The condition of French maize crops plunged to a 13-year low during a record heatwave last week, data from farm office FranceAgriMer showed on Friday.
The share of the grain maize crop in good/excellent condition had fallen to 58% by June 29 from 76% the previous week, FranceAgriMer said in a weekly cereal report.
That was the lowest score for the period since 2013.
Crop observers say that maize has been the cereal most vulnerable to the torrid weather because it is in the midst of its growing season.
French growers are projecting a potential 30% drop in maize production this year, with weather damage coming on top of a sharp fall in planting.
Hot, dry weather forecast for the coming two weeks in France could further stress maize as it enters the pollination stage.
As of Monday, 16% of the grain maize crop had reached pollination, FranceAgriMer said.
For wheat and barley, crop conditions also deteriorated last week while harvesting accelerated.
For soft wheat, France's most produced cereal, 68% of crops were in good or excellent condition, down from 74% a week earlier, though still up from 67% a year ago.
The soft wheat harvest was 26% complete, well ahead of a five-year average of 5% for the same week.