar
   
• A combination of any of these factors

For example, closely spaced and crowded canola plants increase early plant-to-plant competition for light. This “reaching” for light may lead to an extension of the growing point above the soil surface. Any time the growing point (rosette) is elevated, the chances for winterkill are increased because overwintering plant parts are in an unprotected position above the soil surface.

Another factor in stem elongation and winter survival is the amount of surface residue present in the seed row. K-State research has shown that residue removal from the seed row is important for keeping the rosette, or crown, close to the soil surface, especially in no-till cropping systems. Appropriate residue management (any method to remove residue from the seed row) greatly benefits winter survival.

Figure 4 (upper and lower panels) shows what can occur when residue blows back into the seed row following planting into heavy corn residue. Because of residue in the seed row, the canola hypocotyl in this case is etiolated, or overextended, and thus vulnerable to freezing temperatures and other biotic stresses. Etiolation is when a plant develops in partial or complete absence of light.

Under these conditions, plants will have long, weak stems and a pale color. In the situation shown below, the canola hypocotyls grew upward through the corn residue, stopping when they reached sunlight above the soil surface. That is where the rosettes were established. As a result, the crop easily succumbed to cold temperatures, and the field in this specific example was lost because of poor residue management.

2017 Planting Dates

Soil moisture conditions dictated planting dates for winter canola in 2017. In many parts of Kansas, dry soils delayed planting until beneficial rains fell in mid- to late-September.

Compared to 2016 when soil moisture was adequate and allowed for earlier planting, planting dates in 2017 were, in some cases, delayed beyond the optimum. Because canola needs adequate fall growth to overwinter, it is important for the crop to be planted into adequate soil moisture. This will help guarantee rapid emergence and establishment in the fall.

Similar to fall 2016, the warm temperatures in early October have benefited canola growth and development (Figure 5). However, the first hard freeze of the season is expected overnight on October 27. This could be a concern as some canola will not have attained the 5 to 8 true leaf stage that is optimal for winter survival. Late-October is typically when canola begins to acclimate to winter conditions, so these temperatures are not out of the ordinary. Low temperatures at or below 30 degrees F are essential for winter hardening.

Will the fields with small canola succumb to winterkill?

It is hard to say because there are a number of factors that can affect winter survival. Good winter survival begins with selecting a winter hardy cultivar. Management of the crop, including selecting an optimum planting date, fertilization and seeding rate, also can affect overwintering. The environment has probably the biggest influence and individual canola fields may see different effects from the cold.

The ultimate low temperature and the duration of below-freezing temperatures are things to keep in mind when weighing what might happen. In addition, better survival is often seen when temperatures gradually drop versus rapidly drop. In the long run, an interaction of all these factors will determine how the crop will overwinter.

Cultivar Differences
in Overwintering potential

Cultivar differences exist for fall vigor, the ability to avoid fall stem elongation, and winter survival, so it is important to consider these traits when considering what cultivar to grow. Hybrid cultivars tend to have quick establishment in the fall because of hybrid vigor. This is an important trait because it results in rapid plant development for overwintering.

However, there can be a tradeoff between good fall vigor and too much fall growth, and this usually has to be managed by agronomic practices such as planting date and seeding rate. Planting later to take advantage of vigor may present some challenges in terms of winter survival if weather conditions are not favorable for fall growth.

The K-State canola breeding program has been selecting for cultivars that avoid fall stem elongation regardless of the planting date or seeding rate and this often translates into better winter survival. These cultivars have prostrate fall growth which keeps the crown (growing point) more protected at the soil surface.

This trait could be especially useful in years when soil moisture conditions are ideal for planting but the calendar indicates it is too early to plant. We hope to broaden the planting window by planting these cultivars earlier while avoiding the risks of fall stem elongation and winterkill.

Another tool under development by private industry and being evaluated by the K-State canola breeding program is the semi-dwarfing trait. The semi-dwarfing trait also helps keep the crown closer to the soil surface regardless of planting date or seeding rate. We have seen enhanced winter survival in the hybrids that possess this trait.

Current Research

K-State agronomists are investigating production practices to help manage fall vigor and growth. We have studies evaluating seeding rate by variety (open pollinated vs. hybrid) in narrow and wide row spacing (9-in and 30-in). In the first year of the studies, we did not see consistent differences in winter survival among the cultivar by seeding rate interactions because the winter temperatures were warmer than normal.

However, in subsequent years, winter survival was greater with reduced seeding rates, and yield was similar to that achieved with higher seeding rates. Other questions we want to address through these studies include:

• How far can we reduce seeding rates and remain profitable?
   
• How do varieties respond to different seeding rates?
   
• What is the optimum seeding rate for a given row spacing by variety interaction?

In collaboration with private industry, we are evaluating different plant growth regulators and their ability to help manage fall growth. Using plant growth regulators to manage fall growth in winter canola is a common practice in the European Union (EU). In addition to the products, we are evaluating at what growth stage and at what rate do we apply these products in the fall.

Having too little or too much fall growth in winter canola depends on an interaction of the variety chosen, management practices, and the weather. Predicting the weather is challenging enough and this can be stressful on producers. Through breeding and production research at K-State, we hope to find improved ways to manage these risks in winter canola.