Mounting concerns over the cost and environmental impact of N fertilizer combined with progressively higher plant densities in maize (Zea mays L.) production systems make progress in maize N use efficiency (NUE) and N stress tolerance essential. The primary objectives of this 3‐yr field study were to (i) evaluate the N responsiveness, NUE, and N stress tolerance of multiple modern maize genotypes using suboptimal, optimal, and supraoptimal plant densities (54,000, 79,000, and 104,000 plants ha−1, respectively) with three levels of side‐dress N (0, 165, and 330 kg N ha−1), (ii) identify key morphophysiological responses to the simultaneous stresses of intense crowding and low N availability, and (iii) consider our results with extensive reference to literature on maize morphophysiological responses to plant crowding and N availability. At optimal and supraoptimal plant densities, maize receiving 165 kg ha−1 of side‐dress N displayed strong N responsiveness, high NUE, pronounced crowding tolerance, and plant density independence. However, crowding tolerance was contingent on N application. Relative to less crowded, N‐fertilized environments, the 104,000 plants ha−1, 0 kg N ha−1 treatment combination exhibited (i) reduced pre‐ and postanthesis plant height (PHT), stem diameter (SD), and total biomass; (ii) greater preflowering leaf senescence and lower R1 leaf areas at individual‐leaf, per‐plant, and canopy levels; (iii) enhanced floral protandry; (iv) lower pre‐ and postanthesis leaf‐chlorophyll content; (v) lower per‐plant kernel number (KNP), individual kernel weight (KW), grain yield per plant (GYP), andharvest index per plant (HIP); and (vi) enhanced per‐plant grain yield variability (GYCV). Genetic efforts to improve high plant density tolerance should, therefore, simultaneously focus on enhancing NUE and N stress tolerance.

A trend toward early planting of soybean [Glycine max (L.) Merr.] in Indiana results in higher yield, but the limit to which a positive response to early planting occurs has not been evaluated. Our objective was to determine how early planting aff ects yield components and seed composition of indeterminate soybean planted in late March through early June in Indiana. Th ree cultivars (Pioneer brand 92M61, Becks brand 321NRR, and Becks brand 367NRR) were sown at six planting dates (late March through early June) in West Lafayette, IN, in 2006 and 2007. Across cultivars, yield in 2006 ranged between 4.24 to 4.43 Mg ha −1 at the planting dates from late March to mid-May, and decreased to 3.36 and 3.56 Mg ha −1 at later planting dates. In 2007, yield ranged from 4.21 to 4.44 Mg ha −1 for the 10 April, 30 April, and 9 May planting dates. Yield was reduced at the late March and early June plantings and ranged from 3.85 to 3.99 Mg ha −1 . Path analysis revealed that pods m −2 had the greatest impact on yield, but seed mass was also an important constituent. Mean oil concentration decreased approximately 12 g kg −1 as planting was delayed in both years. In 2006, average seed protein concentration varied by planting date. In 2007, mean protein concentration increased 14 g kg −1 as planting was delayed. Delaying planting until late May or early June altered seed composition slightly, but significantly reduced yield. Planting in April or early May is an eff ective management strategy to increase soybean yield in Indiana.

Phosphorus and K fertilization increases alfalfa (Medicago sativa L.) yield and stand persistence, but the changes in yield components as affected by P and K fertility level are not known. Our hypothesis is that P and (or) K fertilization will increase one or more alfalfa yield components, and those component responses may change with stand age. The objectives of this field study were to determine the impact of P and K fertilization on alfalfa forage yield and yield components during the initial 3 yr after establishment. Treatments were the factorial combinations of four P rates (0, 25, 50, and 75 kg P ha−1) and five K rates (0, 100, 200, 300, and 400 kg K ha−1) arranged in a randomized complete block design with four replications. Forage harvests occurred four times annually, and yield, mass shoot−1, and shoots area−1 were determined. Plant populations were determined in early December and late May each year. Incremental additions of P and K increased alfalfa yield in each year. Potassium fertilization did not influence plant population, while robust P‐responsive alfalfa plants apparently crowded out smaller, less vigorous plants thus decreasing plants m−2 Stand assessments based on shoot counts, or aboveground plant counts may not accurately indicate alfalfa yield potential. Shoots plant−1 was not affected by application of either nutrient, while shoots m−2 generally declined with increased P and K fertilization. Improved forage yield of P‐ and K‐fertilized plots was consistently associated with greater mass shoot−1 Because fertilizer‐responsiveness is closely associated with greater mass shoot−1, cultivars possessing this trait may be relatively more productive under well‐fertilized conditions.

Addition of P and K fertilizer can increase alfalfa (Medicago sativa L.) yield and stand persistence, but the yield components associated with P‐ and K‐induced variation in agronomic performance are not clear. Our objectives were: (i) to determine the impact of P and K nutrition on productivity of a relatively old alfalfa stand; and (ii) determine which yield components are associated with changes in alfalfa forage yield. Treatments were a factorial combination of four P and five K rates replicated four times. Forage harvests occurred four times annually. Plant populations were determined in early December and late May each year. When compared to unfertilized plots, addition of P and K increased forage yield each year. Fertilization with P decreased plants m−2 at all K application rates, but especially in plots fertilized with P, but not K. By comparison, plots fertilized with K, but not fertilized with P, had the higher plant population densities. Although regression analysis eventually revealed a positive association between forage yield and shoots m−2 in 2003 and 2004, the greatest forage yields were not obtained in plots with the greatest plant population densities, shoots plant−1 or shoots m−2 Regression and path analysis revealed that improved forage yield in P‐ and K‐fertilized plots was consistently associated with greater mass shoot−1

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