A short flowering period may be key in maximising green area duration, but how else can growers boost rates of photosynthesis during this time? Abby Kellett reports.
Results from last year’s first ever oilseed rape Yield Enhancement Network (YEN) competition combined with recent research into OSR flowering highlights the advantage of a short flowering period.
Analysis of the YEN results in 2017 revealed the top performing crops had 10 extra days from the end of flowering through to desiccation – enabling plants to fill seeds more effectively. These seeds were typically bolder and with a higher oil content to increase gross output per hectare.
Therefore, one of the key targets for growers is to consolidate flowering into a shorter but more intense period, says Georgina Wood, Syngenta’s OSR field technical manager.
She says: “Studies have shown that, unlike cereals where plants can move stored energy around and up to the grain heads, in oilseed rape all yield and seed oil content is set by light capture and utilisation after flowering.
“It can be hugely beneficial to start flowering a few days later when the risk of frost is reduced, and then finish earlier to open up the green leaf area to more light.
“The flowers themselves reflect light away from the crop, so actually reduce a plant’s photosynthetic activity. The shorter the duration of flowering, the more light the crop can absorb.”
To help facilitate a shorter flowering period, she suggests using a PGR at flowering, providing crops are not stressed. In doing so, growers can expect to achieve a more consistent crop, so even though the onset of flowering on the main raceme may be delayed, the side shoots should flower at the same time, leading to a more intense but shorter flowering period.
“The PGR encourages stronger side branching and more accessible photosynthetic green area,” she adds.
Ensuring crops reach a green area index (GAI) of 3.5 by mid-flowering is also key in maximising the yield potential of the crop. However, this season the growth of many crops has been stunted due to wet and cold soil conditions, therefore some crops have struggled to produce sufficient levels of biomass, according to Yara agronomist Natalie Wood.
She says: “We have a lot of smaller canopies this season so it is even more important to maintain green leaf area for as long as possible in order to optimise the rate of photosynthesis at this critical time. Applying a well-timed foliar nitrogen dressing towards the end of flowering can help extend the life of the canopy.”
An ADAS review of 27 experiments showed that applying late foliar N at 40kg N/hectare increased gross output yield by 0.25 tonnes/ha on average, with some responses as high as 0.7t/ha, she adds.
Late foliar N should be applied anytime between flowering and two weeks after the end of flowering, so it is possible to tank mix with a sclerotinia spray and save on application costs, she advises.
While flowering sprays often focus on sclerotinia control due to its potential to cause significant crop damage, outbreaks tend to be less frequent than light leaf spot, which affects all parts of the plant and can result in yield losses of up to 50% if left uncontrolled.
Inadequate control also increases infection pressure in next season’s rape crops when infected air-borne ascospores are released from crop debris and stubbles at harvest. Furthermore, the disease is no longer confined to northern regions of the UK, according to Hutchinsons.
Curative treatments have little activity against light leaf spot or sclerotinia, so well-timed protectant sprays are the only in-season way of reducing infection risk and protecting developing pods.
Hutchinsons northern regional technical manager Cam Murray says: “A prothioconazole/tebuconazole-based fungicide at early stem extension is often the first line of defence to prevent disease travelling up stems. This should be supported with two flowering fungicides; the first of which is best applied at early flowering, followed by a second spray around three weeks later to safeguard crops throughout the flowering period, which typically lasts about six weeks but varies depending on conditions.”
While prothioconazole remains the strongest option against light leaf spot, a non-triazole alternative such as azoxystrobin can boost sclerotinia protection and may also improve green leaf duration, Dr Murray adds.
Given the increasing levels of triazole resistance in light leaf spot, growers are advised to use alternative chemistry where two or more triazoles have already been used in spray programmes.
Fungicide timing during the flowering period is vital for good sclerotinia control, as products are protectant with little or no curative activity and because the period when the crop is at risk is longer than the period of fungicide activity.
Monitoring services based on weather and crop data can help inform spraying decisions. ADAS pathologist Caroline Young says: “Sclerotinia infection in oilseed rape requires three main factors: the presence of sclerotinia inoculum, warm and humid weather conditions and crops in-flower. When each of these factors occur simultaneously, crops are vulnerable to infection.”
ADAS and BASF are using weather and crop information from seven sites across the UK to help inform growers of when the risk of sclerotinia infection is high.
“We are providing growers with a weekly update on inoculum levels in different regions and information on weather conditions likely to promote infection,” says Dr Young.
OSR is at the highest risk from infection when relative humidity is greater than 80% and air temperatures are at or above 7degC for more than 23 hours.
“We also report on the germination of sclerotia in the soil and the inoculum on the petals. When the sclerotia germinate, they develop little apothecia-like mushrooms which release the spores. The spores land on the petals and we can do petal tests to check the proportion of petals infected and when we see very high levels of around petals infected – about 95% - we know there the risk of infection is high,” she adds.
For sclerotinia monitoring bulletins go to www.agricentre.basf.co.uk
Sclerotinia risk infection forecasts are available at cereals.ahdb.org.uk/monitoring/sclerotinia/sclerotinia-risk-report.aspx
These include weekly reports on forecasted sclerotinia infection risk, alongside commentary on other disease risk factors – such as petal stick and spore release information – during the main risk period.