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Winter-planted sorghum and weedy oats

In many environments and seasons, planting sorghum in the traditional window of September or October can result in crops flowering and trying to fill grain when the temperatures are very high and soil moisture is depleted.

Having experienced such seasons frequently over recent decades, some growers are considering planting at the minimum recommended soil temperature of 13 degrees C, as early as August (depending on the locality), to avoid these harsh finishes. In doing so, these early planted crops will likely encounter winter weeds such as sterile oats (also known as black oats and wild oats).

Queensland Alliance for Agriculture and Food Innovation (QAAFI) researchers Gulshan Mahajan and Bhagirath Chauhan, investigated the impact of wild oats in a winter-sown imi-tolerant sorghum crop and tested the efficacy and crop safety of several pre-emergent herbicide options.

Their study found that three options provided excellent control and increased yield in imi-tolerant sorghum. The first was atrazine (Group 5 [C]) alone, the second was imazamox plus imazapyr (Group 2 [B] e.g. Intervix) alone and the third option was sequential applications of atrazine applied at planting followed 16 days later by imazamox plus imazapyr.

These three options reduced the wild oats biomass by 93, 96 and 100 per cent and increased sorghum yields by 116, 136 and 140 per cent, respectively, compared with the nontreated control.

These herbicides are registered for use in sorghum against wild oats and almost eliminated weed seed set. Note: Intervix is only registered for use in imi-tolerant sorghum. Other pre-emergent herbicides registered for wild oat control in other crops were either phytotoxic to the imi-tolerant sorghum or provided only a moderate level of weed control.

The results demonstrated that wild oats can establish in winter-sown sorghum crops and substantially reduce crop yield if left uncontrolled, potentially adding over 900 seeds per m2 to the seed bank. Most of the wild oats plants had dropped their seed before sorghum maturity, suggesting that its complete control is essential to reduce seed build-up in the soil.

While atrazine and Intervix applied singularly provided excellent weed control, repeated use of either herbicide will apply high selection pressure and could result in resistance unless other weed control tactics are also implemented. This study demonstrated the value of ‘sequential’ herbicide applications from different mode of action groups where the atrazine (applied at sowing) followed by Intervix (applied 16 days after sowing) provided the best control.

WeedSmart northern extension agronomist Paul McIntosh says seed quality is a major consideration when sowing into soil cooler than 16 degrees C. “Some growers were successfully sowing sorghum in later winter several decades ago, particularly in the western Darling Downs and North-west NSW,” he says. “The key is to ensure soil temperatures are rising, sow high-quality seed with excellent germinability and minimise any stress on the young seedlings. Aim to establish the highest plant population suited to your area and ensure all the agronomic parameters are met, such as adequate soil moisture and fertility.”

While sowing sorghum in late winter is not a general recommendation, this study has demonstrated that it is an option for growers to trial and that herbicide options are available to manage wild oat that is still likely to germinate at this time of year.

Along with mixing and rotating herbicides, diversity in crop rotations (including altering sowing times) is another WeedSmart Big 6 tactic to manage the weed seed bank.

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