Just before Christmas I heard a radio programme about the Druid celebration of the twelve days around the winter solstice. They celebrate the twelve days when the sun stands still.
This got me looking up data on the angle of the sun and daylight hours. Between 16th and 26th December 2013, the angle of the sun was almost static in Cambridge and the length of daylight varied only by about a minute. The same is true, of course, for the summer solstice and this coming June, the angle of the sun will not change at all in Cambridge between 16th and 26th and daylight length will vary by less than a minute. The nearly 17 hours of light per day coincides with the maximum rate of grain fill in wheat. This is not a coincidence; breeders have selected lines that are filling grain at this time.
We are still, in terms of daylight length, in the depths of winter but growth is occurring. I’ve recently heard from one farmer who drilled a large area of wheat around 11th December. This New Year morning he sent me a photo showing that it’s almost emerged. Coincidentally, the wheat variety is Solstice.
The late drilling date was an attempt to reduce blackgrass populations. Three applications of glyphosate were made, the last one immediately prior to sowing. I saw the fields just before Christmas and the blackgrass looked worryingly green but the glyphosate has now done its job.
Weather during the winter does not have a consistent effect on the efficacy of all herbicides. Some will provide little or no control and sometimes the efficacy of other herbicides is enhanced. Isoproturon applied on a cold but bright frosty morning, when the soil was frozen, was often magically effective. The most effective blackgrass control I ever saw from a particular ‘fop’ herbicide was when it was applied on a cool but very foggy day.
The activity of isoproturon in such conditions may be explained by the fact that it is a photosynthetic inhibitor and the bright conditions enhanced its activity whilst the cool conditions slowed down the breakdown of it and its metabolites in the plant. The activity of the ‘fop’ may be explained by excellent penetration into the blackgrass plant over a longer period of time because the spray did not dry on the leaf. These examples suggest that warm growing conditions are not always the pre-requisites for good herbicide activity. Glyphosate is not bad on annual weeds during the depth of winter but it can take a month or so to get good knockdown. Warm growing conditions seem more essential to control perennial weeds.
There is debate about the impact of cool and short days on the activity of the ALS inhibitor herbicides, particularly the sulfonylureas. When Lexus (flupyrsulfuron) was in its pomp, there was a dip in its control of blackgrass in mid-winter. The current debate is on the impact of cold weather and/or short days on the control of blackgrass with Atlantis (iodosulfuron and mesosulfuron). We’ve not had sufficient experience without the complicating factor of resistance to come to firm conclusions. Certainly it works more effectively in warm conditions when the weed is actively growing. It is also surprisingly effective in relatively cool conditions in early March, suggesting that rapidly increasing lengths of daylight also encourage control.
Danish researchers are convinced that the length of daylight explains why they can use super-low doses of ALS inhibitor herbicides to control broad-leaved weeds in the spring. I have looked up the tables and their daylight length exceeds ours from late March onwards and becomes significantly longer at the end of April when they apply most of their broad-leaved weed herbicides. We tend to apply our broad-leaved herbicides two or three weeks earlier because of our milder winters. The Druids burn Yule logs to brighten the shortest days of winter but I suggest the light they emit would not be enough to lift the performance of ALS herbicides on broad-leaved weeds!