Water quality is the key to achieving proper performance of many herbicides, particularly with glyphosate and many grassy weed products. The following link provides more information on spray water quality:
Custom applicators (and growers in general) should double check to ensure the right product is being applied to the right field. Ensure legal land descriptions and field directions are 100% accurate to eliminate misapplication errors. Follow label directions for proper product rates and application techniques. Although good weed control has occasionally been reported in the past using reduced product and/or water volume rates, remember that the performance will be affected by factors such as growth stage and environmental conditions. Unless the product is applied according to label directions, the applicator will have little recourse in the event of poor weed control or crop damage.
Place a 4 ft X 6 ft tarp on the ground prior to spraying or simply shut off booms for a few yards to create a weed check. This will allow for performance comparisons. This is especially important since GPS technology has all but eliminated unintentional checkstrips!