Her­bi­cide use in dry crops

Warwick Daily News - South West Queensland Rural Weekly - - Rural Weekly - PAUL MCIN­TOSH

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.

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