resulting in an estimated 1066 kg ha Ϫ1 (17 bu ac Ϫ1 ) yield loss over 354 commercial fields. Lauer and Rankin Corn (Zea mays L.) grain yields are known to vary from plant to (2004) and Liu et al. (2004) had differing results, noting plant, but the extent of this variability across a range of environments has not been evaluated. This study was initiated to evaluate by-plant that PSV did not significantly alter grain yields in Wiscorn grain yield variability over a range of production environments consin and Ontario, Canada, respectively. Nafziger et and to establish the relationships among mean grain yield, standard al. (1991) noted that uneven emergence of corn can deviation, coefficient of variation, and yield range. A total of fortyoccur when soils are dry at the time of planting and six 8-to 30-m corn transects were harvested by plant in Argentina, could lead to decreased grain yields. It is generally ac-Mexico, Iowa, Nebraska, Ohio, Virginia, and Oklahoma from 2002 cepted that when adjacent plants differ by more than to 2004. By-plant corn grain yields were determined, and the average two leaf stages, the younger plant may not develop to individual plant yields were calculated. Over all sites in all countries its fullest potential. A two leaf stage difference can result and states, plant-to-plant variation in corn grain yield averaged 2765 from delayed emergence ranging from 5 to 10 d, which kg ha Ϫ1 (44.1 bu ac Ϫ1 ). At the sites with the highest average corn can cause a 1% yield loss for each 1-d delay (Robert grain yield (11 478 and 14 383 kg ha Ϫ1 , Parana Argentina, and Phillips, NE), average plant-to-plant variation in yield was 4211 kg ha Ϫ1 (67 bu L. Nielsen, Purdue University, personal communication, ac Ϫ1 ) and 2926 kg ha Ϫ1 (47 bu ac Ϫ1 ), respectively. As average grain 2004). Tollenaar and Wu (1999) found increased stress yields increased, so did the standard deviation of the yields obtained tolerance in corn when plant-to-plant variability was within each row. Furthermore, the yield range (maximum corn grain lower. In general, these statistics identify a twofold probyield minus the minimum corn grain yield per row) was found to lem: first, the need to homogenize plant spacing and increase with increasing yield level. Regardless of yield level, plantemergence and second, the need to recognize differto-plant variability in corn grain yield can be expected and averaged ences in yield potential that clearly exist by plant. more than 2765 kg ha Ϫ1 over sites and years. Averaging yield over Some technologies in precision agriculture have been distances Ͼ0.5 m removed the extreme by-plant variability, and thus, driven commercially. The most notable has been comthe scale for treating other factors affecting yield should be less than bine yield monitors. Depending on combine speed, 0.5 m. Methods that homogenize corn plant stands and emergence may decrease plant-to-plant variation and could lead to increased Published in Agron.
Field trials were established to compare alum-treated poultry litter (ATPL), normal poultry litter (NPL), and triple superphosphate (TSP) as fertilizer sources for corn (Zea mays L.) when applied at rates based on current litter management strategies in Virginia. Trials were established in the Costal Plain and Piedmont physiographic regions near Painter and Orange, VA, respectively. Nitrogen-based applications of ATPL or NPL applied at rates estimated to supply 173 kg of plant-available nitrogen (PAN) ha(-1) resulted in significantly lower grain yields than treatments receiving commercial fertilizer at the same rate in 2000 and 2001 at Painter. These decreases in grain yield at the N-based application rates were attributed to inadequate N availability, resulting from overestimates of PAN as demonstrated by tissue N concentrations. However, at Orange no treatment effects on grain yield were observed. Applications of ATPL did not affect Al concentrations in corn ear-leaves at either location. Exchangeable soil Al concentrations were most elevated in treatments receiving only NH4NO3 as an N source. At N-based application rates, the ATPL resulted in lower Mehlich 1-extractable P (M1-P) and water-extractable soil phosphorus (H2O-P) concentrations compared to the application of NPL. A portion of this reduction could be attributed to lower rates of P applied in the N-based ATPL treatments. Runoff collected from treatments which received ATPL 2 d before conducting rainfall simulations contained 61 to 71% less dissolved reactive phosphorus (DRP) than treatments receiving NPL. These results show that ATPL may be used as a nutrient source for corn production without significant management alterations. Alum-treated poultry litter can also reduce the environmental impact of litter applications, primarily through minimizing the P status of soils receiving long-term applications of litter and reductions in runoff DRP losses shortly after application.
tional factors that can impact tiller density include soil characteristics, crop residues, planter problems, and soil Adequate tiller density is critical for attaining optimum grain yield drainage patterns (Flowers et al., 2001). Scharf and in winter wheat (Triticum aestivum L.). To ensure maximum tiller development, several states in the Mid-Atlantic recommend split-
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