A BASF-organised oilseed rape briefing focused on the influence of weather on the crop’s yields since 1979, highlighting how some weather patterns significantly benefit yield, and also how growers might mitigate against the negative effects of adverse conditions. Dominic Kilburn writes.
While a gap remains between yields achieved in Recommended List trials and average on-farm yields, encouragingly there has been a steady rise in oilseed rape yields during the past 15 years, predominantly through improved canopy management and overall agronomy. However, researchers have highlighted that there are noticeable fluctuations in oilseed rape yields on account of seasonal weather effects – some in recent years causing as much as a 1t/ha reduction in average yield.
According to ADAS crop scientist Christina Clarke, speaking at a BASF-organised briefing earlier in the spring, there have been obvious fluctuations in yields when extreme weather patterns occurred, such as between 2011–2013, but it posed the question as to whether crops could be managed better to ensure resilience to cope with seasonal effects.
To explore this further, a new ADAS study commissioned by BASF has identified key weather patterns which account for 37 per cent of the variation in oilseed rape yields, and suggested reasons for this.
To complete the work, ADAS analysed Defra oilseed rape yield statistics between 1979–2017, in correlation with Met Office data, with the objective to investigate which weather factors are associated with high or low oilseed rape yields. “We wanted to look at the effects of the weather on the crop at a particular growth stage and to see what difference this had on final yield, as well as propose crop management approaches for maximising yield when weather conditions are not conducive for high yield,” she explained.
High yielding factors
According to Dr Clarke, in summarising the weather factors that are associated with high OSR yield, she said that high temperatures in October, a dry December, a sunny and dry April and a wet, cool and dull May accounted for as much as 37 per cent of yield variation against the long term national yield trend.
“A warm October with an increased temperature of 2ºC is associated with a yield increase of 0.17t/ha.
“The higher temperature produces increased autumn growth resulting in vigorous, strong plants entering winter which are less affected by harsh winter conditions. With good biomass and better rooting, the crop can get off quickly in the spring,” explained Dr Clarke.
In order to mitigate the effects of a cold October therefore, Dr Clarke said that autumn plant vigour must be maximised by timing sowing to benefit from sufficient available soil moisture. Producing a good tilth in the seedbed and ensuring good seed-to-soil contact is also key, she pointed out.
“Varieties with high early vigour should also be considered,” she added.
According to Dr Clarke, a dry December (with reduced average rainfall of 50mm) is associated with a yield increase of 0.11t/ha because of the avoidance of waterlogging over winter, which can inhibit and slow root growth.
However, in order to manage crops when a wet December occurs, growers should ensure that their field drainage systems are in good working order, and minimise soil compaction.
In addition, the same factors highlighted previously for a ‘cold October’ in terms of crop establishment, will help mitigate the effects of a wet December.
The study highlighted that a dry and sunny April with reduced rainfall of 50mm was associated with a yield increase of 0.2t/ha, likely due to drier conditions helping to avoid an over-large canopy and sunny weather increasing pod and seed set in early crops. “It’s so important for light to get down through the crop canopy as its interception is crucial from mid flowering onwards for seed set and fill,” she advised.
She also suggested that dry and sunny weather in April could possibly lower the risk of sclerotinia.
To mitigate the effects of a wet April, growers should try to increase light penetration by preventing over-large canopies – targeting a GAI of 4–4.5 at the time of flowering – by delaying nitrogen fertiliser applications and use of a PGR when GAI is exceeded.
Careful timing of fungicides to minimise sclerotinia is also important.
Wet and cool May
The study found that a wet, cool and dull May, with an average increased rainfall of 15mm, 1ºC cooler and reduced sun hours by 15 per cent was associated with an increase in yield of 0.12t/ha.
“It’s probable that cooler conditions delay the onset of crop development during the important growth stage and therefore allowing a longer duration for the crop to set more pods and seeds,” Dr Clarke pointed out.
“Wetter conditions also provide greater water supply for seed filling during summer,” she added.
In the case of a hot and dry May, she suggested that growers should maximise photosynthesis and canopy duration using fungicides and late foliar N to delay senescence. “Mid-flowing (late april/early May) is key for promoting seed number through light interception, and then seed size later. The more the period is lengthened then the more time there is to fill,” she said.
PGRs can stimulate lower branching to increase the numbers of pods and seeds, Dr Clarke concluded.