Tolerance of field maize hybrids to rimsulfuron

The response of eighteen maize (Zea mays L.) hybrids to rimsulfuron [(1-(4-6-dimetoxipyrimidin-2-yl))-3-((ethylsulfonyl)-2-pyridylsulfonyl) urca] was evaluated under field conditions on the campus of Université Laval (Québec, Canada). These hybrids were selected based on a wide range of maize heat unit (MHU) requirements. The response of hybrids to post emergence treatment of rimsulfuron (20, 40, and 60 g a.i ha^-1) was compared to an untreated hand-weeded control. The data recorded included visual shoot phytotoxicity, plant height, and above-ground dry matter.

Visual phytotoxicity varied among the same hybrid with respect to the date of evaluation, from 32% to 46% in 1992, and from 11% and 23% in 1993. According to Principal Component Analysis (PCA), the first component accounted for 820/0 ofthe total variance, with high weights ('eigenvectors') on the visual phytotoxicity 14, 21, 28, and 35 DAT. This signified that comparision among hybrids for sensitivity to rimsulfuron could be carried out by means of visual scores 14, 21, 28 or 35 DAT. Hybrids react differently to different doses of rimsulfuron. Some hybrids tolerated a high dose ofrimsulfuron, while others were severely injured at a low dose.

A hybrid phytotoxicity score (HPS), based on the mean of visual scores at 14, 21, 28 and 35 DAT, averaged across all three application doses, was used to group the hybrids into three categories according to their degree of tolerance:

I:Tolerant (HPS < O); II. Moderately tolerant (HPS ≡ O); and III.Sensitive (HPS > O).

Tolerance to rimsulfuron varied also depending on genotypes which was positively correlated to MHU  requirements. Hybrids of zone 1 of Québec (> 2700 MHU) were more tolerant (based on the HPS) than hybrids of zone 2 (2500 to 2700 MHU) and zone 3 (2300 to 2500 MHU).

Key words: herbicide tolerance, phytotoxicítyscore, sulfonylurcas, maize heat units, multivariate analysis.