WIDE variation in growing season rainfall will result in a patchy harvest for many grain growers in Victoria's Wimmera and Mallee.

GrainCorp said this week there had been about 19,000 tonnes of grain, mostly canola and barley, delivered to its Victorian sites.

Weather is threatening the harvest, with NSW farmer John Ridley saying forecasts of 75mm in the West Wyalong region just after headers have begun harvesting a big concern.

Mr Ridley, of Burcher, said if the forecast rain eventuated, it would be the third year in a row a downpour had hit the harvest just as it began.

Victorian Department of Primary Industries Swan Hill senior agronomist Rob Sonogan said Mallee farmers were harvesting canola and some barley, but wheat was still ripening.

Mr Sonogan said initial yields had been satisfactory, except in the state's far northwest where frost had further affected canola yields that had struggled in the dry.

Canola yields ranged from 400-600kg a hectare across the Mallee.

"Farmers are generally more than content with the way the crops have finished in the cooler than average daytime temperatures," he said.

Mr Sonogan said cereals were likely to average 1-1.2 tonnes a hectare on less growing season rainfall than in the big drought year of 1982, when growers harvested nothing.

He said this demonstrated the benefits of changes in agronomic practices over the past 30 years.

Piangil farmers Ross Kentish and Neville Taylor were out harvesting windrowed canola last week.

Mr Taylor said the strong, gusty southwesterly winds were making it difficult to avoid picking up sand with the canola.

Mr Kentish estimated he and his son, Mark, had delivered about 200 tonnes of canola to the GrainCorp silo at Piangil before their header broke down.

They sowed 1400ha of canola this year, more than usual, largely because of a desire to rotate some paddocks out of cereals after eight years of continuous cropping.

Southern Farming Systems Gippsland branch president Ben Morris said many crops in his district had suffered from the reverse problem - too much rain.

Rain during sowing wiped out significant areas of canola and wheat, and crops that did survive inundation were expected to produce below average yields.