Plant Population Influences on Post-Silking N Uptake Vary with Hybrid Eras and N Rates.

In previous research, post-silking N uptake (PostN) has been observed to be dependent on soil available N during the grain filling period and on hybrids. However, relative hybrid impacts on PostN could also be dependent on plant populations. Crowding stress at high density can significantly decrease PostN, and so the negative impact of high plant populations can be more severe at low N fertilizer rates. A 2-year (2013 and 2014) split-split plot experiment was conducted at 2 Purdue farms – West Lafayette (ACRE) and Wanatah (PPAC) in Indiana. Two nitrogen fertilizer rates (55N and 220N kg ha^-1) were the main effects, 3 densities (54,000P; 79,000P; 104,000P pls ha^-1) were the sub effects, and 8 DeKalb commercial hybrids (released from 1967 (XL45), 1975 (XL72AA), 1984 (DK636), 1996 (RX730), 2003 (RX752RR2), 2003 (RX752), to 2005 (DK61-72), 2005 (DK61-69)) were the sub-sub effects. Grain yield, when averaged across all treatments and both years, were 12 Mg ha^-1 at ACRE was versus 9 Mg ha^-1 in PPAC. Lower PostN (25 versus 52 kg ha^-1 at PPAC versus ACRE when averaged across all treatments)reflected more N deficiency stress at PPAC  caused by excessive growing-season rainfall in both years (averaged 664 compared to 435 mm in ACRE).. At ACRE, higher densities decreased average PostN by 16 kg N ha^-1 from low to medium density, and by an additional 8 kg N ha^-1 from the medium to high density. Average PostN at ACRE equaled 47% of final Grain N at maturity at low density, just 37% at medium density and only 33% of final Grain N at high density. PCA analysis (averaged over years) indicated that the 220N and 54,000P treatment most favored PostN in both locations. Grain yield (averaged over hybrids and years) was highest at 220N and 79,000P at ACRE but at the 220N, 54,000P combination treatment at PPAC. The most recent (2005) era hybrids – DKC61-69 and DKC61-72 -performed the best in grain yield and PostN relative to older hybrids at both locations. The oldest hybrid in the trial (1967 – XL45) achieved lowest grain yield in all environments and tended to have a greater stem versus leaf biomass ratio at silking in both locations. Hence, PCA analysis confirmed that newer hybrids had a great PostN even at low N fertilizer rate and high density.