THE CLIMATE KELPIE BLOG: Rainfed cotton—the goldilocks crop

Posted by BCG on 9th December 2021

It appears rainfed cotton could be the goldilocks crop when it comes to planting…  it requires detailed planning and conditions that are just right to get it up and away.

HMAg agronomist Rob Holmes from Moree in the northern region of New South Wales, said cotton needs warm, wet soil to germinate and survive: “Cotton seedlings are very weak so the conditions for sowing cotton need to be just right. They have zero tolerance to frost. We regularly use the Global Forecast System [GFS] and Bureau of Meteorology [BoM] from the end of September to mid-to-end of November to try and ensure the seedlings have the best chance of establishing. We have warm fronts that come through here, followed by cool fronts, after each warm front the cooler front gets warmer as we move closer to summer. As the seedlings need warm soil to establish, approximately 12 degrees and rising, we can’t risk the cool front being too low otherwise crop establishment will suffer.”

Cotton seedlings

The catch, Rob said who works with 40 plus growers in the area, is that as growers are waiting for the cool fronts to rise to conducive levels, their soil moisture is drying up: “This is where we use GFS and BOM rain forecasts to see if there is any rain coming in the next couple of weeks. If there is, growers may wait and plant after the rain, which could mean they’re sowing in November which is the ideal window to sow as plants will likely reach maturity in April, the driest month, and therefore less likely to be downgraded to due rain causing damage to the lint (cotton fibre). The November sowing also means the cotton would be in boll fill after the proper heat of summer.”

If there is no rain forecast and the profile is drying up, rainfed cotton growers may still choose to plant provided temperatures are favourable which would result in an early maturing crop—a risk that many take. If, however the forecasters are predicting cooler conditions, these growers are likely to delay sowing. “Growing cotton, particularly rainfed cotton is as much about managing your risks as it is about growing the crop. The stakes are high for rainfed cotton growers: Frost and it’s dead, too cold a soil and it’s dead, lack of moisture, dead.”

Cotton is effectively a ‘desert plant’ that thrives on bright sunshine and soil moisture, however the seedling is vulnerable to cold conditions and disease until it gets established.

“We also can’t sow it dry. If we did and then got 20 to 30mm of rain like we often do here during summer, followed by a 42-degree summer day, the soil crusts over and the delicate seedlings don’t have the vigour to get out of the ground. So once again, the crop could be a failure,” Rob said.

“As you can see it’s a very tedious crop to get going but once it’s established, you practically can’t kill it,” Rob said, “but it still needs the right conditions to yield.” For the rest of the cotton growing season growers rely less heavily on forecasting tools “as cotton has such a long growing season (six months), rainfed growers find it fruitless to utilise other forecasting tools”.

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Managing risk

In addition to not to planting at all, rainfed cotton growers also employ other strategies to manage the risks created by climate variability.

“We have an average annual rainfall of 550mm to 600mm but we can get that all in one week or not at all,” Rob said. “Our rain is also ‘generally speaking’ not season based but falls evenly across the year.”

Growers are also interested in forecasting tools which predict how much summer rainfall we will get, for example they will be watching closely whether forecasters are predicting a La Niña or El Niño as this may affect how many hectares of cotton they decide to plant each summer, or the likelihood of success.  “However La Niña /El Niño normally affects late winter and spring rain more than summer in our region so it’s still a bit of a guess,” Rob said.

“If growers decide not to plant, they will then manage that paddock through the summer to conserve moisture for next year’s winter crop, usually meaning the paddock will have been fallowed for a whole year and therefore setting up next year’s winter crop.”

Most rainfed cotton growers also spread their risk by growing winter crops such as wheat, barley, faba beans and chickpeas and run livestock to ensure their business has more than one income stream.

Growing crops in a winter and summer cropping zone in soils with +200mm Plant Available Water (PAW), the crop residue load can be the difference between high fallow efficiency, particularly as rain is falling in storms with greater intensity. Much of the risk management employed by growers is done in the preceding crops to ensure success when growing a high-value, high returning crop such as cotton. Every drop of PAW in the profile and falling in-crop counts.

Seasonal models are also used by growers and advisors to assess risk each month in the cotton industry’s monthly publication: CottonInfo Moisture Manager. The publication’s author, Jon Welsh, said: “While the two most recent La Niña’s (201718 and 2020) have been popgun events delivering little, model guidance certainly has been more convincing and it’s nice to see the BOM look back at the model failure in November 2020 to see what went wrong.”

Rob Holmes in a cotton crop west of Moree, soon to be defoliated with BoM scientist, Dr Jing-jia Luo (pic courtesy Jon Welsh).

Irrigated cotton

Farmers who grow cotton on irrigation of course use forecasting tools differently. While all cotton growers need rain for their crop to grow well, those with a license, or more specifically, those with WATER on their license, can choose when to irrigate their crops.

Therefore, while irrigated cotton growers do use forecast models such as the GFS temperature global forecaster, they will use it differently to rainfed cotton growers Rob explains: “The growers with irrigation will be using forecast models to determine whether they will use their water to get their crop established or save it for later in the year.

Irrigated cotton growers will also use GFS throughout the season to help them determine when to irrigate: “While heat can affect cotton yield with plants dropping flowers and bolls during intense periods of heat, having an adequate profile of water reduces stress on the plant which means it’s less likely to drop flowers or bolls meaning yield impacts are lessened. Therefore, those irrigating crops will be keeping a close eye on the short-term temperature forecasts to ensure that if they have the water, they supply their cotton with water before/during these high temperatures.”

Tracking the MJO can also help track heat waves and potential rain events when cloudiness may build to Australia’s north and the tropics are more active. Heat waves can dramatically increase crop ETo and shorten irrigations which can help manage soil water deficits. Knowing the phases of the MJO can be a useful guide in-season.

Irrigators also use a vast array of soil moisture monitoring sensors that aid in determining how and when the crops are using the soil moisture.  This then helps when deciding when to irrigate efficiently.

More information:

Rob Holmes: 0429 849 174

Jon Welsh: 0458 215 335

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