Weed control in lentil is difficult because lentil is a poor competitor with weeds and few POST broadleaf herbicides are available. Imadazolinone-tolerant lentils have more herbicide options, but the optimum timing for herbicide application is not known. The critical period of weed control (CPWC) is the period in a crop's life cycle when weeds must be controlled in order to prevent yield loss. The objective of this research was to determine the CPWC for lentil. We made lentil remain weedy or weed-free from 0 to 11 aboveground nodes to investigate the durations of weed interference and weed-free period, respectively. It was found that lentil has a CPWC beginning at the five-node stage and continuing to the 10-node stage. There was an inverse relationship between weed biomass and lentil yield; that is, lentil yield was highest when weed biomass is minimal. We propose that the CPWC begins when weeds start to accumulate significant biomass and ends with crop canopy closure. Therefore, to maximize lentil yields, growers should consider using a POST residual herbicide that can control weeds during the CPWC.

Conventional lentil, because it is relatively noncompetitive, requires effective weed control. In conventional lentil, metribuzin should be applied by the four-node stage to avoid crop injury. This is earlier than the critical period of weed control (CPWC) of lentil, which is between the five- and 10-node stage. However, imidazolinone herbicides potentially can be applied later in imidazolinone-resistant lentil, which might allow lentil to be kept weed-free for the CPWC. The objective of this experiment was to determine the best herbicide choice and application timing necessary to achieve the CPWC in lentil. To do this we tested herbicides differing in efficacy and residual control. The herbicides imazethapyr/imazamox, imazamox, and metribuzin + sethoxydim were applied at the two- and six-node lentil stage. Of the three herbicide treatments, metribuzin + sethoxydim resulted in grain yield that was on average 31% lower than the other herbicides. This occurred because of greater broadleaf biomass (composed primarily of wild mustard) in lentils treated with these herbicides regardless of application timing. Because of this, the CPWC was not attained with metribuzin + sethoxydim. Late applications of imazethapyr/imazamox or imazamox resulted in grain yields 30% higher than with early application of these herbicides. Early applications of the imidazolinone herbicides gave poor control of grass weeds (wild oat and green foxtail), but late applications resulted in grass weed control equivalent to metribuzin + sethoxydim. Imazethapyr/imazamox or imazamox should be applied at the five- to six-node stage of lentil to achieve the CPWC.