Phosphorus Application Influences the Critical Period of Weed Control in Lettuce.

Field studies were conducted in 2010 and 2011 at Belle Glade, FL to evaluate the influence of phosphorus (P) application (98, 196, and 293 kg P ha^-1) on the critical period of weed control (CPWC) in lettuce. Natural populations of mixed weed species were allowed to interfere with lettuce in a series of treatments of both increasing duration of weed interference and duration of weed-free period imposed within 98, 196, and 293 kg P ha^-1 levels added to the soil. The beginning and end of the CPWC for each P level based on a 5% acceptable marketable fresh lettuce yield loss level was determined by fitting log-logistic and Gompertz models to represent increasing duration of weed interference and duration of weed-free period, respectively. The beginning of the CPWC in lettuce was estimated to be 2.2, 2.3, and 2.9 WAE at 93, 196, and 293 P kg ha^-1, respectively which corresponded to the four- to six-leaf stage of lettuce development. The end of the CPWC in lettuce was 6.8, 5.7, and 5.2 WAE at 93, 196, and 293 P kg ha^-1, respectively which corresponded to the cupping to heading stage of lettuce development. Beginning of the CPWC was delayed at high P level (293 kg P ha^-1) while the end of the CPWC was hastened at high P level. Our study shows that reduced levels of P fertilization in lettuce will result in the need for more intensive weed management practices to attain acceptable yields.