宽幅播种提高不同播期小麦产量与氮素利用率

为明确在较宽播期范围内可实现小麦高产高效稳产的播种方式及其理论基础,采用宽幅播种和常规条播2种播种方式,设计10月3日(早播)、10日(传统播期)、17日(晚播)和24日(再晚播)共4个播期处理(分别用D1、D2、D3、D4表示),研究了播种方式与播期互作对小麦产量和氮素吸收利用的影响。相对于常规条播,宽幅播种通过提高单位面积分蘖数和穗数,平均提高产量16.68%;通过提高氮素吸收效率(吸氮量/供氮量)、稳定或提高氮素利用效率(产量/吸氮量),平均提高氮素利用率(产量/供氮量)16.64%。随播期推迟,2播种方式下单位面积穗数、单穗籽粒质量分别呈降低和升高趋势,相对于D1和D2播期,宽幅条件下D3、D4播期的成熟期穗数下降比例显著低于条播,并与其单穗籽粒质量提高的比例相当,进而实现9.00 t/hm2水平的高产稳产;常规条播下晚播因穗数大幅下降导致减产,平均减产0.34 t/hm2。随播期推迟,2播种方式下氮素吸收效率和氮素利用效率分别呈降低和升高趋势,相对于D1、D2播期,宽幅条件下D3、D4播期氮素吸收效率下降的幅度与氮素利用效率提升的幅度相当,因此仍可维持较高的氮素利用率;常规条播下晚播处理氮素吸收效率下降的幅度显著高于氮素利用效率提升的幅度,进而导致氮素利用率平均降低1.01 kg/kg。相对于常规条播,小麦生产上采用宽幅播种,在高产高效的同时可实现较宽播期范围内产量和氮素利用率的稳定。     

不同氮肥水平下水稻产量以及氮素吸收、利用的基因型差异比较

采用田间试验在施氮量为06、0、120、1802、40、3003、60.kg/hm27个水平下研究了不同水稻子粒产量、产量构成因子以及氮素吸收和利用的差异。结果表明,水稻品种4007的子粒产量在各个施氮水平下显著高于品种ELIO对氮肥的响应度高。施氮水平显著影响子粒产量构成因子。有效穗数与子粒产量存在显著正相关:ELIO和4007的相关系数(r)分别为0.839**和0.933**,表明有效穗数对水稻子粒产量起着非常重要的作用。本试验条件下,ELIO和4007获得最高产量所需的有效穗数分别为332、561个/m2;两者的氮素吸收效率在各施氮素水平下差异很小,均随着施氮量的增加而增加,而氮素利用效率均随着施氮量的增加而下降。4007的氮素利用效率在各个施氮水平下显著高于ELIO,较高的氮素收获指数(NHI)是主要原因之一。水稻氮素利用效率与成熟期茎秆、叶片的氮含量显著负相关,说明开花期后植物将吸收的氮素从营养器官有效地转运到子粒中是氮素利用效率高的重要原因之一。     

Assessment of nitrogen uptake and utilization in drought tolerant and Striga resistant tropical maize varieties

Maize (Zea mays L.) is an important food crop in the Guinea savannas of Nigeria. Despite its high production potential, drought, Striga hermonthica parasitsim, and poor soil fertility particularly nitrogen deficiency limit maize production in the savannas. Breeders at IITA have developed drought- and Striga-tolerant cultivars for testing, dissemination, and deployment in the region. Information on the response of these cultivars to N fertilization is, however, not available. This study evaluated grain yield, total N uptake (TNU), N uptake (NUPE), N utilization (NUTE), and N use efficiency (NUE) of selected maize cultivars along with a widely grown improved maize cultivar at two locations in the Guinea savannas of northern Nigeria. Maize grain yield increased with N application. The average grain yield of the maize cultivars was 76% higher at 30, 156% higher at 60, and 203% higher at 120 kg N ha^?1 than at 0 kg N ha^?1. This suggests that N is a limiting nutrient in the Nigerian savannas. Five drought-tolerant cultivars produced consistently higher yields when N was added at all levels. These cultivars had either high NUPE or NUTE confirming earlier reports that high N uptake or NUTE improves maize grain yield. The study also confirms earlier reports that maize cultivars that are selected for tolerance to drought are also efficient in uptake and use of N fertilizer. This means that these cultivars can be grown with application of less N fertilizer thereby reducing investment on fertilizers and reduction in environmental pollution.     

Nitrogen Use Efficiency and Herbage Production of an Established Grass Sward in Relation to Moisture and Nitrogen Fertilization

Herbage growth and nitrogen (N) use efficiency in grazed or mown grasslands are generally low, associated mostly with poor response to fertilizer N. The aim of the present investigation was to examine the short-term response of grass to fertilizer N with respect to herbage yield and nitrogen use efficiency (NUE) in order to provide a better basis for improving the efficient use of fertilizer N in grassland ecosystems. Both NO₃ ^? and NH₄ ⁺ sources of N were applied to an established grass sward with three moisture levels, i.e., natural conditions (63% water-filled pore space, WFPS), near field capacity level (71% WFPS), and slightly wetter than field capacity (84% WFPS). Herbage yield, i.e., dry matter (DM), N uptake, N recovery efficiency, yield efficiency, and physiological efficiency were determined over a 7–28 d period. Addition of N fertilizers significantly increased the herbage yield and N uptake of grass sward over that of the control. In the plots where NO₃ ^??N was added as the N source, DM yield was between 1760–1870 kg ha^?1, N recovery efficiency was between 24%–43%, and yield and physiological efficiency were in the range of 2.1–3.2 and 6.4–8.8 kg DM kg^{? 1} N, respectively. In NH₄ ⁺?N added plots, the DM yield was between 3190–3700 kg ha^{? 1}, N recovery efficiency was between 39%–48% while yield and physiological efficiency were in the range of 3.5–5.6 and 9.0–11.6 kg DM kg^{? 1} N, respectively. Results indicated that total DM yield, N uptake, and NUE depend on the source of N and the level of moisture in the field. Assimilation of N is also affected by the stage of plant development after N fertilization. About 50%–54% of applied N was recovered in the initial 14 and 21 d after fertilizer application and thereafter translocation of N slowed. A fall in herbage production and minimal response to N fertilizer has been observed at 84% WFPS, while the maximum herbage yield and N recovery efficiency was recorded in soil near or below field capacity. The grass sward with added NH₄ ⁺?N produced a larger yield and had higher NUE relative to the sward with NO₃ ^??N. Results confirm that applied N was not utilized efficiently by grass sward and a decrease in N uptake and its utilization seem to be the key factors responsible for the poor herbage productivity often observed in pastoral agriculture. These results suggest that both moisture and N source have a substantial effect on herbage yield and N utilization by plants and therefore should be considered for efficient management of N fertilization and recommendations for grass sward.     

Durum Wheat Genotypic Variation of Yield and Nitrogen use Efficiency and Its Components Under Different Water and Nitrogen Regimes in the Mediterranean Region

Farmers in dry areas of the Mediterranean region are reluctant to apply nitrogen (N) fertilizer to rainfed wheat because of frequent drought. So, it is desirable to select varieties with high nitrogen use efficiency (NUE). The objectives of this study, conducted in 2009/10 and 2010/11 in Syria, were to study the response of genotypes of durum wheat to low and high N applications and to evaluate the contribution of N uptake efficiency and utilization efficiency to NUE under rainfed and supplemental irrigation conditions. Under the rainfed regime, grain yield decreased significantly in year 1 and year 2 when applying N fertilizer at a high rate. The early maturing genotypes gave the highest average yields under rainfed and irrigated regimes. High N fertilizer rate decreased significantly NUE from 36.1 to 24.3 in year 1 and 37.0 to 5.8 in year 2. Under irrigation, NUE fell from 84.6 to 67.1 in year 1 and from 117.7 to 33.3 in year 2. The contribution of N uptake efficiency and utilization efficiency to NUE varied from one year to another. In year 1, the contribution of N utilization efficiency was more important at all nitrogen levels; while the opposite was observed in year 2 when more N was applied. The fractions of the genotype sum of squares, respectively, for N uptake efficiency and utilization efficiency were in average 0.15 and 0.78 in year 1 and 0.75 and 0.25 in year 2. From this study we conclude that high N levels in the soil exacerbate the effect of water stress on productivity and NUE of wheat. Early maturing new genotypes tend to be better adapted and to use nitrogen more efficiently under limited water conditions. N utilization efficiency contributes more to NUE under high N availability than N uptake efficiency and vice-versa.     

Response of Wheat Cultivars to Nitrogen and Sulfur for Crop Yield,Nitrogen Use Efficiency,and Protein Quality in the Semiarid Region

ABSTRACT

The effects of nitrogen (N at 0, 100 and 180 kg N ha-1) and sulfur (S at 0, 20, 40 and 60 kg S ha-1) on crop yield, nutrient uptake, nitrogen use efficiency (NUE), and amino acid composition of two bread wheat cultivars, ‘Shehan’ and ‘Enkoy,’ grown in Andisols and Cambisols in randomized blocks with three replications were evaluated. Both cultivars responded significantly (P < 0.05) to N and S applications and S application with N improved the NUE by 28%. The yield increase for the two cultivars by N and S application ranged between 0.8 to 2.4 Mg ha^?1. The N concentration increased significantly from N0 to N2 in both cultivars. Sulfur fertilization increased the concentration of cysteine and methionine by 27% and 14%, respectively, as compared to N alone. The grain yield, NUE, N, and S uptake, and the S-amino acid concentration of ‘Enkoy’ were significantly higher than ‘Shehan’ cultivar.     

腐植酸与氮肥配施对冬小麦氮素吸收利用及产量的影响

研究腐植酸与氮肥配施对冬小麦产量、氮素吸收及经济效益的影响,可为提高氮肥的增产效益,减少氮肥对生态环境的污染提供理论指导。在河南褐土区冬小麦-夏玉米轮作制度下,于2014年开始在河南省南阳市卧龙区开展田间定位试验,共设置单施磷钾肥、常规施肥、单施腐植酸、常规施肥+腐植酸、常规施肥减氮15%+腐植酸、常规施肥减氮30%+腐植酸6个处理,分析不同氮肥与腐植酸配施下冬小麦产量和氮肥利用的特征。结果表明,腐植酸与氮肥配施可以有效提高冬小麦的产量及其构成要素,促进植株对氮素的累积,提高氮肥利用率。其中,常规施肥减氮15%+腐植酸处理下冬小麦产量、籽粒氮含量、籽粒氮累积量、地上部总氮累积量、氮肥利用效率和纯收益均增加,与常规施肥相比,冬小麦产量增加4.96%,氮肥利用效率增加23.42%,纯收益增加2.18%。常规施肥减氮30%+腐植酸条件下冬小麦产值和收益降低。因此,在施用腐植酸的基础上,配施适量氮肥才能获得较高的产值和收益。常规施肥减氮15%+腐植酸是本研究区域最佳的施肥模式,对实现现代化农业生产的高产高效、资源节约和生态环境保护具有重要意义。     

Nitrogen Use Efficiency of Rice Under Cadmium Contamination:Influence of Rice Cultivar Versus Soil Type

There is a need for rice cultivars with high yields and nitrogen (N) use efficiency (NUE), but with low cadmium (Cd) accumulation in Cd-contaminated paddy soils. To determine the relative effects of rice genotype, soil type, and Cd addition on rice grain yield and NUE, a pot experiment consisting of nine rice cultivars was conducted in two types of paddy soils, red soil (RS) and yellow soil (YS), without or with Cd spiked at 0.6 mg kg^-1. The N supply was from both soil organic N pools and N fertilizers; thus, NUE was defined as the grain yield per unit of total crop-available N in the soil. Cd addition decreased grain yield and NUE in most rice cultivars, which was mainly related to reduced N uptake efficiency (NpUE, defined as the percentage of N taken up by the crop per unit of soil available N). However, Cd addition enhanced N assimilation efficiency (NtUE, defined as the grain yield per unit of N taken up by the crop) by 21.9% on average in all rice cultivars. The NpUE was mainly affected by soil type, whereas NtUE was affected by rice cultivar. Hybrid cultivars had higher NUEs than the japonica and indica cultivars because of their greater biomass and higher tolerance to Cd contamination. Reduction of NUE after Cd addition was stronger in RS than in YS, which was related to the lower absorption capacity for Cd in RS. Canonical correspondence analysis-based variation partitioning showed that cultivar type had the largest effect (34.4%) on NUE, followed by Cd addition (15.2%) and soil type (10.0%).     

施氮对单作和套作小麦产量和氮素利用特征的影响

小麦/玉米套作是四川主要的旱作模式,研究小麦的氮素吸收利用效率及套作玉米对小麦的影响有助于进一步提示套作小麦的增产优势、养分高效利用及了解玉米小麦间相互作用机理。本研究通过田间试验研究了不同氮水平下[0 kg(N)·hm-2、60 kg(N)·hm-2、120 kg(N)·hm-2和180 kg(N)·hm-2,分别记为N1、N2、N3和N4]小麦单作、小麦/空带和小麦/玉米套作3种模式中小麦的产量、氮素吸收利用特征和玉米对小麦的影响。结果表明:在不同的氮处理下,与单作小麦相比,小麦玉米套作的小麦始终表现出明显的产量优势,其生物量和籽粒产量比单作小麦平均增加15.7%和17.8%;套作小麦边行优势明显,其边行的地上部生物量、产量、吸氮量和氮肥偏生产力比单作行分别增加23.8%、27.3%、48.9%和19.1%,说明套作小麦比单作小麦对氮利用效率更高。不施氮(N1)和低氮(N2)处理小麦/玉米套作模式中小麦的生物量、产量比小麦/空带模式平均低6.5%和5.7%,但在中氮水平(N3)时小麦/玉米套作模式中小麦产量、地上部生物量、地上部吸氮量和氮肥偏生产力分别比小麦/空带模式高14.1%、5.0%、6.8%和4.5%。说明在小麦/玉米套作模式中套入玉米在施氮不足时小麦生长受到抑制,而在施氮充足时小麦生长得到促进。因此,套作小麦有边行优势和产量优势,小麦行间套作玉米时需要配施一定量的氮肥以消除小麦、玉米间的氮素竞争从而促进小麦的生长。     

Is it necessary to adjust nitrogen recommendations for tillage and wheat residue management in irrigated sweet corn?

ABSTRACT

Conservation tillage practices have gained interests. A 2-year field study (2014–2015) was conducted to evaluate four N rates (0, 69, 138, and 207 kg N ha^?1) effects on irrigated sweet corn (Zea mays L.) grown with or without wheat (Triticum aestivum L.) residue removal and conventional (CT), reduced (RT), or no-tillage (NT) practices near Shiraz, Iran. After 2 years, maximum marketable yield occurred at 156 and 159 kg N ha^?1 under CT and NT, respectively, while yield was tended to be increased with increasing N rates under RT. Increasing N rate increased total plant N uptake, shoot, and grain N accumulation. The lowest nitrogen use efficiency (NUE) was obtained under NT, while RT and CT either showed similar effects or RT was superior over CT. Soil total N was greater under CT and residue retention showed 18% and 14% higher N concentration than residue removal in 2014 and 2015, respectively. Soil organic matter was the highest (2.59%) under RT with residue retention and 138 kg N ha^?1. Conservation tillage needs more N rather than CT during transition from conventional to conservation agriculture practices, but it is based on the short-term results and evaluation of long-term experiment is highly recommended.     

川东北麦秆还田对水稻氮素吸收转运的影响

为合理利用秸秆,于2014和2015年两个水稻生长季,在大田条件下,以当地平均施肥量为标准,设置不同量的化肥配施秸秆处理,研究秸秆还田下水稻氮素吸收转运特征。结果表明:较纯化肥处理,秸秆替代一部分氮肥处理对水稻产量、地上部氮素积累量、氮素收获指数及氮肥生产效率无显著影响(P0.05);在不同程度上降低氮素在穗部的分配比例、营养器官吸收氮素向穗部转运量、转运率、转运氮对籽粒氮素贡献率;在不同程度上提高成熟期营养器官氮素积累量,显著提升抽穗后氮素吸收量及其对籽粒氮素贡献率(P0.05)。综合氮素吸收转运及利用效率,川东北稻麦轮作区水稻季在化肥减施30%基础上,麦秆还田量以6 000 kg/hm~2为宜。     

The effect of form and level of inorganic N on nitrogen use efficiency of sugarcane grown in pots

To-date, assessments of nitrogen use efficiency (NUE) of sugarcane have not included the contribution of its components, nitrogen uptake efficiency (NUpE) and nitrogen utilization efficiency (NUtE). This study determined these values, based on biomass and plant nitrogen (N) content, in two four-month-old pot-grown genotypes. The treatments included six N regimes, with nitrate (NO₃^—N) or ammonium (NH₄⁺-N) supplied alone, or as NO₃^?-N for the first 6 weeks and then NH₄⁺-N until harvest, each as 4 or 20 mM. Regardless of the N form, NUE was higher at four than at 20 mM due to significantly higher NUpE at low N supply. The results indicated that there was luxury N uptake and preference for NH₄⁺-N nutrition, which resulted in the highest determined NUE. There were significant differences between genotypes in biomass, morphological growth parameters, N uptake, total plant N and NUE, the latter matching previously established sucrose yield-based NUE field rankings.     

Winter wheat fertilizer nitrogen use efficiency in grain and forage production systems

Abstract

Nitrogen use efficiency (NUE) is known to be less than fifty percent in winter wheat grain production systems. This study was conducted to determine potential differences in NUE when winter wheat (Triticum aestivum L.) is grown strictly for forage or grain. The effects of different nitrogen rates on plant N concentrations at different growth stages and on grain yield were investigated in two existing long‐term winter wheat experiments near Stillwater (Experiment 222) and Lahoma (Experiment 502), OK. At both locations in all years, total N uptake was greater when wheat forage was harvested twice (Feekes 6 and flowering) compared to total N uptake when wheat was grown only for grain. Percent N content immediately following flowering was much lower compared to percent N in the forage harvested prior to flowering, indicating relatively large losses of N after flowering. Averaged over locations and years, at the 90 kg N ha^?1 rate, wheat produced for forage had much higher NUE (82%) compared with grain production systems (30%). While gaseous N loss was not measured in this trial, the higher NUE values found in the forage production systems were attributed to harvesting prior to anthesis and the time when plant N losses are known to be greater.     

Developing nitrogen management strategies under drip fertigation for wheat and maize production in the North China Plain based on a 3‐year field experiment

The intensive winter wheat (Triticum aestivum L.)–summer maize (Zea mays L.) cropping systems in the North China Plain (NCP) rely on the heavy use of mineral nitrogen (N) fertilizers. As the fertigated area of wheat and maize in the NCP has grown rapidly during recent years, developing N management strategies is required for sustainable wheat and maize production. Field experiments were conducted in Hebei Province during three consecutive growth seasons in 2012–2015 to assess the influence of different N fertigation rates on N uptake, yield, and nitrogen use efficiency [NUE: recovery efficiency (RE_N) and agronomic efficiency (AE_N)]. Five levels of N application, 0 (FN₀), 40 (FN_40%), 70 (FN_70%), 100 (FN_100%), and 130% (FN_130%) of the farmer practice rate (FP: 250 kg N ha^?1 and 205.5 kg N ha^?1 for wheat and maize, respectively), corresponding to 0, 182.2, 318.9, 455.5, and 592.2 kg N ha^?1 y^?1, respectively, were tested. Nitrogen in the form of urea was dissolved in irrigation water and split into six and four applications for wheat and maize, respectively. In addition, the treatment “drip irrigation + 100% N conventional broadcasting” (DN_100%) was also conducted. All treatments were arranged in a randomized complete block design with three replications. The results revealed the significant influence of both N fertigation rate and N application method on grain yield and NUE. Compared to DN_100%, FN_100% significantly increased the 3‐year averaged N recovery efficiency (RE_N) by 0.09 kg kg^?1 and 0.04 kg kg^?1, and the 3‐year averaged N agronomic efficiency (AE_N) by 2.43 kg kg^?1 and 1.62 kg kg^?1 for wheat and maize, respectively. Among N fertigation rates, there was no significant increase in grain yield in response to N applied at a greater rate than 70% of FP due to excess N accumulation in vegetative tissues. Compared to FN_70%, FN_100%, and FN_130%, FN_40% increased the RE_N by 0.17–0.57 kg kg^?1 and 0.03–0.34 kg kg^?1and the AE_N by 4.60–27.56 kg kg^?1 and 2.40–10.62 kg kg^?1 for wheat and maize, respectively. Based on a linear‐response relationship between the N fertigation rate and grain yield over three rotational periods it can be concluded that recommended N rates under drip fertigation with optimum split applications can be reduced to 46% (114.6 kg N ha^?1) and 58% (116.6 kg N ha^?1) of FP for wheat and maize, respectively, without negatively affecting grain yield, thereby increasing NUE.     

Response of triticale,wheat, and rye to N in a greenhouse

Abstract

Triticales (X Triticosecale, Wittmack), 6TA 131, 6TA 298 and 6TA 298 and 6TA 385, Arthur wheat (Triticum aestivum L.), and Abruzzi rye (Secale cereale L.) were grown in a soil treated with 0, 50, 100, and 200 ppm N. The objective of this study was to determine the influence of N rate on dry matter yields, N uptake, and N utilization efficiency.

The dry matter and N uptake increased while the N utilization efficiency decreased with increasing N fertilizer applied to the soil for all the cultivars. At high N rate, rye had higher dry matter and N uptake than the triticales and wheat; no consistent differences were obtained between triticale and wheat. However, rye and triticale tended to be more efficient in utilizing absorbed N than wheat at the lower N rates.     

Early nitrogen deficiency favors high nitrogen recovery efficiency by improving deeper soil root growth and reducing nitrogen loss in wheat

ABSTRACT

A two-year field and micro-plot ¹⁵N-labelled experiment was conducted under two levels of N application rate (240 and 180 kg N ha^–1) with three basal N application stages [seeding (L0), four-leaf stage (L4), and six-leaf stage (L6)] to investigate the effects of reducing basal N application amount and postponing basal N fertilization period on wheat growth and N use efficiency (NUE). No significant differences were observed in grain yield, root growth and root morphology between the N180L4 and N240L0 treatments, while the root-shoot ratio of N180L4 was significantly improved. Postponing basal N application period increased the residual basal ¹⁵N in soil and reduced basal ¹⁵N loss, and N180L4 treatment favored the highest ¹⁵N recovery efficiency (NRE), mainly due to reduced ¹⁵N loss. Grain yield and basal NRE were significantly positively correlated with root dry weight in deeper soil layers (40–60 cm), and the contribution of root growth to improved grain yield and NRE increased with the downward distribution of the roots. Therefore, postponing the basal N fertilization period under N deficiency promotes deeper root growth during the post-jointing period and increases basal N uptake, as well as reducing basal N loss and increasing grain yield and NUE.     

Relationship between Nitrogen Use Efficiency and Response Index in Winter Wheat

Although nitrogen use efficiency (NUE) of small grains is well documented at 33% worldwide, there has been little research relating NUE to yield factors. This study examined the relationship between NUE and the response index at harvest (RI_HARVEST) of winter wheat (Triticum aestivum L.). Yield data from a long-term fertility study established at Lahoma, Oklahoma in 1971 was used to explore the relationship. In this report, six nitrogen (N) rates at non-limiting levels of P and K were evaluated. Regression analysis showed a positive relationship between NUE and RI_HARVEST for all years across all N rates (r² = 0.37). But this relationship was improved (r² = 0.45) when both RI_NDVI and RI_HARVEST were included in the model. The linear relationship between NUE and RI_HARVEST was significantly improved, when yield data and corresponding NUE were separated according to the annually applied fixed N rate. As the N rate increased the resulting slope of the relationship between NUE and RI was reduced. These analyses also demonstrate that temporal variability in NUE exists and that NUE can be predicted.     

Changes in N leaching and crop production as a result of measures to reduce N losses to water in a 6‐yr crop rotation

The effects of various measures introduced to increase nitrogen (N)‐use efficiency and reduce N losses to water in a 6‐yr crop rotation (winter wheat, spring barley, green manure, winter wheat, spring barley, spring oilseed rape) were examined with respect to N leaching, soil mineral N (SMN) accumulation and grain yield. An N‐use efficient system (NUE) with delayed tillage until late autumn and spring, direct drilling of winter wheat, earlier sowing of winter and spring crops and use of a catch crop in winter wheat was compared with a conventional system (CON) in a field experiment with six separately tile‐drained plots in south‐western Sweden during the period 1999–2011 (two crop rotation cycles). Total leaching of NO₃‐N from the NUE system was significantly 46 and 33% lower than in the CON system during the first and second crop rotation cycle, respectively, with the most pronounced differences apparently related to management strategies for winter wheat. Differences in NO₃‐N leaching largely reflected differences in SMN during autumn and winter. There was a tendency for lower yields in the NUE system, probably due to problems with couch grass. Overall, the measures for conserving N, when frequently used within a crop rotation, effectively reduced NO₃ concentrations in drainage water and NO₃‐N leaching losses, without severely affecting yield.     

Genetic variation in nitrogen‐use efficiency of tef

Tef (Eragrostis tef (Zucc.) Trotter) is the ancient and most important cereal food crop of Ethiopia. A set of 20 tef genotypes was investigated in field experiments at three environments in Ethiopia to estimate genetic variation in nitrogen (N)‐use efficiency and in characters related to N accumulation as well as their relationships to grain yield. In each environment, genotypes representing both widely grown landraces and recently released cultivars were grown under three N‐fertilizer rates (0, 4, and 8 g m^–2 N). In grain yield, modern cultivars were superior to landraces, whereas in other characters, differences were less clear. The variation in grain yield was significantly related to the variation in total grain N and total plant N. Grain yield weakly correlated with N‐utilization efficiency and N harvest index. Broad sense heritability was higher for grain yield, total grain N, total plant N, and N harvest index than for N‐use, N‐uptake, and N‐utilization efficiencies. The contribution of uptake efficiency to the variation in N‐use efficiency decreased from 75% to 55% and that of utilization efficiency increased from 22% to 43% at the 4 to 8 g m^–2 N‐supply rate change. This study clearly suggests that tef N‐use efficiency would be increased by selecting genotypes with greater uptake efficiency at low N‐supply levels.