Efficacy of POST herbicides on weeds during compensatory growth

Death of the apical shoot and subsequent compensatory growth from previously inhibited axillary buds is a possible result of POST weed control practices. The objective of this research was to determine the efficacy of glyphosate and lactofen on ivyleaf morningglory, common waterhemp, and giant ragweed during mechanically stimulated compensatory growth. Plants were grown in the greenhouse to a height (or lateral growth) of 15 cm; at which time, the apical shoots were removed by cutting just above the cotyledonary node. Plants were allowed to regrow to 15 cm and treated with lactofen or glyphosate. Herbicide treatments were also applied to intact plants that were 15 cm in height for comparison. Weed response to herbicides during mechanically stimulated compensatory growth varied by weed species and herbicide. Giant ragweed under compensatory growth was less sensitive to both glyphosate and lactofen compared with intact plants. Comparison of GR₅₀ (the herbicide dose that reduced dry weight by 50%) values indicated that ivyleaf morningglory under compensatory growth was 1.5 times more sensitive to glyphosate than intact plants. Conversely, previously injured ivyleaf morningglory plants were less sensitive to lactofen than intact plants. The GR₅₀ for glyphosate applied to intact common waterhemp plants and plants under compensatory growth was similar. However, common waterhemp plants under compensatory growth were more sensitive to lactofen at the three lowest rates evaluated compared with intact plants. In summary, the efficacy of foliar herbicides applied to weeds that exhibit compensatory growth may be different from weeds under a normal growth state.Nomenclature: Glyphosate; lactofen; common waterhemp, Amaranthus rudis Sauer AMATA; giant ragweed, Ambrosia trifida L. AMBTR; ivyleaf morningglory, Ipomoea hederacea (L.) Jacq. IPOHE.