One Years Weed, 10 Years Seed!
June is a good time to take stock of the year gone by.
Black grass and ryegrass are so successful as grass weeds because like them, cereals are grasses.
What is good for winter wheat and winter barley establishment is also good for grass weeds. September-early October drilling gets your crops off to a great start but also sets you up for a bigger grass weed burden.
Drilling from mid-October onwards will reduce the grass weed burden but also makes for more difficult cereal establishment. We have seen too many poorly established late sown cereal crops from the last two wet Autumns. These less competitive crops can still end up infested with grass weeds, albeit fewer grasses in number, but with more seed heads given the extra space around them.
We have over-relied on selective herbicides such as Atlantis which by their very nature run a fine line between controlling grass weeds and damaging the crop. And every year of herbicide use, naturally selects the resistant individuals which produce thousands of seeds per plant in preparation for the next crop, driving the problem harder.
But, in making life hard for grass weeds, we inevitably run the risk of poor cereal crop establishment.
Spring cropping undoubtedly helps to reduce the grass weed burden. But once again, forcing spring crops into less-than-ideal conditions leads to poor establishment, less competitive crops and grass weeds continuing to seed.
What is the answer to this dilemma?
Be ruthless in preventing seed return. The appearance of patches sprayed off in crops is now seen as acceptable farming not failure by those looking over the hedge.
Where levels are low, hand rogueing is well worth the effort.
Map areas of infestation for future reference.
In the ‘nightmare’ scenario, extreme grass weed problems require extreme solutions. Going forward with cropping you must stop further seed return. Avoid winter cropping badly infested fields.
So double spring cropping, ideally with competitive spring barley will recover the position in a badly infested field. Ignore what the gross margins say on the spreadsheet. They are meaningless if you cannot crop the field with winter sowings longer term. If you have livestock or an AD plant nearby, maize is a very good spring crop option for breaking the germination cycle of black grass. Ryegrass is a very different beast and is equally happy to germinate from spring sowings, albeit lower numbers. Oilseed rape is the fastest way to multiply up grass weed problems as grass weeds just love that early sowing. It is no longer a cleaning crop. By the time soils are cold enough for propyzamide applications, the grass weed roots are out of sight. Control from earlier application of Centurion Max is waning fast, expect 50% control at best.
Cover crops do not solve grass weed problems. The main benefit comes from the timing of the crop sowing in the spring and the ability to spray off overwintered grass weeds with glyphosate. Normally two doses are required predrilling.
Stale seedbeds help to get the numbers down but without late summer moisture it is a painful waiting game. You must hold your nerve where grass weed numbers are high. And shallow cultivation post-harvest is the key, 50mm(2”) maximum which is asking a lot of any cultivator or operator to work at that depth. Ask yourself if you were sowing a grass ley what would you do to get the grass to establish? Shallow cultivate to conserve moisture and roll after sowing. So why do any different to get grass weeds to grow??
‘Pressing the re-set button’ with ploughing is fine provided you have not been mixing the grass weed seed through the profile through previous cultivations and you plough it back up again. True inversion ploughing with no future deep cultivation can buy you a few years. Headland ploughing is always difficult and less effective.
For lower grass weed or patchy problems where Autumn sowing is still possible, getting the seed rate right for the soil conditions gives you a sporting chance of a competitive crop. Varying seed rate at drilling using knowledge of your fields or precision farming methods will help. Higher seed rate in the worst grass weed areas reduces the seed return but does not solve the problem.
Maximise the control from residual herbicides by keeping the grass weeds on or near the surface, mix and stack the actives used and pray for rain post application, but not too much to wash herbicides around the seed zone and damage the crop! With everything in your favour, you will get around 80% control. Remember, you need 98% control to stand still on grass weed numbers.
Avadex is increasingly important against black grass with still no known resistance. Application timing is tight so applying from the back of rolls or drills gets the job done when labour and time are short without waiting for contractors. Liquid Avadex Factor is a good stacking option with other pre ems but is not as effective as Avadex Excel granules. A seed depth of 40mm becomes critical using Avadex for winter wheat, less so for barley. But as with all pre ems, soil moisture is key to success. See the relevant chart on the NIAB TAG poster below.
Once Black grass or ryegrass has emerged, the only effective chemical treatment (but for how long?) is glyphosate.
And patience is a four-letter word Autumn or Spring. We can all farm well with hindsight.