Field studies were established in 2015 at Bowling Green and Lexington, KY to evaluate industrial hemp (Cannabis sativa) tolerance to various herbicides. Hemp was planted into conventionally tilled soils in mid to late June at a seeding rate of 39 kg/ha in Bowling Green and 22 kg/ha in Lexington. Five herbicide active ingredients were applied preemergence (PRE) the day of planting and six postemergence (POST) treatments were applied to 30 cm hemp with a CO2-backpack sprayer delivering 140 L/ha. Plots were 3.1 m wide by 6.1 m long and were sprayed with a 2.1 m boom sprayer leaving a 0.46 m visual check on either side of the sprayed area. A weed free check and a non-treated control were included and all treatments were replicated four times in a randomized complete block design. Hemp phytotoxicity was evaluated at 14 days after treatment for both PREs and POSTs. Hemp above-ground biomass, weed above-ground biomass, and seed yield were also evaluated. PRE herbicides did not injure hemp as much as POST herbicides, especially at the Bowling Green location. Mesotrione was the most injurious PRE evaluated (> 90%) while bromoxynil and MSMA applications resulted in low phytotoxicity (< 15%). Above-ground biomass was higher in the PRE treated plots, with the exceptions of bromoxynil and MSMA. Weed above-ground biomass was higher in the POST treated plots with the exception of mesotrione. At Bowling Green, PRE herbicides resulted in comparable yields to the weed-free check, except mesotrione. Metolachlor increased seed yield compared to the weed-free check and MSMA and bromoxynil had comparable yields to the weed-free check at both locations. Results identified possible herbicides to include in a future integrated pest management weed control program for industrial hemp.

Some of the factors influencing the performance of bromoxynil, 4(2,4-DB), or a companion crop for weed control in seedling alfalfa were studied in the greenhouse and at field locations in Farmington and Logan , Utah. The effect of application rate , stage of growth, temperature, and soil moisture on the phytotoxicity of bromoxynil and 4(2,4-DB) to alfalfa and weeds were studied. Alfalfa yields were increased by 4(2,4-DB) at all rates and stages of application. All rates and stages of bromoxynil treatment except the one-fourth pound per acre three to four trifoliate application resulted in alfalfa yields below that of the control. The use of a companion crop was not conducive to the growth and development of alfalfa although it did control the weeds . Bromoxynil at all rates and at both stages of application resulted in effective mustard control. Mustard control in the 4(2,4- DB) plots was excellent at the early stage of application but required three- fourths pound per acre for control at the later stage of weed growth . Pigweed control was rather ineffective in bromoxynil plots at Logan where moisture was optimum, but effective in plots at farmington where moisture was limited for 18-20 days following application. The density of the pigweed stand in bromoxynil treatments at Logan was attributed to an influx of weed growth following initial control of weeds . Control of pigweed by 4(2 , 4- DB) was in excess of 90 percent at the four to five leaf stage of weed growth.

Master of Science / Department of Agronomy / Curtis R. Thompson / Curtis R. Thompson / Postemergent herbicide options for grain sorghum are limited and increasingly challenged by the development of herbicide resistant weeds. The herbicide pyrasulfotole & bromoxynil (P&B) was evaluated for potential use in grain sorghum and for control of a suspected HPPD-resistant Palmer amaranth population. Field experiments were conducted near Manhattan and Rossville, KS, to evaluate grain sorghum response to P&B with and without 2,4-D applied to growth stages from 1-leaf through the flag leaf stage and tankmixed with 2,4-D ester, amine, or dicamba applied to 3- and 6-leaf sorghum. The addition of 2,4-D ester did not reduce sorghum injury from P&B alone. Increasing the rate of P&B increased injury. Treatments applied to 1- and 4-leaf sorghum were injured the most. All P&B treated sorghum, regardless of timing, yielded 8 to 20% less than the untreated check. Pyrasulfotole & bromoxynil applied alone or with dicamba injured sorghum less than 2,4-D applied at 3- or 6-leaf. Increasing the rate from 140 to 280 g ha[superscript]-1 2,4-D amine or ester increased injury by 6 to 11%. Yields were lowest when P&B was applied with 2,4-D amine at 140 g ha[superscript]-1 and 2,4-D amine or ester at 280 g ha[superscript]-1 compared to all other treatments. Increasing the rate of growth regulator herbicides decreased yields by 8% and did not reduce crop injury from P&B alone. Greenhouse and field experiments were conducted to evaluate the response of two suspected P&B-resistant (R1 & R2) and one susceptible (S) Palmer amaranth population to P&B, atrazine, and tembotrione. Herbicides were applied when plants were 7 to 19 cm tall. The S population was controlled with less than field use rates. A resistance index (RI) of 4.8 to 11.0 was determined for R1 and R2 in greenhouse and field experiments. Tembotrione controlled 100% of S in all experiments, while providing 63 to 86% injury to R1 and R2 populations. Atrazine did not control the resistant populations. Pyrasofotole & bromoxynil will be an valuable tool for weed control in sorghum, however, Palmer amaranth populations exist that will not be controlled.