Influence of Nitrogen Rate, Seeding Rate, and Weed Removal Timing on Weed Interference in Barley and Effect of Nitrogen on Weed Response to Herbicides

Field experiments were conducted at the Montana State University Southern Agricultural Research Center, Huntley, MT, in 2011 through 2013 to determine the effect of nitrogen (N) rate, seeding rate, and weed removal timing on weed interference in barley. A delay in weed removal timing from the 3- to 4-leaf (LF) stage to the 8- to 10-LF stage of barley resulted in up to 3.5-fold increase in total weed biomass and 10% reduction in barley biomass, and this was unaffected by a N rate that ranged from 56 (low) to 168 (high) kg ha⁻¹. The effect of N rate on barley biomass was more pronounced when weed removal was delayed from the 3- to 4-LF stage to the 8- to 10-LF stage of barley and in nontreated plots. Increasing the barley seeding rate from 38 to 152 kg ha⁻¹ increased the barley plant density by 50%, biomass by 13%, and grain yield by 29%, averaged over N rates and weed removal timing. On the basis of 5 and 10% levels of acceptable yield loss, the addition of ≥112 kg N ha⁻¹ delayed the critical timing of weed removal by at least 1.3 wk in barley, compared with the 56 kg N ha⁻¹ rate. A medium or high N rate prevented reduction in barley grain quality (plumpness and test weight) observed when the seeding rate was increased from 38 to 76 or 152 kg ha⁻¹ at the low N rate. In a separate greenhouse study, the effect of N rate on the effectiveness of various herbicides for controlling wild oat, green foxtail, kochia, or Russian thistle was investigated. Results highlighted that wild oat or green foxtail grown under 56 kg N ha⁻¹ (low N) soil required 1.4 to 2.6 times higher doses of clodinafop, fenoxaprop, flucarbazone, glyphosate, glufosinate, pinoxaden, or tralkoxydim for 50% reduction in shoot dry weights (GR₅₀) compared with plants grown under 168 kg N ha⁻¹ (high N). Similarly, a reduced efficacy of thifensulfuron methyl + tribenuron methyl, metsulfuron methyl, or bromoxynil + pyrasulfotole was observed (evident from the GR₅₀ values) for kochia or Russian thistle grown under low- vs. high-N soil. Information gained from this research will aid in developing cost-effective, integrated weed management (IWM) strategies in cereals and in educating growers on the importance of fertilizer N management as a component of IWM programs. Nomenclature: Bromoxynil + pyrasulfotole; clodinafop; fenoxaprop; flucarbazone; glyphosate; glufosinate; metsulfuron methyl; pinoxaden; thifensulfuron methyl + tribenuron methyl; tralkoxydim; green foxtail; Setaria viridis (L.) Beauv.; kochia, Kochia scoparia (L.) Schrad; Russian thistle, Salsola tragus L.; barley, Hordeum vulgare L.; wild oat, Avena fatua L.