FEBRUARY 2018 SEEDWORLD.COM / 61 Flowers from Giant Ragweed (Ambrosia trifida) discovered in the United States in the late 1950s at a pine nurs- ery where triazine herbicides had been used repeatedly. In addition, field bindweed resist- ant to 2,4-D was reported in Kansas in 1964. “This explains what we are seeing out in the field now. We need to broaden our view of what we see in weed resist- ance and look to the horizon in order to see what is coming. Now, more than ever, it’s important to reinforce best practices to manage weed resistance,” Waldstein says. Waldstein recommends several best practices for managing resistance. Identifying the target weeds is the first. “We have grown to rely on glyphosate which was extremely effective on many species of weeds when it first came out, so we did not need to be that great of an agronomist or weed scientist to identify the target weeds,” he says. “Now that we have widespread glyphosate resist- ance in many of our key weed species, we need to under- stand the chemistries that are available to control weeds so we can develop an effective weed control program.” Four big weeds that have developed resistance are: •  Horseweed (marestail): found commonly in no-till cropping systems, it was the first weed to develop glyphosate resistance in agronomic crops in the United States, Young notes. •  Palmer amaranth: accord- ing to the Weed Science Society of America, Palmer amaranth takes the prize as the most troublesome weed in the U.S. • Waterhemp: a type of pig- weed related to Palmer ama- ranth, is an annual plant that produces prolific amounts of very small seeds. •  Giant ragweed: can read- ily be found along fence- rows of fields and pastures. Increasingly, it is becom- ing established in fields. Herbicides commonly used, like glyphosate, provide only partial control. Layering residual herbi- cides is a major management strategy Waldstein recom- mends. “It is important to start with a pre-emergent herbi- cide that lays a good residual foundation. Both pre and post applications have become increasingly important in recent years,” he says. Layering residual pre and post herbicides is espe- cially important for weeds like waterhemp and Palmer amaranth, which continue to emerge throughout the season. It is not uncommon to see clean fields in May and June, but in July and August these weeds seem to appear out of nowhere because of a lack of a good residual weed control program. Later in the season, Waldstein advises growers make post applications not only with burn- down chemistries that kill the