No abstract

The sensitivity of soybean [Glycine max (L.) Merr.] main stem node accrual to ambient temperature has been documented in greenhouse‐grown plants but not with field‐grown plants in the north‐central United States. Biweekly V‐node and R‐stage, stem node number, internode length, and other traits were quantified in an irrigated split‐plot, four‐replicate, randomized complete block experiment conducted in Lincoln, NE, in 2003–2004. Main plots were early‐, mid‐, late‐May, and mid‐June sowing dates. Subplots were 14 cultivars of maturity groups 3.0 to 3.9. Node appearance was surprisingly linear from V1 to R5, despite the large increase in daily temperature from early May (10–15°C) to July (20–25°C). The 2003 and 2004 May planting date regressions exhibited near‐identical slopes of 0.27 node d−1 (i.e., one node every 3.7 d). Cold‐induced delays in germination and emergence did delay the V1 date (relative to planting date), so the primary effect of temperature was the V1 start date of linearity in node appearance. With one exception, earlier sowings led to more nodes (earlier V1 start dates) but also resulted in shorter internodes at nodes 3 to 9 (cooler coincident temperatures), thereby generating a curved response of plant height to delayed plantings. Delaying planting after 1 May led to significant linear seed yield declines of 17 kg ha−1 d−1 in 2003 and 43 kg ha−1 d−1 in 2004, denoting the importance of early planting for capturing the yield potential available in soybean production, when moisture supply is not limiting.

At any given time, the leaf area index (LAI) of a soybean [Glycine max (L.) Merr.] crop consists of the summed contributions of each trifoliolate leaf present at each main stem node and on branches. No data are available on nodal LAI distributions in modern indeterminate (IN) or semi-determinate (SD) cultivars grown in irrigated, early-planted, high-yield production systems. Th e impact of stem termination type and row spacing on that distribution was investigated in such environments at Lincoln, NE in 2003, 2004, and 2005. Trifoliolate LAI at each stem node followed a temporal pattern of rapid increase (aft er leaf initiation) to a peak before declining due to senescence-driven leaf abscission, thus emulating, on a shorter time-scale, the canopy LAI pattern. Th e post-peak decline in nodal LAI was rapid in nodes initiated prebloom (i.e., nearly 100% abscission before seed-fi ll), but was gradual in nodes initiated aft er pod initiation (i.e., little abscission before plant maturity). Nodal LAI peaked at the eighth node of the IN cultivar, but rapid leaf expansion at prefl owering nodes of the SD cultivar led to a broad peak spanning the fi ft h to eighth node. Simulation of the Beer-Lambert law of light attenuation in both canopies revealed that light penetration was deeper in the IN canopy than in the SD canopy. Although higher plant density suppressed branching (and thus branch leaf area) in the SD cultivar, this was not observed in the IN cultivar. Th ese fi ndings suggested that nodal LAI development can be used to mechanistically model canopy LAI.Abbreviations: DAE, days aft er emergence; IN, indeterminate; LAI, leaf area index; MG, maturity group; Rn, reproductive stage for which R1 is begin bloom, R2 is full bloom, R3 is begin pod, R4 is end pod, R5 is begin seed-fi ll, R6 is end seed-fi ll, R7 is physiological maturity (one mature brown pod per plant), R8 is genotype maturity (95% of the pods in the crop are a mature brown; SD, semideterminate; Vn, vegetative stage for which n is the main stem node number.

The Agricultural Production Systems sIMulator (APSIM) was used to evaluate two alternative approaches for extending the cover crop growing window into corn (Zea mays L.) and soybean (Glycine max L.) crop rotations in Nebraska, USA. We evaluated how: (i) shifting corn planting dates (mid-April to early-June) and (ii) altering comparative relative maturity (CRM) corn hybrids (80 to 115 days) influence cover crop biomass and corn yields over a 30-year period. The APSIM model was tested using experimental data and was then used to simulate a range of cover crop planting and termination scenarios. Our results showed no significant yield differences within the same corn relative maturity when planted on April 20 and May 13 but that yield declined when planted in June. During a six week fall cover crop planting window (September 15-October 31), every day before October 31 that the cover crop was planted resulted in additional 62 kg ha −1 of biomass. We also simulated a one month spring termination window (April 1-April 30) and, every day delay in cover crop termination resulted in per day additional 35 kg ha −1 of biomass. Cover crop biomass accrual was highly dependent on weather, where for identical fall planting dates, a warm wet season accrued approximately four times more biomass than a cool dry season. Although we found significant yield differences between early, medium and late season CRMs, earlier fall cover crop planting associated with either earlier spring corn planting or planting an early to medium season variety leads to tenfold greater cover biomass. Delayed corn planting by mid-May had no yield penalty relative to April planting, and could facilitate four-fold greater cover crop biomass (cover crop terminated April 30 instead of April 1). Our results demonstrate that earlier cover crop planting in fall or later cover crop termination in spring can result in significantly more biomass which can be balanced with yield goals.