Effects of application date and rate of foliar-applied glyphosate on pine seedlings in Turkey

Glyphosate is the herbicide most extensively used for site preparation and conifer release. It is a broad-spectrum herbicide and therefore crop safety is a critical issue. This study assessed the early effects of 14 different treatments, including no weed control, manual weed control, and 12 foliar-applied herbicide treatments at low, intermediate, high, and highest application rates and application timing on glyphosate phytotoxicity of containerized seedlings of Austrian pine ( Pinus nigra J.F. Arnold.), Scots pine ( P. sylvestris L.) and maritime pine ( P. pinaster Aiton), conifer species widely used for afforestation and supplementary plantings in Turkish forestry. In general, Scots pine seedlings were tolerant to glyphosate compared to the other species. Glyphosate phytotoxicity varied significantly according to the time and rate of application. Seedlings were relatively tolerant to glyphosate in April whereas they were intolerant in May. The highest herbicide rate (1.2% v:v) was consistently phytotoxic to all species. Moreover, the effect of herbicide rate on seedling survival and growth varied significantly according to application date (i.e., application rate × date interaction). Seedlings appeared tolerant to glyphosate at low and intermediate rates (0.2, 0.4% v:v) between mid-spring and mid-summer, whereas they demonstrated significant sensitivity to the highest rate across all time periods. Glyphosate at the high rate (0.8% v:v) was particularly more phytotoxic when applied in May. Application of glyphosate at rates up to 0.8% could be recommended for weed control without significant pine damage in mid-spring when the needles presumably have a dense leaf epicuticular wax layer limiting herbicide penetration. Applications of 0.8 and 1.2% v:v are not recommended during May–June.