Arable

Controlling black-grass whatever the weather Anyone dealing with black-grass probably has a mental list of ‘good’ and ‘bad’ seasons for weed control. There are several reasons why control varies year-to-year but autumn weather conditions are high on the list. “Autumn weather dictates when you drill and influences the activity of preemergence herbicides,” says Bayer’s Darren Adkins. “During autumn, you have to respond to conditions to maximise control because the weather is a rule unto itself. “Obviously, a wet autumn can make drilling wheat in the second half of October difficult, on the other hand dry conditions can reduce the efficacy of pre-em chemistry. What we are hoping for is the sweet spot with just enough moisture for herbicides to work and crops to establish well.” It’s quite easy to imagine what this ‘sweet spot’ is like because autumn 2016 had near perfect conditions for late drilling and herbicide applications across much of the country. Conditions were settled and quite dry but there was enough rainfall at the right times for crops and herbicides. The favourable conditions in autumn 2016 are demonstrated by results from Bayer field trials which report the best ever results for pre-emergence applied Liberator (flufenacet + diflufenican) in over a decade of trials.

Soil moisture Sufficient soil moisture helps maximise the efficacy of the flufenacet in Liberator. The active is taken in by the weed roots so soil moisture allows it to move throughout the germination zone. Another consideration is the residual activity of the herbicide; it continues to be active for a number of weeks. The exact time depends on the

temperature with warmer conditions promoting faster breakdown of the active. “With the shift to later drilling, it would be very unusual to have insufficient soil moisture for pre-ems to work in crops drilled in October, on top of that soil temperature should be cooler by then so the herbicide will give prolonged residual activity,” explains Mr Adkins. “But for people drilling wheat in September and in winter barley crops there is more of a risk of reduced herbicide performance because of dry soils.” The danger from dry soil is shown by results from the 2011/12 season. The summer and autumn was very dry (map top) and, as a result, Liberator performance (0.6-litres/ha at true pre-em timing) in trials was way below anything seen before or since. “2011/12 was an anomaly with unusual conditions as, apart from 2011, the level of control doesn’t vary that much year-to-year. Last season, was particularly good but not a massive leap from previous seasons. In many ways, the more interesting story is why do final black-grass levels vary so much after a successful pre-emergence programme?”

Different control The last two autumns both had good conditions for late drilling and residual herbicides in many parts of the country; however, the final levels of black-grass were very

different. On most farms, 2015/16 was a ‘bad’ year while 2016/17 was ‘good’ but the interesting thing is that autumn conditions and herbicide performance were very similar so the decisive factor came afterwards. Climate data (below) shows that winter 2015/16 was exceptionally mild with rain during the spring at the post-em timing. It seems that the warm winter was perfect for black-grass to continue to grow and outcompete wheat crops by tillering vigorously. Winter 2016/17 was what you might call ‘normally mild’; The summer and autumn of 2011 was very dry and, there were some frosts which as a result, pre-em herbicide performance in trials slowed black-grass growth, was way below anything seen before or since. and finished off some plants weakened by the herbicide, and in the spring crops tillered more vigorously which competed with blackgrass growth. But when the crop is drilled, we don’t know what winter holds in store so what can growers do to get the best control? “Whatever the conditions, the basic plan doesn’t change for dealing with black-grass – aim to drill after the main flush of black-grass then apply Liberator and any mix partners within 48 hours. If it’s dry, make sure there’s Climate data shows that winter 2015/16 was enough moisture for the exceptionally mild. residual to work and be wary when conditions are in our favour, we of repeated stale seedbeds that might can get good control. Now we need dry the soil out. If it’s wet, compaction to keep up the pressure on blackduring late drilling might become grass and hope that the weather an issue. helps us this season as well.” ■ “The last two years show that

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