Global positioning and tractor guidance systems could, in a few years, give growers a new edge for more economical control on velvetleaf, johnsongrass, and other weeds in field crops, according to a northern California farm advisor. Doug Munier, who is based at Orland in Glenn County, says if only weedy portions of a field were sprayed, herbicide costs would be significantly reduced. "If you have a material that would otherwise cost $30 per acre and apply it to only 10 percent of the field, you have a $3 per acre treatment." Munier, outlining field-mapping advances to colleagues at the recent conference in San Jose of the California Weed Science Society, said, "This may open up possibilities for field-crop growers to very effectively use some of the more expensive postemergence herbicides on problem weeds." Velvetleaf, he added, is an example of a tough weed that has invaded the Sacramento Valley. He theorizes that with mapping a grower could first spray scattered patches and return — without having to rely on memory -- the following year and spray those same sites again, whether the weeds were visible or not at the time. Munier said the computerized technology, like herbicide-resistant crops, will be key in replacing manual hoeing in field crops. What’s more, he noted, it restores some of the familiarity farmers once had with individual fields, now lost with far larger farm acreages. Three components The site-specific technology is based on three components: the hardware for GPS, the software to manage data, and the hardware to manage control of the sprayer. GPS equipment, including handheld units accurate to four to five feet, is still pricey, but costs are dropping as more units are produced. Growers can map weeds, by species, in their own fields with the equipment, he said. "If you are going to the trouble of manual mapping, it’s most useful to map the perennial weeds that slowly become established. Identify the early infestations, and eliminate or confine them." Once the weed sites, which can number in the thousands, are entered into the equipment, the grower can survey points for each species and determine where best to apply certain herbicides. If a weed is widespread across a field, the site-specific method has less potential, and the entire field would likely have to be sprayed. Human perception, he said, can underestimate a weed population across a field and underscores the need for more exact methods. Munier asked two farm managers well-acquainted with a field to estimate the number of spots of johnsongrass in the field. One said 20, the other said 30, but Munier said he mapped more than 100.