Weedy rice (Oryza sativa f. spontanea) is one of the three worst paddy weeds in most rice growing areas.  The unexpected heavy infestation is derived from a persistence of soil seed bank of weedy rice, which the shattered seeds chiefly feed back to.  Information on soil seed bank dynamics is imperative to predict the infestation of weeds.  In the present paper, the effect of rotary tillage on weedy rice seed bank structure was studied first, and a burial experiment of marked seeds was conducted to observe the overwintering survival, seed viability and seedling emergence of weedy rice.  The results showed that the proportion of weedy rice seeds in deeper soil increased but seedling emergence decreased with increasing plowing depth.  The viability of weedy rice seeds decreased as the burial duration time extended but more slowly in deeper soil layers.  Additionally, there was no significant effect of burial density on seed viability.  Moreover, the logistic model fitted well (R²≥0.95, P≤0.01) with the depressive trends of seed viability with increasing burial time under all burial depths and densities which can provide us further information about seed survival.  In field experiments, number of seedling emergence significantly decreased as seed burial depth increased, conversely, proportion of seedling emergence increased as seed burial density decreased.  This study has important implications for determining strategies for weedy rice management by exhausting its seed bank through the alteration of tillage practices.

Crop type is one of main factors influencing weed community structure.  However, the identity of weed communities associated with the cultivation of different crops in farmlands remains largely unclear.  A field survey of weed seed banks was conducted in 2 280 fields at 228 sites of 62 locations representing three different types of farmland (95 paddy, 73 summer-ripe, and 60 autumn-ripe farmlands) along the bank of the Yangtze River in Anhui Province, China.  A total of 43 families and 174 species of weeds were found in these weed seed banks.  A comparison of the composition of weed groups in the seed banks showed that the species number and density percentage of grass, sedge and broadleaf weed groups were similar among the different types of farmland.  The seed banks of all three farmland types shared 71 common weed species, accounting for 40.80% of the total number of species.  These common weeds, which were both associated and not associated with crops, accounted for 91.71% of the total dominance degree among all farmland types.  The crop-associated weed species were distributed in all soil layers of each farmland type.  The Shannon-Wiener index H´ (description of species diversity which is more sensitive to dense species) and Pielou’s evenness index J (description of species evenness) in summer-ripe farmland were similar to those in autumn-ripe farmland but differed from those in paddy farmland.  However, the Simpson’s index D (description of species diversity which is more sensitive to sparse species) was similar among all three farmland types.  The results of similarity comparison indicated that although the aboveground weed community differed among the different cropping patterns, the weed species composition in the soil seed bank was still similar.  Consequently, our results demonstrate that after the implementation of long-term monoculture patterns, weed species compositions in the soil seed bank in different farmlands become homogenized regardless of the crop type. 

 

To ascertain the influence of wheat stubble management options and chemical weed control methods on weed growth and productivity of dry direct-seeded fine rice, a two years’ field study was undertaken at the Agronomic Research Farm, University of Agriculture, Faisalabad, Pakistan in 2013 and 2014.  Different wheat stubble management methods, viz., incorporation, burning and retention were executed during seed-bed preparation.  While, herbicide treatments comprised of a weed check, weed free, pendimethalin followed by tank mixture of fenoxaprop p-ethyl+ethoxysulfuron ethyl, and bispyribac sodium followed by tank mixture of fenoxaprop p-ethyl+ethoxysulfuron ethyl.  Results revealed that weed control efficacy of both herbicide treatments ranged from 84 to 94%.  Herbicide treatments significantly reduced weed density (88–90%) and dry weight (86–88%), while improved the rice growth attributes compared with weed check.  Application of bispyribac sodium followed by tank mixture of fenoxaprop p-ethyl+ethoxysulfuron ethyl in stubble retention recorded 226 and 273% increase in kernel yield over weedy check in 2013 and 2014, respectively.  In stubble incorporation, pendimethalin followed by tank mixture of fenoxaprop p-ethyl+ethoxysulfuron ethyl was more effective in increasing (256–293%) rice yields over weedy check.  Among different treatment combinations, the maximum net benefits (1 397.49–1 472.22 USD ha^–1), net returns (636–700 USD ha^–1), benefit cost ratio (1.77–1.83) and marginal rate of return (2 187–2 330%) were recorded with the application of bispyribac sodium followed by fenoxaprop p-ethyl+ethoxysulfuron ethyl in stubble retention.  In crux, application of bispyribac sodium followed by tank mixture of fenoxaprop p-ethyl+ethoxysulfuron ethyl in stubble retention is efficient approach to control weeds, and get maximum rice productivity and net economic returns under dry seeded system.

Compared to single-trait transgenic crops, stacked transgenic plants may be more prone to become weedy, and transgene flow from stacked transgenic plants to weedy relatives may pose a potential environmental risk because these hybrids could be more advantageous under specific environmental conditions.  Evaluation of the potential environmental risk caused by stacked transgenes is essential for assessing the environmental consequences caused by crop-weed transgene flow.  The agronomic performance of fitness-related traits was assessed in F₁+ (transgene positive) hybrids (using the transgenic line T1c-19 as the paternal parent) in monoculture and mixed planting under presence or absence glufosinate pressure in the presence or absence of natural insect pressure and then compared with the performance of F₁– (transgene negative) hybrids (using the non-transgenic line Minghui 63 (MH63) as the paternal parent) and their weedy rice counterparts.  The results demonstrated that compared with the F₁– hybrids and weedy rice counterparts, the F₁+ hybrid presented higher performance (P<0.05) or non-significant changes (P>0.05) under natural insect pressure, respectively, lower performance (P<0.05) or non-significant changes (P>0.05) in the absence of insect pressure in monoculture planting, respectively.  And compared to weedy rice counterparts, the F₁+ hybrid presented higher performance (P<0.05) or non-significant changes (P>0.05) in the presence or absence of insect pressure in mixed planting, respectively.  The F₁+ hybrids presented non-significant changes (P>0.05) under the presence or absence glufosinate pressure under insect or non-insect pressure in monoculture planting.  The all F₁+ hybrids and two of three F₁– hybrids had significantly lower (P<0.05) seed shattering than the weedy rice counterparts.  The potential risk of gene flow from T1c-19 to weedy rice should be prevented due to the greater fitness advantage of F₁ hybrids in the majority of cases. 

Excessive use of nitrogen (N) fertilizers in agricultural systems increases the cost of production and risk of environmental pollution.  Therefore, determination of optimum N requirements for plant growth is necessary.  Previous studies mostly established critical N dilution curves based on aboveground dry matter (DM) or leaf dry matter (LDM) and stem dry matter (SDM), to diagnose the N nutrition status of the whole plant.  As these methods are time consuming, we investigated the more rapidly determined leaf area index (LAI) method to establish the critical nitrogen (N_c) dilution curve, and the curve was used to diagnose plant N status for winter wheat in Guanzhong Plain in Northwest China.  Field experiments were conducted using four N fertilization levels (0, 105, 210 and 315 kg ha⁻¹) applied to six wheat cultivars in the 2013–2014 and 2014–2015 growing seasons.  LAI, DM, plant N concentration (PNC) and grain yield were determined.  Data points from four cultivars were used for establishing the Nc curve and data points from the remaining two cultivars were used for validating the curve.  The N_c dilution curve was validated for N-limiting and non-N-limiting growth conditions and there was good agreement between estimated and observed values.  The N nutrition index (NNI) ranged from 0.41 to 1.25 and the accumulated plant N deficit (N_and) ranged from 60.38 to –17.92 kg ha⁻¹ during the growing season.  The relative grain yield was significantly affected by NNI and was adequately described with a parabolic function.  The N_c curve based on LAI can be adopted as an alternative and more rapid approach to diagnose plant N status to support N fertilization decisions during the vegetative growth of winter wheat in Guanzhong Plain in Northwest China.