Not surprisingly, smaller weeds are easier to control. The assumption when using a glyphosate based herbicide is that when weeds get bigger, no worries, just add more glyphosate. A recent research trial conducted by Dr. Peter Sikkema (Ridgetown Campus, University of Guelph) would suggest otherwise.
Take a look at the photo below. Glyphosate tolerant corn with annual weeds at 10 cm (4″) in height and sprayed with a product rate equivalent to 750 mL/ac of Roundup Original (360 g/L). To steal a line from Larry David, “pretty, pretty good”!
However, when we allow those same annual weeds to get another 10 cm (4″) tall, we can double the rate, but with less than stellar results.
Bottom line: spraying annual weeds with glyphosate when they are small ensures better control, requires less herbicide and saves money.