Beating the screenings bogey by Alex Nicol

Harvesters in field

Wheat growers in southern Australia who had unacceptably high levels of screenings last year should be looking to balance their nitrogen supply to better match the seasonal rainfall. The advice comes from the CSIRO's Anthony van Herwaarden who says that the 10 days before and after flowering can have a significant effect in determining grain size.

"If the wheat plant is short of water, it will set more grains which have a tendency to be small and it doesn't matter how the season finishes, the size of these grains won't increase," he says. He advises checking the screenings to see if they are small but filled grain rather than pinched grain.

"If the grain is small but well filled, then the dry spell around flowering in early October was probably the culprit. If the grain is shrivelled, that's a classic sign of haying off and may be the result of earlier flowering, pushing the crop too hard, or any disease that reduced the crop's ability to use moisture."

Balance needs to be right

Dr van Herwaarden recommends working to get the balance between nitrogen and water supply right for the season. He urges growers to be flexible and not to put all their nitrogen fertiliser on at the start of the season.

"Growers could manage several of their best paddocks more intensively to more reliably achieve higher protein segregations. The soil moisture profile at booting is vital. If there's insufficient moisture at that stage, then any extra nitrogen will be working against the crop if the season remains dry.

"Instead, at the beginning of the season, growers should aim for their average crop yield, deep soil test and calculate a nitrogen budget, then apply up to three-quarters of the nitrogen needed for that crop at sowing.

"If they keep the remainder in reserve until they have more information on the season and the soil moisture situation between booting and flowering, they'll have a better chance of reducing the incidence of screenings."

Simple moisture test

Suggesting that wheat crops in southern Australia use moisture at "about the rate of pan evaporation", Dr van Herwaarden says a simple rule of thumb for estimating the amount of moisture in the soil bank would help growers in their decision making.

"We can use the pan evaporation rates measured by the Bureau of Meteorology to estimate how much moisture the crop is using. We propose that if the water content of the soil — determined from the deep-N test at the start of the season — is added to the rainfall since sampling, we have an estimate of water supply. If we subtract from this total the pan evaporation, then we end up with an estimate of the moisture left in the soil at that point in the season.

"If the soil is estimated to contain good reserves of water between booting and flowering, then growers would have more confidence in topdressing some paddocks with N to maintain yield and protein content."

Program 3.4.2 Contact: Dr Anthony van Herwaarden 02 6246 5097