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Accurate prediction of phenological development in maize (Zea mays L.) is fundamental to determining crop adaptation and yield potential. A number of thermal functions are used in crop models, but their relative precision in predicting maize development has not been quantified. The objectives of this study were (i) to evaluate the precision of eight thermal functions, (ii) to assess the effects of source data on the ability to differentiate among thermal functions, and (iii) to attribute the precision of thermal functions to their response across various temperature ranges. Data sets used in this study represent >1000 distinct maize hybrids, >50 geographic locations, and multiple planting dates and years. Thermal functions and calendar days were evaluated and grouped based on their temperature response and derivation as empirical linear, empirical nonlinear, and process-based functions. Precision in predicting phase durations from planting to anthesis or silking and from silking to physiological maturity was evaluated. Large data sets enabled increased differentiation of thermal functions, even when smaller data sets contained orthogonal, multi-location and -year data. At the highest level of differentiation, precision of thermal functions was in the order calendar days < empirical linear < process based < empirical nonlinear. Precision was associated with relatively low temperature sensitivity across the 10 to 26°C range. In contrast to other thermal functions, process-based functions were derived using supra-optimal temperatures, and consequently, they may better represent the developmental response of maize to supra-optimal temperatures. Supra-optimal temperatures could be more prevalent under future climate-change scenarios, but data sets in this study contained few data in that range. EEA Balcarce Fil: Kumudini, S. The Climate Corp; Estados Unidos Fil: Andrade, Fernando Hector. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce-Unidad Integrada-Universidad Nacional de Mar del Plata. Facultad de Ciencias Agrarias; Argentina Fil: Boote, K.J. University of Florida. Department of Agronomy; Estados Unidos Fil: Brown, G.A. Breaking Ground; Estados Unidos Fil: Dzotsi, K.A. University of Florida. Department of Agricultural and Biological Engineering; Estados Unidos Fil: Edmeades, G.O. Hemmans; Nueva Zelanda Fil: Gocken, T. Monsanto; Estados Unidos Fil: Goodwin, M. Monsanto; Estados Unidos Fil: Halter, A.L. Dupont-Pioneer; Estados Unidos Fil: Hammer, G.L. University of Queensland; Australia Fil: Hatfield, J.L. USDA-ARS. National Laboratory for Agriculture and the Environment; Estados Unidos Fil: Jones, J.W. University of Florida. Department of Agricultural and Biological Engineering; Estados Unidos Fil: Kemanian, A.R. Pennsylvania State University. Department of Plant Science; Estados Unidos Fil: Kim, Sung Hyun. University of Washington. College of the Environment. School of Environmental and Forest Sciences; Estados Unidos Fil: Kiniry, J. United States Department of Agriculture. ARS; Estados Unidos Fil: Lizaso, J.I. Universidad Politécnica de Madrid. Departamento de Producción Vegetal; España Fil: Nendel, C. Leibniz Centre for Agricultural Landscape Research. Institute of Landscape Systems Analysis; Alemania Fil: Nielsen, R.L. Purdue University. Department of Agronomy; Estados Unidos Fil: Parent, B. INRA. Laboratory d’Ecophysiologie des Plantes sous Stress Environnementaux; Francia Fil: Stӧckle, C.O. Washington State University. Biological Systems Engineering; Estados Unidos Fil: Tardieu, F. INRA. Laboratory d’Ecophysiologie des Plantes sous Stress Environnementaux; Francia Fil: Thomison, P.R. Ohio State University. Department of Horticulture and Crop Science; Estados Unidos Fil: Timlin, D.J. USDA-ARS. Crop Systems and Global Change Lab; Estados Unidos Fil: Vyn, T.J. Purdue University. Department of Agronomy; Estados Unidos Fil: Wallach, D. INRA. Agrosystèmes et développement territorial; Francia Fil: Yang, H.S. Universidad de Nebraska - Lincoln. Department of Agronomy and Horticulture; Estados Unidos Fil: Tollenaar, M. The Climate Corp; Estados Unidos

Research attention to improving source and sink strength in maize production is requisite for enhancing yield. Improvement in source strength has been achieved with higher post-silking dry matter accumulation, whereas historical improvement in sink strength has been mostly attributed to increasing kernel number (KN) per unit area, in part because KN is known to be more vulnerable to abiotic stresses compared to kernel weight (KW). However, KW can also vary widely as it is dependent on both genotype and dry matter accumulation during the post-silking period. In order to illustrate the consequences of breeding efforts over a 4-decade period for enhancing source and sink strength at varying nitrogen rates and plant densities, a 2-year and 2-location study was conducted in 2013 and 2014. Eight commercial hybrids from DeKalb released from 1967 to 2005 were compared at 2 nitrogen rates (55 and 220 kg N ha −1 ) and 3 plant densities (54,000 (D1), 79,000 (D2) and 104,000 (D3) plants ha −1 ). Breeding progress increased grain yield per hectare (GY) by an average of 66 kg ha −1 year −1 , and grain yield per plant (GYP) by 0.91 g plant −1 year −1 across all treatments and environments. This yield increase with hybrid improvement was attributed more to an increase in KW (1.29 mg kernel −1 year −1 across all treatments and both locations), than to any increase in KN. The overall source-sink ratio (SSR − ratio of post-silking dry matter accumulation to kernel number per plant) also increased by an average of 1.25 mg kernel −1 year −1 across all treatment and locations. The hybrid improvement in SSR was more pronounced at the high N rate or low plant density. Post-silking dry matter accumulation (PostDM) increased by an average of 54 kg ha −1 year −1 across all treatments and locations. KW was highly correlated with ear growth rate (EGR) during grain fill. New hybrids had much higher KW gain per unit of EGR. Newer hybrids also had a longer active grain filling period, but the correlation of post-silking dry matter accumulation to the duration of active grain filling period was weak. This study showed that the breeding progress for yield gain in these DeKalb hybrids was achieved by (i) longer duration of the grain filling period plus longer leaf stay green that accompanied a higher PostDM of newer hybrids, (ii) enhanced source to sink strength during grain filling by a higher SSR in newer hybrids, (iii) improved efficiency for transferring source from cob and husk to grain by increasing KW gain per unit of EGR, and (iv) enhanced stress tolerance in newer hybrids to maintain grain yield even under high density.