What’s be­hind canola seed­ing rate rec­om­men­da­tions?


Suf­fi­cient plant es­tab­lish­ment is key to en­sur­ing a canola crop has the high­est po­ten­tial for suc­cess. A sim­ple seed­ing rate cal­cu­la­tion can be per­formed to de­ter­mine the amount of seed re­quired to op­ti­mize yields and max­i­mize re­turn on in­vest­ment for seed costs. How­ever, while the cal­cu­la­tion is sim­ple, de­ter­min­ing some of the val­ues to plug into the equa­tion may be less so. De­ter­min­ing av­er­ages of the less spe­cific vari­ables within fields or on farms could go a long way to im­prov­ing the ac­cu­racy of any seed­ing rate cal­cu­la­tion. were tar­geted. Add to this seedling sur­vival rates that ranged from 40 to 60 per cent, and, the seed­ing rate cal­cu­la­tion was all but im­prac­ti­cal for canola.

To nar­row this range, it be­came com­mon prac­tice to sug­gest 100 to 140 plants/m2 as a more rea­son­able tar­get. As seed costs climbed, new hy­brid va­ri­eties were de­vel­oped and more re­search was con­ducted; the tar­get plant pop­u­la­tion range rec­om­men­da­tion was fur­ther nar­rowed and re­duced to 70 to 100 plants/m2.

With the ad­vent of bet­ter seed place­ment tech­nol­ogy and im­proved hy­brid vigour, the canola seedling sur­vival range has also been nar­rowed to 50 to 60 per cent of planted seeds. While not per­fect, the tight­ened tar­get plant-stand and seedling sur­vival ranges have com­bined to make seed­ing rate cal­cu­la­tions a much more use­ful tool.

One fac­tor that has not changed over time, is the plant den­sity, be­low which there is a sig­nif­i­cant re­duc­tion in yield po­ten­tial. That thresh­old re­mains at 40 plants/m2. It is worth not­ing that even when plant pop­u­la­tions are near this level (but not be­low), high yield po­ten­tials can only be reached in the ab­sence of crop stresses such as in­sects, weeds, dis­eases, spring frost, hail and ex­cess heat. As well, a longer grow­ing sea­son is re­quired to al­low the crop to fully ma­ture.

When de­ter­min­ing an ap­pro­pri­ate seed­ing rate, con­sid­er­a­tion must be given to each of these fac­tors. For ex­am­ple, low plant stand den­si­ties of canola can tol­er­ate less pres­sure due to flea bee­tles, frosts or hail. Plant re­duc­tions from in­sect feed­ing, spring frosts or hail can quickly re­duce stands be­low that re­quired for max­i­mum yield and in the case of in­sects, in­crease the re­liance on in­sec­ti­cide for con­trol.

Low plant stand den­si­ties of canola will take longer to cover the ground and the plants are not able to com­pete as well with weeds. Low seed­ing rates can in­crease the time over which weed growth must be con­trolled by her­bi­cides re­sult­ing in an in­creased num­ber of ap­pli­ca­tions within a sea­son. As well, since these in­creased ap­pli­ca­tions of her­bi­cides are re­quired to be made in-crop, there is lit­tle op­por­tu­nity to change her­bi­cide modes of ac­tion. This has the po­ten­tial to de­velop or in­crease her­bi­cide re­sis­tant weed pop­u­la­tions.

Fungi­cide and har­vest tim­ing de­ci­sions can also be made more dif­fi­cult with low plant pop­u­la­tions. Fewer plants in a given area will re­sult in in­creased branch­ing on each plant which will in turn in­crease the flow­er­ing pe­riod and time to ma­tu­rity. When the flow­er­ing pe­riod is in­creased, there is less phys­i­o­log­i­cal uni­for­mity re­sult­ing in po­ten­tial losses due to un­pro­tected crop, or in­creased fungi­cide, fuel and time needed for split ap­pli­ca­tions. In­creas­ing the time to ma­tu­rity in­creases risk of ex­ces­sive shat­ter­ing losses or high green seed counts be­cause har­vest tim­ing is less pre­cise, de­layed or pro­longed.

A con­found­ing fac­tor, not pre­vi­ously dis­cussed, is that newer va­ri­eties of canola seed are be­ing pro­duced with sig­nif­i­cantly larger thou­sand seed weights. A con­se­quence to this is that more weight of seed is re­quired to tar­get cur­rent op­ti­mal plant stand rec­om­men­da­tions. This im­plies added cost for seed that is pur­chased by weight. In or­der to sta­bi­lize seed costs, the trend has been to adopt a “one size fits all” seed­ing rate, re­gard­less of seed size. How­ever, re­cent re­search has in­di­cated that these larger seeds have no sig­nif­i­cant im­pact on emer­gence or yield and thus, us­ing a sin­gle seed­ing rate has the po­ten­tial to be prob­lem­atic if plant stands drop be­low op­ti­mal.

As tech­nol­ogy and ge­net­ics im­prove it may be pos­si­ble that re­search could fur­ther nar­row the ranges for op­ti­mal plant den­sity and seedling sur­vival rate. How­ever, these will al­ways re­main ranges since they are de­ter­mined over large ar­eas with dif­fer­ing soil types, cli­matic con­di­tions and seed­ing sys­tems. Mea­sur­ing and record­ing these val­ues on in­di­vid­ual fields, us­ing a con­sis­tent seed­ing sys­tem, has the po­ten­tial to de­velop a field spe­cific av­er­age that will dras­ti­cally im­prove pre­ci­sion in the seed­ing rate cal­cu­la­tion and gain con­fi­dence when de­ter­min­ing re­turn on in­vest­ment.

For more in­for­ma­tion con­tact the Agri­cul­ture Knowl­edge Cen­tre.

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