The Susceptibility of Annual Weeds and Canada Thistle to MCPA Applied at Different Times of Day

Lambsquarters (Chenopodium albunm L.), redroot pig- weed (Amaranthus retroflexus L.), and white mustard (Brassica hiirta L.) were highly susceptible to 2-methyl-4-chlorophenoxyacetic acid (MCPA) regardless of the pretreatment light exposure to which they were subjected. These broadleaved weeds were slightly more susceptible to MCPA applied after a prolonged period of darkness. Neither total carbohydrate, reducing sugar, protein, nor amino acid contents at the time of spraying were associated with the susceptibility of mustard plants to MCPA. Floral bud develop- ment on Canada thistle was greatly reduced by MCPA applied at any time of the day to plants 9 and 12 inches tall and was prevented on plants 3 and 6 inches tall. The susceptibility of broadleaved annual weeds and Canada thistle to MCPA was not affected by variations in temperature and humidity at time of spraying.