Landmark state agronomist Matthew Sparke said lentils suffered the most damage from last week's heatwave but some barley and wheat crops had also been affected.

"Farmers need to go out and assess their crops individually because the damage appears to be based on a series of variables," he said.

"Variables range from soil type to the crop's developmental stage, how much moisture was available and grain varieties.

"This was an event we haven't seen for some time, a prolonged hot spell at a time when there is no chance for plants to recover."

Mr Sparke said areas south of Horsham would probably be hardest hit by the hot spell because many crops were still flowering when it occurred.

Carl Sudholz of Natimuk said he started harvesting canola yesterday and didn't expect to see a lot of heat damage in that particular crop.

But Mr Sudholz said his lentil, barley and wheat crops would be a different story.

"We haven't started harvesting the other crops yet but it is very likely there will be quite a bit of damage in the lentils, it is just hard to know how much at this stage," he said.

"We will probably go down a quality grade with the barley - we were on track for malting barley but it will probably be feed quality now.

"It is a bit early to tell if the heat has really damaged the wheat, the yield is probably set but the quality might be decreased."

Telangatuk farmer Ian Dyer said his wheat and barley crops were still expected to finish well courtesy of an isolated shower from a thunderstorm before the heatwave.

"We aren't harvesting yet but things are looking pretty good because we had 120 points of rain in a thunderstorm so we had enough moisture to cope with the high temperatures," Mr Dyer said.

"We had concerns for the wheat and barley because we actually hadn't had a lot of the rain other people had throughout the season, but then we just got it at the right time so we have been fortunate."