Peter Griffith

Predicting harvest date for oilseed rape is always tricky and this year will be no exception. However, researchers have found that there is a strong correlation between temperature and the time taken for seeds to develop. Recent work by a team of French researchers has looked at the whole development of the oilseed rape crop from seed to seed in an attempt to model crop growth. One factor that has come out of the research is that from the end of flowering until when seed is ready to harvest takes around 950 day degrees.

Day degrees are a way of working out thermal time and are calculated by taking an average of the minimum and maximum temperature in any one 24 hours. The figure of 950 is very comparable to a study in the Netherlands done in the late 60’s when crops were monitored across northern Europe. Work in the UK also shows that temperature is the main factor determining the duration of seed development.

So can this information be used to predict harvest date this year? Taking met office data for recorded temperatures in May and June at specific sites across the UK it is possible to calculate the number of day degrees that crops have accumulated this year. Thus if a crop in Oxfordshire finished flowering in late May then it is highly likely that the most mature seeds will be ready to harvest soon after the 20 July.

However, it is worth remembering that flowering in oilseed rape follows a set pattern. Flowering starts on the main stem working from the base of the inflorescence working up (acropetal). So the most mature pods will be at the base of the main raceme. Flowering on the side branches starts with those at the top of the crop and moves down to lower branches (basipetal). As harvest approaches the most mature pods will be at the base of the inflorescence on the main stem whilst the most immature will be on the top of the inflorescence on the last formed side branch. It is the ability to keep forming side branches, with side branches on side branches that gives the crop a long flowering period. Photoperiod and temperature determine the length of this branching process.

The data used in this exercise is designed to see if there would be much regional variation in crop development. England is divided into eight regions, Scotland into two and Wales treated as one block.

Table of Regions and weather stations used to calculate thermal time.

The data from the weather stations for each region was averaged to give a daily regional figure and then summed to show a weekly total (Fig 1).

Figure 1 – Weekly accumulated day degrees by region for May and June 2013

image expired

The graph shows the degree of variation between the regions on a weekly basis ranging from a low of 13 day degrees during the third week of May to a high of 20 day degrees in the second week of June. This is equivalent to extending the week by two or three days extra growth.

Running the model.

Predicting a definitive harvest date from this data is expecting too much, but by running the model the data gives a range of pod maturity dates based on when flowering ended. The table below gives a date when the pods are likely to be fit to harvest based on end of flowering date. The prediction assumes a hot July and August with an average daily temperature in the region of 19 – 21 degrees (min 16 Max 24). It is only a prediction based on the model output.

Predicted seed maturity dates based on end of flowering date

Back to that crop in Oxfordshire, if the main raceme finished flowering say on 21 May then these pods will mature around 17 July (950 day degrees later). But if side branches didn’t finish flowering until 21 June the pods from these flowers will probably not mature until 3rd August.

References

• Annals of Botany 107, (2011)
• Field Crops Research 54, (1997)
• Field Crops Research 67, (2000)
• Plant Physiol. 90, (1989)
• Met Office climate data (2013)
• Stellingen behorend bij het proefschrift van Barbara Habekotté: (1996)
• HGCA Oilseed rape Guide
• Canola growth & development (2011) State of New South Wales Department of Primary Industries