Thinking about oilseed rape – and neonics

Last week we finished harvesting our rapeseed, including the field that was half neonic and half not – it was discussed in some detail here. It was done in two bites, separated by 4 days, as one half was a lot riper than the other. The overall yield was a poor 2.56t/ha, but this was largely down to about a quarter of the field having been severely pigeon damaged in the cold spring. Anyway, here’s the yield map,

Dark red (showing where the pigeons did their worst) is less than 2t/ha, with yields increasing up to around 4t/ha with the light yellow and very small bit of green. This yield map has been calibrated with weighbridge results, so should be pretty accurate

I started to write this post analytically, but it was messy, waffly, and almost entirely without merit. So I’m going to cut it much shorter.

The neonic treated side of the field looks to have yielded more than the untreated – it is the right half as shown above divided by the thin blue line. There are several differences between the two sides of the field

  • Seed rate – higher in the treated section
  • Drilling date – later in the treated section
  • Seed treatment – neonic in the treated section (amazing)

My personal feeling is that it is the drilling date which has caused the yield effect, as we generally found here (as did other farmers anecdotally) that the later a field was drilled/harvested, the better it has yielded. I’d put this down to the weather in the spring, and later crops getting more sunshine at the important times.

However…I can not, and will not, say for sure that’s what has done it. It is very frustrating to have these other two differences now. I believe there are scientific studies into the yield effects of neonic treatments which will be published soon.

The other interesting thing that come up this year is what happens when you look at the other field that I measured in the original neonic post back at the start of the year. It too yielded a similar amount, 2.6t/ha. It too had very bad pigeon damage.

Dark red (showing where the pigeons did their worst) is less than 2t/ha, with yields increasing up to around 4t/ha with the light yellow and very small bit of green. This yield map has been calibrated with weighbridge results, so should be pretty accurate

The scale here is the same – dark red is less than 2t/ha, with yields increasing up to around 4t/ha with the light yellow and then up to over 6t/ha on the blue. White was so bad it was not even harvested.

The area I tested, and found to have low levels of CSFB (cabbage stem flea beetle) larvae (3.6 and 1.5 larvae per plant), was actually one of the worst yielding areas of the field. This was the bottom left corner, where the yields were around 2-2.5t/ha. There does not seem to be any difference between the areas that were sprayed once, or twice, with a pyrethroid insecticide.

What I find amazing about this map though is how it totally confounds what I felt about the year. I thought the lacklustre yields were all down to the weather conditions, but in this field we have significant areas yielding over 6t/ha. Obviously I was wrong, and the weather was OK if everything else worked too.

Do I think it was CSFB? No. Here’s why – take a look at this photo from Google Maps.

Screen Shot 2016-08-05 at 11.41.19

This photo was taken a couple of years before we started farming here, when this field was split in two. It is very, very clear that the difference in yield between the two old fields is HUGE. The bottom field was actually rapeseed the year before we took it over, in harvest 2011. So it has had a 4 year break from OSR – but still something is causing it to yield much lower than the top field which has had a longer (but unknown) break.

I do not personally believe that CSFB, or any other insect, respects historic field boundaries: the answer must lay in the soil. Is it disease? Nutrients? Structure? I don’t know. What I do know is that without this demonstration, I would be blaming weather, when that’s clearly not (entirely) the case. It is a cautionary tale to not single out any particular factor without clear evidence. I could have quite easily blamed CSFB, as many people are this year. To be clear though, we have one field with very low larvae numbers, some of which is yielding 2t/ha, and some 6t/ha. On the other hand we also have a field where neonic dressed seed yielded more than undressed.

So who the *&$% knows? Not me.

[In case you were wondering, this is still the un-waffly version]

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