The effects of glyphosate herbicide applied at different rates and dates on the seedlinds of austrian pine (Pinus nigra J .F. Arnold.), scots pine (Pinus sylvestris Line.) and maritime pine (Pinus pinaster Aiton.)

Use of herbicides for site preparation and weed control in conifers and broad-leaved species in forestry is extensively increasing. Glyphosate is one of the most frequently used herbicides for site preparation and weed control on conifer sites in forestry. It is a broad-spectrum herbicide and therefore, crop safety is a critical issue. The glyphosate tolerance of conifer seedlings varies by species and date and rate of application. This study assessed the phytotoxicity of 14 different treatments, including no weed control, manual weed control, and 12 foliar-applied chemical weed control (glyphosate) applied, two consecutive years, on in young containerized seedlings of Austrian pine (Pinus nigra J.F.Arnold.), Scots pine (P. sylvestris Line.) and maritime pine (P. pinaster Aiton), the conifer species widely used for afforestation and supplementary plantings in Turkish forestry. Chemical treatments included the low, intermediate, high, and highest application rates (0.2, 0.4, 0.8, and 1.2% v:v, respectively) applied in three different dates (early April, May and June) on the glyphosate phytotoxicity. Two sets of analyses were carried out with and without nonherbicidal (the control and manual weed control) treatments. Significant treatment differences (p ≤ 0.05) were found in both analyses. In addition to the main treatment effects, significant application date × rate interaction effects were found for the pine species in the second analysis which included only herbicide treatments. The glyphosate phytotoxicity in the young pine seedlings varied according to the application date and rate. The effect of herbicide rate on seedling survival and growth also significantly varied according to application date (i.e., application rate × date interaction). The Austrian, maritime and Scots pine seedlings appeared to be tolerant to glyphosate at low and intermediate rates (0.2-0.4% v:v or 172-344 g a.i. ha-1) in the period between mid-spring and mid-summer whereas they demonstrated significant sensitivity to the highest rate (1.2% v:v or 1032 g a.i. ha-1) across all dates. Application of glyphosate at the rate of 0.8% v:v (689 g a.i. ha-1) is recommended only in mid-spring when the needles presumably have a dense leaf epicuticular wax layer with limited herbicide leaf penetration.