Plant densities of two to four plants per square foot can be adequate to produce a viable crop, provided weed competition
can be effectively controlled.
Young, developing canola is not very competitive at the best of times. A thin stand makes the situation worse.
An ideal canola stand is seven to 14 plants per square foot. A thin stand is anything below that level. With a thin stand you have two choices: give up and reseed, or do what’s necessary to give the crop a competitive advantage. You need to scout more diligently and be ready to spray for weeds, insects and disease.
1. SHOULD YOU RESEED?
To aid in this decision, look at the cause and severity of the damage, soil moisture, weed competition (amount and type), reseeding costs and calendar date.
If hail or frost have knocked back the crop, don’t make any snap decisions. Canola seedlings damaged by frost, wind or hail need several days to recover before accurate assessments of survival can be made. Severe damage to cotyledons and true leaves that cause yellowing, browning or blackening does not mean that seedlings are dead. If the growing points and hypocotyls (the stem from the seed to the above-ground growing point) remain intact and turgid, those plants should survive.
Recommendations are to reassess those plants four to 10 days after the damage has occurred. At this time, small leaves will begin to emerge from the growing points, indicating that the plants will survive. If no new leaves emerge, survival is unlikely.
As a reasonable guideline, plant densities of two to four plants per square foot can be adequate to produce a viable crop, provided weed competition can be effectively controlled.
The Canola Growers Manual, available online at www.canolacouncil.org,has two examples where good yields resulted from thin stands. In the first example, the farmer had a plant density range of one to four plants per square foot early in the season. The field was not reseeded. It averaged four plants per square foot at harvest in mid-September and yielded 38 bushels per acre.
In the second example, frost hit May 23, 2004 in a canola field seeded April 29. In this particular case, half the field was reseeded on May 25 and was compared to the original frost-damaged portion of the field. The original frost-damaged half the field yielded 45.8 bushels per acre compared to the reseeded half, which yielded 38.4 bushels per acre. This worked out to a $72 per acre drop in profitability, including reseeding costs.
2. WATCH FOR INSECTS
The lower the plant density, the fewer insects needed to reach the action threshold for control. In the Canola Council’s Canopy Manipulation Trial in 2002 at Yorkton, Sask., flea beetle pressure was moderate to severe flea beetle. Plots with two to three plants per square foot reached the action threshold of 25 per cent defoliation eight days after emergence. In comparison, plots with eight to nine plants per square foot reached the action threshold at 20 days after emergence.
Thin stands are more susceptible to plant losses from insects since any losses are more likely to reduce yield. Thin stands often require additional application of a foliar insecticide to control damaging insects.
3. CONTROL WEEDS
Weeds can dramatically reduce yields through competition with the crop for light, moisture and nutrients. This competition reduces canola plant growth and leaf area resulting in increased flower, pod and seed abortion.
Depending on weed type, density and stage of development, yields can be reduced by five to 50 per cent. Figures from the Canola Growers Manual show that five thistles per square metre will reduce canola yields by four per cent or so. Twenty thistles per square metre reduce yields by 25 per cent. With volunteer wheat, 20 plants per square metre reduce canola yields by 15 per cent. With volunteer barley, 20 plants reduce canola yields by 25 per cent.
You may need to spray more than once until full canopy closure is achieved.
4. THIN STANDS WILL HAVE VARIABLE MATURITY
While canola plants will branch out to compensate for a thin stand, this extra branching can delay seed maturity up to 21 days depending on environmental conditions. Extra branching calls for careful harvest management.
Swathing when seed colour is at 50 to 60 per cent change on the main stem will reduce quality concerns and improve yield potential on areas with marginal increases in secondary branches (two times the normal). If there is a substantial increase in secondary branches (three to four times the normal), check the whole plant to determine maturity. Look for an average of 30 to 40 per cent seed colour change. Make sure much of the immature seed in the side branches is at least firm and dark green to help reduce yield and quality losses.
This is taken from a Canola Council of Canada factsheet called “Managing poor plant stands in canola.” To see the factsheet online, go to www.seedsmart.organd click on “Stand establishment resources” near the top.