Low December rainfall, sunshine in April and a cool May will give oilseed rape yields the biggest boost, a study has found.
Changing weather patterns are one of the biggest challenges facing farmers, but new information from ADAS and BASF suggests growers can, to some extent, mitigate against poor weather through good winter soil management and optimising inputs in the summer.
The study, which reviewed the impact of weather incidents and associations with yield between 1979 and 2017, found specific weather conditions at different times throughout an oilseed rape plant’s lifecycle can contribute to 37% of yield variation.
Dr Christina Clarke, ADAS says: “We have seen a steady rise over the last 15 years in OSR yields, but we are still seeing this gap between what is being achieved on farm and what is being achieved in RL trials.”
Reviewing temperature, rainfall and sun hours from Met Office mean weather data, the study looked at what combination of weather parameters explains variation in yield – and how growers can still maximise yield potential in years of poor weather.
Increased temperatures of 2degC in October were found to be associated with a yield increase of 0.17t/ha, which Dr Clarke put down to increased autumn growth resulting in vigorous and strong plants going into the winter.
“Good seedbed preparation is important, creating a good tilth to maximise seed to soil contact and improve the water-holding capacity of the soil. Time sowing to make the most of the moisture in the soils so, if you have good seed to soil contact, that will get germination off quickly as plant growth starts to slow. Choosing varieties with high vigour will mitigate against cooler autumn temperatures,” says Dr Clarke.
This is because plants with bigger biomass are not so affected by cold and frosty conditions and can better defend against pest pressure.
“Going forwards into the spring, the crop also already has the resources in the canopy to ’kick off’ quickly when sun hours and temperatures start to increase,” Dr Clarke adds.
In December, a reduction in average monthly rainfall of 50mm was found to be associated with yield increases of 0.11t/ha, thought to be due to less waterlogging and soil overfilling.
“Waterlogging leads to reduced aeration in the soil, inhibiting growth. Unlike cereals, oilseed rape has no root aerenchyma, the large air space in the root that allows oxygen to flow from the root to the shoot, so low oxygen reduces the functioning of the stomata and plant growth slows,” says Dr Clarke.
This results in stunted root growth which will affect water and soluble nutrient capture later in the season, adds Dr Clarke. It is a key time for root growth because, once the crop starts to flower, energy is put into seed production and root growth slows, which is why good plant establishment is key, says Dr Clarke.
“Reducing compaction and preventing reduction in soil aeration will help, but it is more important to reduce the compaction at deeper soil layers,” Dr Clarke advises.
“Cover crops create natural channels and pores which are essential in improving root growth in deep soil layers. Crop residues will also promote earthworms.”
A ‘warm’ March, with increased average minimum temperatures of 2degC, saw yield increases of 0.10t/ha.
“This is likely to be due to earlier onset of spring growth without interruptions in growth and reduced risk for frost damage,” says Dr Clarke. “During this period, we would recommend an early nitrogen application to help stimulate early growth and minimise risk of light leaf spot by optimising fungicide strategy.”
A dry, sunny April with reduced average rainfall of 50mm was associated with a yield increase of 0.20t/ha because drier conditions delay uptake of nitrogen, preventing overlarge canopies. The sunny weather is a critical point because it helps to increase pod and seed set, the major yield determinant in OSR.
Increased rainfall of 15mm in May, combined with 1degC cooler temperatures and a 15% reduction in sun hours, were associated with yield increases of 0.12t/ha. However reduced sun hours were unlikely to have a direct positive impact on yield, says Dr Clarke.
“It is more of a case of increased rain and cloud and dull conditions coming hand in hand. These cooler conditions allowed longer canopy duration to delay onset of crop senescence to set and fill more seeds. Wetter conditions later on is really important for seed fill, going back to sunny April where the crop set lots of seeds, it is important you have a water supply later on in the season to then fill those seeds.”
To buffer against a dry and hot May in the later growth stages, growers should maximise photosynthesis and canopy duration potential using fungicides and later foliar nitrogen to delay senescence and increase light interception, particularly in the leaves, says Dr Clarke.
Survival of leaves is important at this stage, because photosynthetic efficiency in leaves is much higher compared to the pods and during this growth stage, oil rich seeds have a high energy demand.
“Use PGRs to stimulate lower branching to increase numbers of pods and seeds. Mid flowering is important in OSR, because this is when seed number is determined. Seed size is determined shortly after and the more that period is lengthened, the more your seeds have time to fill – as long as there is enough water and the canopy is green enough.
Plant growth regulators (PGR) should be applied if canopies are overly large to reduce apical dominance and encourage stronger branching and better light penetration, according to Clare Tucker, BASF.
Ms Tucker says: “In high yielding crops, 60% of that yield is coming from the lower canopy, so we need to make the most use of light to this area.
“Early sown crops at low seed rate are more likely to be of reasonable population and will not lodge but, if they have a big canopy, they will need canopy manipulation.
“Big crops will just reflect light from the canopy. This means it does not get down to the leaves and you get early loss of leaves over the summer. If you can generate your optimum canopy at flowering, which means you have good light penetration, then leaves will go over the summer for as long as possible.”
Ms Tucker says OSR plant growth regulator Caryx (metconazole + mepiquat chloride) can be applied to actively growing crops with a 0.8 GAI at the beginning of stem extension.
“Applying Caryx at the start of stem extension will give you shorter crops. If they have really big canopies and they are at high risk of lodging, we suggest earlier timing at stem extension, whereas if they are more like GAI 0.8 then that might mean at yellow bud timing where there is more canopy effect, especially if you have flea beetle.”
Delaying senescence will have a direct impact on yield, according to YEN data which shows the % highest yielding 50% of crops had an extra 10 days between flowering and desiccation, with as much as 1t/ha difference, compared to the lowest yielding 50%.
“This research gives growers the option to mitigate and buffer against non-ideal conditions,” says Dr Clarke. “Vigorous plants, minimising risk of waterlogging and canopy management will all help to buffer against non-ideal conditions.”
BASF are currently undertaking studies to determine whether Caryx applications can compensate against the impact of flea beetle.
“If you can encourage secondary branching, that may help to compensate against flea beetle. What we suggest for people worried about flea beetle is to see how the crop is growing from March through to April, then at yellow bud timing you will get a better idea of what damage you have, so you have more time to wait and monitor the crop,” says Ms Tucker.
If cabbage stem flea beetle is not an issue, then managing secondary branching is still beneficial to get a more even spread of yield across the whole plant, she adds.