The effect of nitrogen fertility rate and seeding rate on yield, nutritive value and economics of forage corn in a low corn heat unit region of Western Canada

Grazing whole plant forage corn (Zea mays L.) during winter by gestating beef cows (Bos taurus) is an alterative feeding strategy in Saskatchewan, Canada. Seeding rate for target plant population and N fertility rates for grazing corn have been based on grain corn research results. The objectives of this study were to determine seeding rate and N fertility rate effects and interaction on forage corn yield and nutritive value. Trials were conducted over three growing seasons (2016, 2017, and 2018) at six sites: Scott, Lanigan, Melfort, Yorkton, Outlook, and Redvers Saskatchewan, that ranged from 2100 to 2450 corn heat unit (CHU) rating. Three N fertility rates, 112, 168 and 225 kg ha⁻¹, and 3 seeding rates, 75,000, 100,000 and 125,000 seeds ha⁻¹, were evaluated with corn hybrids that were adapted to each site. Dry matter (DM) forage yield at the half-milk line stage was determined and samples analyzed by NIRS at a commercial lab for nutritive value. Increased seeding rate from 75,000 to 125,000 seeds ha⁻¹ resulted in increased DM yield by 1.0 ± 0.1 Mg ha⁻¹ and increased total digestible nutrients (TDN) yield while crude protein (CP) and P concentrations decreased. Fiber, TDN, and carbohydrate concentrations were not affected by seeding rate. Increased N fertility from 112 to 225 kg ha⁻¹ resulted in increased DM yield by 0.6 ± 0.1 Mg ha⁻¹, and increased CP yield, and TDN yield. CP and soluble protein concentrations increased while neutral detergent fiber digestibility (NDF-dᵢg) concentration decreased with increased N fertility. Mineral concentrations also increased with increasing N fertility. There were no seeding rate x N rate interactions for yield or nutritive value. Site-years varied in most yield and nutritive value variables. N rate x site-year interactions were significant for DM yield, CP yield and TDN yield because the effect of N fertility was significant at only one third to one half of the site-years. Cost of production per tonne of DM or TDN increased with increased seeding rate but neither was affected by N fertility rate. Net revenue was reduced from $522 ha⁻¹ to $462 ha⁻¹ by increased seeding rate while there was no effect of N fertility rate on net revenue. Current seeding rate and N fertilizer rates for grazing corn in Saskatchewan will provide producers with the highest net return.