Weed Interference and Glyphosate Timing Affect Corn Forage Yield and Quality

Received for publication July 28, 2004. Corn (Zea mays L.) planting and the first harvest of perennial forages overlap in the northeastern USA in some years. The use of glyphosate [N-(phosphonomethyl)glycine]–resistant corn may help dairy producers lessen their workload during this time by allowing for a timely glyphosate application after completion of the first harvest of perennial forages. We evaluated 92 and 103-d hybrids to determine the impact of season-long weed interference and the optimum timing for glyphosate application on corn forage yield and quality. Season-long weed interference vs. weed-free corn in two competitive growing seasons reduced dry matter (DM) accumulation at silking (R1 stage) by 50 to 65%, DM yield by 70 to 75%, and calculated milk yield by 75 to 80%. Season-long weed interference reduced milk per megagram, a forage quality index, by 10% in a dry year by preventing grain formation. Glyphosate application at the three to four leaf stage (V3–V4) vs. weed-free corn resulted in similar DM accumulation at the R1 stage, DM content at harvest, DM yield, forage quality, and calculated milk yield in both years. Glyphosate application at the V5–V6 stage vs. weed-free corn increased milk per megagram by 7% in the dry year, but resulted in 20 to 25% less DM yield in both years. Dairy producers in the northeastern USA should apply glyphosate by the V3–V4 stage in competitive growing conditions, regardless of hybrid maturity, which may overlap with the first harvest of perennial forages in some years.