Herbicide use in dry crops
WHEN did we start using residual herbicides in our cropping scenes in the northern region? Let me refresh your memory. The old 24-D as a knock-down product is probably the first herbicide many of us recall, which was initially discovered and used in the 1940s. Then the triazine group of herbicides was discovered in the 1950s with products such as Atrazine, and closely related products such as substituted urea actives such as Diuron, at a similar time. All had some knock-down capabilities we discovered later on, however their big introduction to the world was as residual type products. In other words they are soil applied basically before the weed seed has either germinated or emerged through the soil surface. These residual herbicides do not kill the seed itself. Very simplistically, the herbicide molecule’s uptake by the weed is first mixing with the soil moisture and then this solution is adsorbed by the root hairs or the shoot moving through the soil profile. So with dry conditions prevailing, there is not much herbicide in these minimalistic soil solutions, so herbicide uptake by a weed right now would be very poor, as it is.
The example you could use here is Balance in your chickpea crop with its complex make-up and its long residual capability in dry times. Balance has a low solubility and high stability in sunlight by the book, however it certainly takes some significant falls of rain to push into the root zone and be adsorbed by the weed roots in the soil solution. By significant, I would judge well over 25mm of rain and more like 50mm of rain in our very dry soils to be fair.
So it is difficult times for our residual herbicides to be effective. Of course, many of us do not like having to commit to these residual herbicides in a fallow decision, as it locks you into a crop that tolerates the herbicide you have chosen for your weed control. For example, if we use Balance with the active ingredient Isoxaflutole in the Group H mode of action. A brilliant weed control product for our chickpea crops and now registered in fallow weed control for activity against fleabane, feathertop rhodes and sowthistle and it can give barnyard grass a real headache with suppression or even better at times. The only problem is the length of time that this herbicide has residual action on weeds, but it can also affect sensitive crops. Not only does the re-cropping of sensitive crops such as grain sorghum have a seven-month interval, it also has a minimum rainfall requirement of 250mm. The other twist is that the 250mm of rain needs to fall consistently over the sevenmonth period for the hydrolysis and microbial breakdown to occur. So a dry growing season for your chickpeas with Balance applied POPSA (post plant, surface applied) as happened in many areas in 2017, then the January plant of grain sorghum could have been very dangerous to commit to. It fulfilled the label advice of seven months, however the 250mm of rain, which may have all happened in late December, then this sorghum crop would have been badly affected by the Balance product applied at chickpea planting time in May. Don’t think I am just singling out Balance, as many of our residual type products do have label guidelines for re-crop intervals, however they also need some common sense. Of course the reason we are using more of these residual type products is the failure of our knock-downs like Glyphosate and other post emergent products. Flame is also a great residual grass control product with the Group B active of Imazapic and is broken down slowly and mostly by microbial degradation in alkaline soils. These soil microbes are relatively large (about 10,000 times bigger than a herbicide molecule) and require water to live in and basically ingest the herbicide molecules floating around in the soil solution. If there is no soil solution, that is a dry soil, then the number of these active eaters diminishes and the herbicide persists.
Of course the why did we start to use our soil residual herbicides more and that answer is easy. There is the choice to use a residual product so the life of the crop is weed-free, hopefully. The other main reason is of course is our knock-down herbicide products are not as effective with herbicide resistance more prevalent every day.
We are in a corner with our 40-year run of popular herbicides in agriculture nearly stalling. To keep it going with our current full suite of herbicides useful availability, we will need to make some drastic changes in our weed control methods. What change will you make in your weed control options and control methods?
HERBICIDE USE: Paul McIntosh discusses the affects of using herbicides such as Balance in chickpea crops.