No abstract

or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Split-row planters compared to grain drills may allow for reduced soybean seeding rates and seed costs because row crop planters result in more uniform seed depth and distance between seeds in a row, improving emergence and uniformity of fi nal stands (Bertram and Pedersen, 2004). Some studies (Weber et al., 1966;Oplinger and Philbrook, 1992) in northern latitudes, however, have reported row spacing by seeding rate interactions with soybean responding more positively to higher seeding rates in narrow vs. wide rows. Other studies in Ohio (Beurelein, 1988) and Ontario, Canada (Ablett et al., 1991) reported no row spacing by seeding rate interactions with across seeding rates. Nevertheless, yield showed a quadratic response to seeding rate (3.04, 3.25, and 3.12 Mg ha −1 at 321,000; 420,000; and 469,000 seeds ha −1 , respectively) with no row spacing interaction. Soybean compensated more at lower seeding rates than at wider rows, but fi eld-scale studies are being conducted to evaluate the economics of both practices.

Crop models can be evaluated based on accuracy in simulating several years' yields for one location or on accuracy in simulating long‐term mean yields for several locations. Our objective was to see how the ALMANAC (Agricultural Land Management Alternatives with Numerical Assessment Criteria) model and a new version of CERES‐Maize (Crop‐Environment Resource Synthesis) simulate grain yield of rainfed maize (Zea mays L.). We tested the models at one county in each of nine states: Minnesota, New York, Iowa, Illinois, Nebraska, Missouri, Kansas, Louisiana, and Texas (MN, NY, IA, IL, NE, MO, KS, LA, and TX). Simulated grain yields were compared with grain yields reported by the National Agricultural Statistical Service (NASS) for 1983 to 1992. In each county we chose a soil commonly used in maize production, and we used measured weather data. Mean simulated grain yield for each county was always within 5% of the mean measured grain yield for the location. Within locations, measured grain yield was regressed on simulated grain yields and tested to see if the slope was significantly different from 1.0 and if the y‐intercept was significantly different from 0.0, both at the 95% confidence level. Only at MN, NY, and NE for ALMANAC and at MN, NY, and TX for CERES was slope significantly different from 1.0 or intercept significantly different from 0.0. The CVs of simulated grain yields were similar to the those of measured yields at most sites. Also, both models were appropriate for predicting an individual year's yield for most counties. Values for plant parameters, such as heat units for development and the harvest index, and values for soil parameters describing soil water‐holding capacity offer users reasonable inputs for simulating maize grain yield over a wide range of locations.

Modern compared with older maize (Zea mays L.) hybrids tolerate more plant density stress, but more information is required on how modern hybrids interact with plant density. Field experiments were established in 1991 (warm‐dry) and 1992 (cool‐wet) to evaluate whole‐plant physiological, dry matter (DM), and grain yield responses of four commercial hybrids at low (4.5 plants m−2), medium (6.75), and high (9.0) plant densities. As plant density increased, leaf CO2 exchange rates (CER) declined 10 to 20% under mild and 20 to 30% under warm dry conditions. Compared with the high plant density, the low plant density had a 40% lower leaf area index from midvegetative to early grain filling, which offset higher photosynthetic efficiency, resulting in lower crop growth rates during vegetative development and 25% less DM accumulation at silking. When averaged across hybrids, the low plant density averaged 15% lower DM and grain yields than the high plant density. Hybrid ✕ plant density interactions were observed for DM and grain yields. The more prolific hybrid showed mostly linear DM and grain yield responses to plant density, whereas a single‐eared hybrid showed quadratic responses in both years. Another single‐eared hybrid, which did not respond to density, had low leaf CER at all densities, a reduction in kernels per plant at the medium density, and increased barrenness at the high plant density in 1991. Apparently, modern hybrids interact with plant density, regardless of growing conditions, and some modern hybrids do not tolerate density stress in dry years.

Dairy producers in the northeastern USA who grow corn (Zea mays L.) forage in narrow rows plant at 125000 plants ha−1 and fertilize at 225 kg N ha−1 because they believe narrow‐row corn yields best at high plant densities and N rates. We evaluated corn in 1996 and 1997 at two row spacings (0.38 and 0.76 m), two harvest densities (80000 and 116000 plants ha−1), and six N rates (0, 50, 100, 150, 200, and 250 kg ha−1) to determine if row spacing × plant density × N rate interactions existed for dry matter (DM) and calculated milk yields. No interactions existed for DM yield, forage quality characteristics, and milk yields. Corn had greater DM and milk yields at 0.38‐ (20.3 and 16.1 Mg ha−1, respectively) vs. 0.76‐m spacing (18.9 and 15.2 Mg ha−1, respectively). Dry matter and milk yields had quadratic‐plus‐plateau responses to N rates with maximum yields (20.6 and 17.1 Mg ha−1, respectively) at an N rate of 150 kg ha−1. Nitrogen accumulation at harvest, which had a row spacing × N rate interaction, had a linear response to N rates at 0.38‐m spacing and a quadratic response at 0.76‐m spacing. Dairy farmers in the northeastern USA can produce corn silage at similar plant densities and N fertility, regardless of row spacing. Dairy producers who have excess animal waste could apply slightly more N to narrow‐row corn silage because it accumulates more N at harvest.