Improving Flowering & Seed Set in Canola

Many growers will have experienced issues with poor flowering or pod set in their canola crops but may not know the reasons behind this. Seasonal factors outside the grower’s control such as soil moisture, rainfall, temperature & frosts can all influence flowering, however nutrition also plays an essential role in crop reproduction & importantly it is something growers can manipulate.

Nutrients including nitrogen, calcium, boron, zinc, copper & molybdenum are all required for optimal reproduction, seed set & pod retention. Nutrient balance is also critical here as high nitrogen (eg. nitrates) will impair fertilization as well as reducing uptake and/or increasing demand of these other critical nutrients.

Calcium is required by all crops for flowering due to its major function in cell wall structure & integrity (structural or physical integrity of reproductive organs is a key element to successful fertilization – see boron & copper). Calcium, being a major nutrient, needs to be addressed primarily pre-sowing either as lime or gypsum application. Calcium however is very immobile so soil availability may be limited & deficiencies may still arise even though exchangeable levels are “adequate”. Many ‘experts’ advise not applying calcium as a foliar spray because it is immobile. Indeed it is (it doesn’t translocate or move around the plant) however it is readily absorbed by leaves, stems, buds and flowers. It is common practice in many horticultural crops to apply bud or flowering sprays containing calcium, boron & zinc to enhance flowering and fruit set. So timing is critical here but foliar calcium will definitely help set if buds & flowers can be targeted (you don’t need high rates either). With excellent growing conditions this season calcium demand for new growth will be high, so supplementing soil calcium with strategic foliar sprays should pay dividends with increased pod retention & yields.

Being a brassica, canola has a very high boron requirement. Like calcium, boron is a structural component of cell walls and therefore deficiencies cause cell breakdown, or specifically in canola, impaired growth of pollen tubes. Boron is also a regulator of auxins, a plant hormone required for optimal flowering. Unlike calcium however, boron is a trace element and also plant mobile in canola, therefore deficiencies can be corrected solely through foliar application if required. Due to its mobility, the window for boron application is a bit wider than calcium, but generally you want to apply it prior to flowering (5-8 leaf) or during bud development or the early bolting stage. When applying boron to the leaves uptake is slow because the cuticle has a negative charge that repels anions (eg. borates, molybdates, phosphates). SJB boron products employ a unique complex formulation to give boron a neutral charge so it moves into the plant more readily than boron salts.

Zinc is essential for the synthesis of auxin, an important plant growth hormone involved in reproduction. Zinc deficient plants will also be stunted and therefore their production of carbohydrates is reduced and unable to meet the high energy requirements needed during flowering & pod set. Zinc is immobile in the soil and only has limited mobility in the plant, so multiple applications may be beneficial in very deficient situations, however canola appears to be reasonably efficient at accessing soil zinc so a single early spray (4-5 leaf) is usually sufficient.

Like calcium & boron, copper is also important for structural integrity due to its role in lignification. Poor lignification means that pollen tubes don’t develop normally, thus fertilization is compromised. Copper is also essential for photosynthesis (starch production) and deficiencies during flowering result in an energy deficit that reduces seed set. High soil nitrate levels increase vegetative growth & copper requirements, so with good moisture and optimal growing conditions this season, potential for deficiencies is high on susceptible soil types. Like zinc, copper is basically immobile so an early application (4-6 leaf) is important to optimise photosynthetic activity prior to flowering.

Molybdenum (Mo) is an important trace element for brassica crops, and the requirement for canola is five times that of wheat. Moly is required for nitrogen utilization by the crop as it produces an enzyme that converts nitrates into amino acids. We have already discussed the potential problems that high plant nitrate levels can cause regarding reproduction and Mo deficiency only compounds this. High nitrates will also affect the hormone balance within the plant so it is likely to remain in the vegetative mode for longer which compromises the reproductive phase of development. Moly deficiencies are most common on acidic soils & may be corrected simply through lime application. Foliar sprays are very effective as Mo is plant mobile and can move to areas of greatest demand. Sprays should be effective from early emergence right through to flowering.

Foliar nutrient options for Canola:

4-8 leaf – LIG-GRAIN PLUS (Zn, Cu, B, Mo) @ 2 L/ha, LIG-ZINC + BORON @ 1 L/ha.

Bud development to flowering – LIG-CALCIUM + BORON @ 2 L/ha, LIGNO-BORON + MOLY @ 1-2 L/ha.