Chemical control of weeds in wheat (Triticum aestivum L.) in Iran

Experiments were conducted in 2006–2007 at six research stations of the Iranian Plant Protection Research Institute to study the efficacy of different herbicides to control weeds in wheat. Treatments included mesosulfuron-methyl plus iodosulfuron-methyl-sodium plus mefenpyr-diethyl (WG) at 45 + 45 + 135 g a.i./ha, respectively, sulfosulfuron at 21, 31.5, 42 and 51 g a.i./ha, chlorsulfuron at 15 g a.i./ha, bromoxynil plus MCPA at 600 g a.i./ha with clodinafop propargyl at 64 g a.i./ha, sulfosulfuron plus metsulfuron-methyl at 36 g a.i./ha, mesosulfuron-methyl plus iodosulfuron-methyl-sodium plus mefenpyr-diethyl (OD) at 15 + 3 + 45 g a.i./ha, respectively, and a full season weed-free control. Herbicides were applied at wheat tillering. Results showed that sulfosulfuron plus metsulfuron-methyl, and bromoxynil plus MCPA with clodinafop propargyl resulted in satisfactory weed control and wheat grain yield at most locations. Weed control efficacy of both formulations of mesosulfuron-methyl plus iodosulfuron-methyl-sodium was variable across locations. Efficacy of the OD formulation appears to depend upon location, so that application of this herbicide at Shiraz and Gorgan resulted in better weed control compared to use of the WG. Satisfactory performance of the OD formulation at Gorgan and Shiraz could be attributed in part to the even pattern of rainfall distribution during the growing season and wheat cultivar used, respectively. With respect to grain yield, however, the OD formulation was better than WG formulation at most of the locations.     

Impact of glyphosate-resistant volunteer corn (Zea mays L.) density,control timing,and late-season emergence on yield of glyphosate-resistant soybean (Glycine max L.)

Glyphosate-resistant (GR) volunteer corn is a troublesome weed in soybean fields in a corn-soybean rotation as well as in corn fields in a continuous corn production system. The objectives of this study were to evaluate the impact of (1) different densities of GR volunteer corn on soybean yields, present as individual plants or clumps, controlled at fourth trifoliate (V4), sixth trifoliate (V6), or full flowering (R2) soybean growth stages, and (2) late-season volunteer corn emergence on soybean yields, after being controlled at different soybean growth stages. Field experiments were conducted in 2013 and 2014 under irrigated conditions in Clay County, Nebraska, and under rain-fed conditions in Lancaster County, Nebraska, USA. To maintain the desired number of isolated volunteer corn plants (1250, 2500, 5000, and 10,000 plants ha⁻¹) and clumps (63, 125, 250, and 500 clumps ha⁻¹), individual seeds and/or corn ears were hand-planted in each plot based on their respective target densities. Volunteer corn was controlled with applications of clethodim at V4, V6, or R2 soybean growth stages. Late-season volunteer corn emergence had no effect on soybean yield with volunteer corn densities and control timings at both locations in 2013 and 2014. During the first year of study at Clay County, volunteer corn densities and control timings had no effect on soybean yield. When volunteer corn was left uncontrolled or controlled at the R2 soybean growth stage, yield was the lowest at highest isolated volunteer corn plants (10,000 plants ha⁻¹) plus clump density (500 clumps ha⁻¹) during the second year of study in Clay County (≤5068 kg ha⁻¹) and during both years of study in Lancaster County (≤1968 kg ha⁻¹).     

Weed emergence as affected by maize (Zea mays L.)-cover crop rotations in contrasting arable soils of Zimbabwe under conservation agriculture

Weed control in smallholder farming systems of sub-Saharan Africa is labour intensive or costly. Many researchers have therefore advocated for the use of cover crops in weed management as an affordable alternative for smallholders. Cover crops may be grown in rotations to suppress weeds and reduce the reliance on herbicides. The use of cover crops creates microenvironments that are either conducive or inhibitive to the emergence of certain weed species. A study, initiated in 2008 in contrasting soils at four different locations of Zimbabwe, investigated the effect of maize (Zea mays L.)-cover crop rotations on the emergence of weeds that showed dominance in those soils. Weed assessments were however, carried out from 2011 to 2014. The weed species Galinsoga parviflora Cav., Commelina benghalensis L., and Richardia scabra L. showed dominance in all four locations with weed densities as high as 500 plants m⁻² being recorded for R. scabra L. in a sandy soil. Maize-cover crop rotations resulted in higher densities of Bidens pilosa compared with maize monocropping (control treatment) due to its high nitrogen (N) requirement to produce more seeds. On the other hand, the integration of cover crops such as pigeon pea [Cajanus cajan (L.) Millsp.] that had poor shading qualities, due to large gaps or spaces and slower initial growth, had limited effects on competitive weeds such as Cyperus esculentus L. which tend to dominate exhausted soils. The density of C. esculentus was 38% greater in maize–pigeon pea rotations compared with the control treatment. Variability between seasons and sites affected emergence of all weeds in the present study, which masked long-term trends. The results suggest that there is need to identify the germination and emergence requirements of specific weeds and select cover crops best suitable for their control. The study provides useful information for farmers and advisors on the best cover crops for control of certain problematic weeds in different soil types of Zimbabwe.     

Control of volunteer cereals with post-emergence herbicides in maize (Zea mays L.)

Volunteer winter cereals are found sporadically in maize (Zea mays L.) fields across southern Ontario. Seven field trials were conducted over a two-year period (2006 and 2007) at four locations to determine the efficacy of five acetolactate synthase (ALS)-inhibiting herbicides for the control of volunteer cereals applied at two post-emergence application timings (2–4 and 4–7 maize leaf tips). The volunteer cereals were a hard red winter wheat (Triticum aestivum L.) (‘Hyland AC Morley’), soft red winter wheat (‘Pioneer 25R47’), soft white winter wheat (‘Pioneer 25W41’), and a autumn rye (Secale cereale L.) (‘FR’) cultivar. Volunteer cereal competition in maize resulted in a yield reduction of up to 44%. Foramsulfuron, nicosulfuron, nicosulfuron/rimsulfuron provided greater than 70% control of the volunteer cereals at 56 days after treatment (DAT), while primisulfuron and rimsulfuron provided greater than 60% control. Volunteer cereal control with early and late application was greater than 82 and 61%, respectively. Hard red winter wheat was the most sensitive to the ALS-inhibiting herbicides with control of 84–93%. Soft red and soft white winter wheat cultivars were intermediate in sensitivity with control of 76–87%, while autumn rye was the least sensitive with control of at 56–71% control at 56 DAT. Maize yields were improved when volunteer cereals were controlled with the use of herbicides compared to the weedy control, but were lower than the weed-free control. Early herbicide application resulted in improved control of volunteer cereals and higher maize yield.     

Integrated effect of allelochemicals and herbicides on weed suppression and soil microbial activity in wheat (Triticum aestivum L.)

To evaluate the allelopathic effects of major crops and weeds, studies were conducted during 2011–12 and 2012–13 by utilizing water extracts of allelopathic plants namely rice (Oryza sativa L.), Parthenium hysterophorus L., Phragmites australis Cav., and Datura alba L. with reduced rates of fenoxaprop-p-ethyl and bromoxynil plus MCPA to control weeds. Application of Phragmites australis and Parthenium hysterophorus along with lower rates of fenoxaprop-p-ethyl and bromoxynil plus MCPA showed promising results by controlling weeds and improving yield. Parthenium hysterophorus extract with half of the recommended dose of fenoxaprop-p-ethyl and bromoxynil plus MCPA reduced weed density by 51 and 50% during year 1 and year 2, respectively, compared with the control. Phragmites australis extract with half of the recommended dose of fenoxaprop-p-ethyl produced grain yield (5.7 and 6.1 t ha⁻¹ during year 1 and 2, respectively) in wheat. However, these results were also at par with Parthenium hysterophorus and Datura alba extracts in combinations with half the recommended doses of the above mentioned herbicides. The study of microbial activity showed higher amount of mineralizable carbon in D. alba with half the recommended dose of fenoxaprop-p-ethyl treated plots (0.073 g during both the years). The lowest amount of mineralizable carbon (0.035 and 0.030 g during year 1 and 2, respectively) was observed in the control plots. The presence of allelopathic plants in field crops and subsequent mixing in soil by tillage may create problems in crop production. Therefore, further studies are suggested to fully explore all the possible interactions among allelochemicals and herbicides.     

The impact of tillage system and herbicides on weed density,diversity and yield of cotton (Gossipium hirsutum L.) and maize (Zea mays L.) under the smallholder sector

The study was carried out to evaluate the impact of tillage system in combination with different herbicides on weed density, diversity, crop growth and yields on 18 farms in Kadoma, Zimbabwe. Experiments were set up as a split plot design with three replications on each farm. Tillage was the main plot (Conservation Tillage (CT), Conventional Tillage (CONV)) and weeding option (hand weeding, cyanazine, atrazine, glyphosate only and mixture of cyanazine + alachlor and atrazine + alachlor) as the sub-plots. Due to the heterogeneous nature of farmers' resource base, the farms were grouped into three farm types: high (Type 1), medium (Type 2) and poorly resourced farmers (Type 3). The hand hoe weeded treatments had 49 percent higher total weed densities in CT relative to CONV, and was statistically similar to the glyphosate treatment. The mixed pre-emergence herbicides reduced the diversity indices by 69 and 70 percent when compared to the hand hoe weeded treatment under CT in cotton and maize, respectively. The effectiveness of all pre-emergence herbicides were not influenced by tillage but were affected by farmers resource endowments with pronounced effect in Farm Type 1. Maximum plant heights of 85 and 238 cm were recorded for mixed pre-emergence herbicides under CT for cotton and maize, respectively. Minimum plant heights of 75 and 217 cm were recorded for the respective hand hoe weeded treatments. The hand hoe weeded treatments resulted in average cotton lint yield of 1497 and 2018 kg ha⁻¹ for maize. The mixed pre-emergence herbicides treatments gave yields of 2138 and 2356 kg ha⁻¹ of cotton and maize, respectively. The higher weed densities in CT under hand weeded treatments underscored the need for other weeding options. Similarly, a mixture of cyanazine + alachlor in cotton and atrazine + alachlor in maize is recommended for suppressing broad and grass weed populations and enhancing yields in CT systems.     

A thermal time analysis of ageing of maize (Zea mays L.) seed can account for reduced germination in hot moist soil

After sowing into moist soil, maize seed imbibe although the soil moisture content may be too low to permit seedling emergence. The hypothesis tested here is that in such circumstances, and especially at high temperature, the seed age in the soil with the result that when the moisture restriction is subsequently removed, germination and seedling emergence are reduced.First, maize seed were artificially deteriorated in the laboratory at four elevated moisture and five constant temperature regimes for four different periods in order to determine the effect of ageing on radicle and coleoptile emergence. Next, the responses when seed were deteriorated at alternating temperatures were shown to be the same when the temperature exposure was expressed as thermal time. A base temperature for thermal ageing time was estimated as about 30°C. Finally, when four Sri Lankan maize cultivars were sown in soil, the seed imbibed and experienced temperatures above 30°C with the result that final seedling emergence was reduced. The effect of ageing in the soil appeared greater than that in the laboratory because the moisture content of seed in the soil was not constant but increasing.     

Narrow row planting increases yield and suppresses weeds in common bean (Phaseolus vulgaris L.) in a semi-arid agro-ecology of Nyagatare,Rwanda

Narrow row planting has potential to increase crop growth and yield by increasing radiation interception (RI) and minimizing intra-specific competition in the crop. It reduces weed growth and competitiveness, making resources that are normally taken up by weeds available for crop uptake. The objective of this study was to assess the effect of row spacing on weed biomass, bean growth and yield in a semi arid agro-ecology at Nyagatare, Rwanda. The study was set up as a randomized complete block design in October–December 2009 and repeated in 2011. Planting patterns at a constant bean population density of 111 000 plants ha⁻¹ random planting (normal practice), narrow row planting (30 cm × 30 cm), medium row planting (45 cm × 20 cm) and wide row planting (60 cm × 15 cm) were treatments tested in this study. The narrow row square planting pattern significantly (P < 0.01) out-yielded the wide and random planting patterns by 22–31% in the wet 2009 season and by 27–70% in the dry 2011 season. Bean plant dry weight (P < 0.01) and number of pods per plant (P < 0.01) was highest in the narrow row and lowest in the random planting pattern in the dry 2011 season. Bean plant dry weight was not significantly affected (P > 0.05) in the wet 2009 season but number of pods plant⁻¹ (P < 0.001) was highest in the narrow row and lowest in the random planting pattern. Weed biomass was significantly lower (P < 0.05) in the narrow row and the random than in the medium and wide row planting patterns at 3, 6 and 9 weeks after emergence in 2009, but the random planting had the highest weed biomass in 2011. The results suggest that the effects narrow row planting in alleviating the negative impact of inter- and intra-specific competition were more strongly expressed in the dry 2011 season than the wet 2009 season when water was probably not a limiting factor to crop growth and yield. The results also indicate that narrow and equidistant planting has potential to increase bean yield by 30%–70%, when compared to random planting (normal practice) while at the same time suppressing weed growth and is recommended for smallholder farmers in Rwanda and other semi-arid areas in sub-Saharan Africa.     

Integration of soil-applied herbicides at the reduced rates with physical control for weed management in fennel (Foeniculum vulgare Mill.)

Fennel has been widely used in traditional medicine for their antimicrobial effects. Since fennel is long duration crop and have slow initial growth, it protection from weed is essential. Experiments were conducted for two consecutive seasons to evaluate the efficacy of soil-applied herbicides at the reduced rates in combination with physical control for weed management and optimizing the yield of fennel. Treatments were type of herbicide (trifluralin and pendimethalin), application dose (recommend dose (R), 75% R, 50% R, and 0% R) and physical weed control (none, one hand-weeding at 50 day after planting (DAP), wheat straw mulch). Weed-free control treatment was also included in each year. The results showed that the use of soil-applied herbicides resulted in reduced weed biomass but did not provide season long weed control without an additional physical control. In both seasons, pendimethalin provided better weeds control than trifluralin. Reduced herbicide rates were found to be more effective when herbicides application followed by hand-weeding than when were used alone or combined with mulch. Experimental results also showed that one time increasing in herbicide rates increased seed yield by 17.5 and 7.5% in 2012 and 16.5 and 6.3% in 2013, when one hand-weeding and mulching were used as supplemental control, respectively. Overall, the 75% of the labeled recommended rate of herbicides followed by one hand-weeding at 50 DAP produced consistently high yields and less weed biomass, reflecting both superior weed control and crop safety.     

Effects of crude oil spillage on growth and yield of maize (Zea mays L.) in soils of midwestern Nigeria

The effect of crude oil spillage on growth, productivity and nutrient uptake of maize (Zea mays L.) was assessed in a pot experiment using an Evwreni manifold sample of a petroleum development company, which had aspecific gravity of 0.8778. The Suwan 1 variety of maize was used in the experiment. In crude oil polluted soils, germination was delayed and the germination percentage was significantly affected by oil pollution. Growth was poor in polluted soils using parameters such as plant height, stem girth,ear height, leaf area at four weeks after planting, leaf area at maturity and average length of primary roots as growth indicators. Grain yield was significantly reduced at 95% level of probability with yield (when compared with the control) reduced by as much as 98.6%, 96.5% and 58.3% for preplant,five weeks after planting (5 WAP) and seven weeks after planting (7 WAP) treatments, respectively. Leaf analysis of the maize plants grown in soilscontaminated with crude oil a week before planting (preplant treatment) revealed mean levels of heavy metals (6.18 ppm Zn²⁺, 0.62 ppm Cu²⁺,26.24 ppm Fe²⁺, 10.84 ppm Mn²⁺, 2.96 ppm Pb²⁺ and 3.88 ppm Co²⁺) which are higher than the maximum permissible levels (MPL) for maize in tropical soils. Maize plants that were polluted at other time intervals showed no significant (p>0.05) variation in heavy metal concentrations when compared with the control, and were considered potentiallysafe for human consumption.     

Effect of nitrogen supply on leaf appearance,leaf growth,leaf nitrogen economy and photosynthetic capacity in maize (Zea mays L.)

Leaf area growth and nitrogen concentration per unit leaf area, N_a (g m⁻² N) are two options plants can use to adapt to nitrogen limitation. Previous work indicated that potato (Solanum tuberosum L.) adapts the size of leaves to maintain N_a and photosynthetic capacity per unit leaf area. This paper reports on the effect of N limitation on leaf area production and photosynthetic capacity in maize, a C4 cereal. Maize was grown in two experiments in pots in glasshouses with three (0.84–6.0 g N pot⁻¹) and five rates (0.5–6.0 g pot⁻¹) of N. Leaf tip and ligule appearance were monitored and final individual leaf area was determined. Changes with leaf age in leaf area, leaf N content and light-saturated photosynthetic capacity, P_max, were measured on two leaves per plant in each experiment. The final area of the largest leaf and total plant leaf area differed by 16 and 29% from the lowest to highest N supply, but leaf appearance rate and the duration of leaf expansion were unaffected. The N concentration of expanding leaves (N_a or %N in dry matter) differed by at least a factor 2 from the lowest to highest N supply. A hyperbolic function described the relation between P_max and N_a. The results confirm the ‘maize strategy’: leaf N content, photosynthetic capacity, and ultimately radiation use efficiency is more sensitive to nitrogen limitation than are leaf area expansion and light interception. The generality of the findings is discussed and it is suggested that at canopy level species showing the ‘potato strategy’ can be recognized from little effect of nitrogen supply on radiation use efficiency, while the reverse is true for species showing the ‘maize strategy’ for adaptation to N limitation.     

Evaluation of some weed control treatments for long season weed control in maize (Zea mays L.) under zero and minimum tillage at Samaru, in Nigeria

The effect of weed control treatments for long season control of weeds in maize under zero and minimum tillage was evaluated at Samara in northern Nigeria. Among the weed control treatments evaluated, soil ridging plus application of either 2,4-D or atrazine at 1.5 kg a.i/ha performed well as they effectively controlled weeds and resulted in better growth and a grain yield that was comparable to the hand weed control. Maize production under minimum tillage was better than under zero tillage.     

EPIC模型对华北平原冬小麦与夏玉米生长和产量模拟的适用性评价

为验证EPIC模型应用于华北地区农业生产的适用性,利用禹城生态试验站的试验观测数据对该模型参数进行了修订,构建了适合华北平原的模型参数数据库,并分别验证了2003-2005年冬小麦、2004年夏玉米的叶面积指数、生物量和产量模拟的精度与适用性。结果表明,EPIC模型对禹城站冬小麦和夏玉米叶面积指数和地上部生物量的模拟值和实测值随时间的变化趋势大体一致,决定系数均高于0.81;EPIC模型对禹城站2004、2005年冬小麦产量的模拟误差分别为1.61%和-2.56%,对2004夏玉米产量的模拟误差为-2.60%。因此,该模型模拟产量的绝对误差低于2.60%,适用于华北平原冬小麦和夏玉米的产量评估。     

小麦生育后期主茎和分蘖次生根对籽粒产量和品质的影响

为通过根系调控提高小麦产量和改善品质,以弱春性优质强筋小麦品种郑麦9023为材料,研究了生育后期主茎次生根和分蘖次生根对小麦籽粒产量和品质的影响。结果表明,小麦分蘖次生根根系活力与根中全氮含量显著高于主茎次生根,而根中可溶性糖含量则低于主茎次生根。随着生育进程的推进,分蘖次生根与主茎次生根间的差异逐渐减小。分蘖次生根对单株成穗数、千粒重及单株产量的贡献大于主茎次生根,而对穗粒数的贡献则显著小于主茎次生根。与对照（维持完整根系）相比,只留主茎次生根处理和只留分蘖次生根处理的籽粒直链淀粉含量与直/支比例显著提高,而蛋白质含量显著下降。因此,生产实践中可以通过调节分蘖次生根与主茎次生根的比例及生理特性来提高籽粒产量和改善品质。     

硫肥对玉米氮、磷、钾吸收利用影响的基因型差异

以我国夏玉米主产区的34个玉米基因型(6个自交系,4个特用玉米和24个杂交种)为试验材料,通过不同硫肥供应水平的大田试验,分析了硫肥对玉米氮、磷、钾吸收利用的影响.结果表明,施用硫肥促进了玉米对营养元素的吸收,植株氮、磷、钾的积累总量平均比未施用硫肥的对照分别增加6.07%、11.89%和19.31%,肥料利用效率提高.硫肥对氮、磷、钾吸收利用的影响不完全相同,并且存在明显的基因型差异施用硫肥后,自交系氮、磷、钾的收获指数和利用效率均有大幅度提高;杂交种的氮素收获指数和利用效率均有所提高,磷素利用效率下降,但是不同基因型的变化程度差异较大;另有17个杂交种的磷素收获指数降低,10个杂交种的钾素收获指数和14个杂交种的钾素利用效率降低.     

大麦单株产量性状的杂种优势及其稳定性分析

为研究不同环境下大麦单株产量性状的杂种优势及其稳定性,以113个(Nasonijo×泰兴9425)DH系配制226个杂交种构建的永久F_2群体及亲本为材料,分别调查参试材料在4个环境下的单株穗数、主穗粒数、千粒重、单株产量和单株生物量,利用方差分析、聚类分析及稳定性分析对大麦单株产量性状的杂种优势及其稳定性进行了分析。结果表明,大麦杂种F_1各被测性状大多表现中亲优势,超亲优势组合出现率相对较低。各被测性状的杂种优势不仅受自身遗传因素影响,而且受试点生态条件及年度气候条件的影响。杂种优势的稳定性因性状不同而异,千粒重和主穗粒数杂种优势的稳定性较好,单株生物量和单株产量杂种优势的稳定性较差,仅有2个组合在多环境下表现出稳定的强杂种优势。     

Effects of Coronatine on Growth,Gas Exchange Traits,Chlorophyll Content,Antioxidant Enzymes and Lipid Peroxidation in Maize (Zea mays L.) Seedlings under Simulated Drought Stress

Abstract

Coronatine is a phytotoxin that affects the accumulation of defence-related metabolites in plants but information on how its effects may be mediated by environmental stress is scanty. An experiment was carried out to determine the changes in growth, gas exchange, relative water content, chlorophyll (Chl) content, antioxidant enzymes and lipid peroxidation in maize (Zea mays L., var. ‘Nongda 3138’) seedlings treated with coronatine under simulated drought stress. Seedlings raised hydroponically in a growth chamber with simulated drought for 8d (long-period drought) or 3d (short-period drought) were treated with or without coronatine at the three-leaf stage. Under the drought condition, treated with coronatine significantly increased the fresh weight and relative water content in leaves of seedling leaves. The increase was accompanied by increased rates of photosynthesis and transpiration, and the maintenance of Chl pigments. Coronatine had no effects on catalase (CAT), guaiacol peroxidase (POD) and glutathione reductase (GR) under normal condition, but it significantly enhanced activities of CAT, POD and GR in stressed seedlings under the long-period drought treatment. Under the short-period drought treatment, the POD and GR activity in the seedlings treated with coronatine were much higher than in those not treated. Malondialdehyde (MDA) increased sharply under drought condition, but treatment with coronatine significantly reduced it by 15%. The total Chl content of leaves under the drought condition was markedly increased by the treatment with coronatine. Seedlings subjected to a short-period drought had reduced water content, but recovered fairly well by the treatment with coronatine with negligible effects on most physiological and biochemical processes. The application of coronatine alleviated the drought stress in maize seedlings and enhanced their tolerance of water stress through changes in physiological and anti-oxidant enzyme activities.     

Association of Puroindoline b-B2 variants with grain traits,yield components and flag leaf size in bread wheat (Triticum aestivum L.) varieties of the Yellow and Huai Valleys of China

A total of 169 wheat (Triticum aestivum L.) varieties (landraces and cultivars) were used to asses the relationship between Puroindoline D1 alleles and Puroindoline b-B2 variants and grain hardness, other grain traits, yield components, and flag leaf size. Results indicated that the average SKCS hardness of Pinb-B2v3 varieties was significantly greater than that of Pinb-B2v2 varieties within the soft Puroindoline D1 haplotype sub-group. Conversely, no statistically significant difference was obtained for SKCS hardness between varieties with the Pinb-B2v3 vs. Pinb-B2v2 alleles within the two hard Puroindoline D1 haplotypes (Pinb-D1b and Pinb-D1p sub-groups). Therefore, the Puroindoline b-B2 gene may have a bigger impact on soft wheat varieties than hard. Across all varieties, thousand-kernel weight, grain weight per spike, grain diameter, grain number per spike, flag leaf width and area of Pinb-B2v3 varieties were significantly greater than those possessing Pinb-B2v2. These results indicated that the Pinb-B2v3 allele was associated with preferable grain yield traits compared to the Pinb-B2v2 allele in bread wheat. This study provides evocative information for better understanding the molecular and genetic basis of wheat grain yield.