Pa. researchers discuss scouting routine for weeds

AFP Correspondent

STATE COLLEGE, Pa. (April 11, 2017) — At Penn State’s recent Agronomy Scout School, Extension associate Dwight Lingenfelter and research technologist Hanna Wells shared their insight on scouting for weeds.
Lingenfelter pointed out that scouting had been common before the introduction of Roundup Ready corn and soybean crops in the late 1990s. As excellent weed control became effective, Lingenfelter said, “Weed scouting almost became a dying art.”
But since herbicide resistance began developing in several key weed species, scouting has become important and necessary again.
Now, weed scouting is not simply scouting for weed species presence and their densities.
Assessing herbicide effectiveness, crop injury from herbicides, and verifying weed shifts and herbicide resistant species is essential.
Lingenfelter, a weed specialist, says, “Routine monitoring of these factors has become imperative to an effective weed management program and to protect crop yields.”
Wells, who performed field scouting for six years, described that role as assessing crop performance and monitoring pest pressure. She noted that Pennsylvania alone has four different growing zones.
At the time she scouted, Wells included in her field equipment a soil probe, both paper and plastic bags, a sweep net, chlorophyll meter, magnifying lens, tape measure, shovel, a seed finder or screw driver, and a penetrometer to measure soil compaction. She also had available personal items such as a lunch, boots, toilet paper and protective gear.
Current sophisticated equipment such as GPS can mark locations of weed infestations to monitor over time. Also, scouting software and apps, plus smartphones and tablets can more easily document and archive information, Lingenfelter added.
He predicted that drones in the future will be more typical and will collect other field information simultaneously. 
“However, physically walking fields and observing the status of the crop will still be a productive effort,” Lingenfelter noted.
Weed scouting in some cases can occur while searching for insects and diseases, or conducting crop stand and yield calculations.
Lingenfelter said that depending on the crop and especially in no-till fields with a burndown or early replant herbicide application, the first weed survey should begin as soon as weeds appear. It should continue until after harvest or killing frost. Three or four times a season should be adequate.
In the mid-season survey, after crop emergence, evaluate pre-emergence herbicide performance and determine if post-emergence herbicides or mechanical controls are needed. If post-emergence herbicides are applied, assess the field seven to 10 days later to evaluate the weed control and determine any crop injury. At that time, Lingenfelter observes, resistant weeds or weed species shifts will begin to become evident.
The final weed survey should be performed in late summer. Observations should be made in several locations and another weed map developed.
Various scouting patterns may be employed. Zig-zag, ‘M,’ or ‘U’ shapes or grids can be used as long as the sample represents the field accurately. Lingenfelter warned against making assumptions based on field edges, along waterways, drier clay, shale knobs, or in low, poorly drained areas.
The severity of any weed infestation can be analyzed by counting the number of weeds per ten feet of row for large infestations or per 100 feet of row for smaller infestations in all sampled areas. Lingenfelter said sample areas should represent no more than five acres, so enough samples must be surveyed to get an accurate count. 
All weed species should be identified and recorded. Dates must be noted. The height and growth of both weeds and the crop should be recorded. Other data such as moisture observations should be added. This information will be useful in developing effective herbicide programs, and planting dates.
Iowa State’s online publication, “Relative emergence sequence for weeds of corn and soybeans,” Lingenfelter said, is helpful for identification and planning.
Wells advised that when working as a crop consultant or scout, “Do not assume, be professional, prepare to be challenged.” She pointed out that checking and following up is far preferable than giving bad advice.
For one’s own crop, Lingenfelter recommended learning to recognize consistent patterns of weeds and species shifts. Also, he stressed that controls are more effective when weeds are young and actively growing.