Seasonal variation in growth characteristics of Lolium perenne and Trifolium repens in swards under different managements

In a range of perennial ryegrass/white clover swards, variation in the surface heights of the grass and clover components, the rates of increase of these surface heights and the specific leaf areas (SLAs) of ryegrass and white clover were described for 1 year. The swards were of an early- or late-flowering (Aurora or Melle respectively) perennial ryegrass variety growing with either a small- or a medium-leaved (Kent or Milkanova) white clover and were either continuously stocked by sheep or continuously stocked apart from a rest period in April-May (Aurora) or May-June (Melle).
The surface heights of grass and clover were not affected by the variety of their companion species, and the surface heights of the two clover varieties were similar. The grass was always taller than the clover, although the magnitude of the difference between the species varied with time of year and the timing of the rest period.
Before the summer solstice the rate of increase in height of grass was greater than that of clover except at cool temperatures (5°C) and warm temperatures (16°C), and in the unrested Melle sward. After the solstice the rates of increase in height, particularly of clover, were lower than the rates seen at similar temperatures before the solstice.
Overall, the SLAs of both clover varieties were greater than those of ryegrass when grown with Aurora but not when grown with Melle, and the SLAs of both species increased during the year. By October the SLAs of both grass varieties were less than those of their companion clovers.
The results are discussed in relation to their implications for the species composition of the swards.     

Animal production evaluation of herbage varieties. 1. Comparison of Aurora with Frances, Talbot and Melle perennial ryegrasses when grown alone and with clover

Comparative herbage characteristics and sheep production (ewes and lambs until weaning and lambs thereafter) from swards of Aurora (very early flowering), Frances (early flowering), Talbot (intermediate flowering) and Melle (late flowering) varieties of perennial ryegrass ( Lolium perenne ) were studied under a continuous variable stocking management based on sward surface height guidelines. The varieties were assessed as grass-only (215 kg N ha⁻¹ fertilizer) and grass/clover (83 kg N ha⁻¹ fertilizer) pastures.
Over 2 years (1985-86) total annual lamb production per hectare from grass-only swards of Aurora was 19% more than that from Frances despite similar herbage productivity. Differences between the varieties in lamb output were more pronounced as grass/clover swards, with Aurora producing 29 and 18% more lamb than Melle and Talbot respectively and with Frances also giving 16% more than Melle. Overall lamb production from grass/clover swards was 10% more than that from the grass-only pastures, mainly due to 69% advantage in individual lamb growth rates after weaning. Herbage organic matter digestibility, during the post-weaning period, was higher on grass/clover than on grass-only swards, and similarly with Aurora versus the other varieties. Under a frequent cut simulated grazing regime the relative herbage productivity of the four varieties differed, with Aurora 13% less productive than Frances.
The results are discussed in relation to the limitations of assessing performance under cutting, and the significance of the interaction in animal productivity between the two sward types.     

A comparison of the herbage productivity of Bromus wildenowii cv. Grasslands Matua with four cultivars of Lolium perenne when grown in association with Trifolium repens

In a small-plot trial five grass varieties bromegrass cv. Grasslands Matua, perennial ryegrass CVS. Melle (diploid) and Bastion, Condesa and Meltra (tetraploid) were established as grass/white clover swards with white clover cv. Menna. Productivity was measured under 6-weekIy cutting both without N fertilizer (N_o) and with 100 kg N ha^?1 applied in spring (N₁₀₀) Evaluation was made over 2 harvest years, 1986–87. Total mean annual production of herbage dry matter (DM) in the first harvest year at No and N_o was 5·07 t ha^?1 and 6·93 t ha^?1 respectively. In year 2, corresponding values were 11·81 and 12·67 t ha^?1. In year 1, Matua swards at N_o and N₁₀₀ yielded 5·08 and 6·65 t DM ha^?1 compared with 507 and 70 t DM ha^?1 for the mean of the four ryegrass varieties. In year 2, corresponding values were 12·90 and 12·29 for Matua and 11·54 and 12·78 for the four ryegrasses. In year 1, the digestable organic matter in the dry matter (DOMD) of the Matua swards was lower than that of Melle, Bastion and Condesa at N_O, particularly at the first cut. In year 2, differences in DOMD between treatments and varieties were not significant. The proportion of white clover was found to be higher in the N_o than the N₁₀₀ treatment, and also higher in year 2 in most treatments. For the _No treatment Matua swards had the highest proportion of white clover in year 1 (32% compared with 24% for the mean of the ryegrass varieties) but the lowest proportion in year 2 (27% compared with 60% for the ryegrasses). For the N_o treatment in year 1 clover production was also 43% higher, on average, from the tetraploid treatments than with Melle as the companion grass; for this comparison in year 2 the differences were not significant. It is concluded that Matua bromegrass/white clover swards receiving no N fertilizer may have a good potential under cutting management. However, the evidence from this trial is that in the second year the proportion of white clover is lower with Matua swards than with perennial ryegrass as the companion grass.     

Responses of Seed Yield and Quality to Nitrogen Application Levels in Two Oilseed Rape (Brassica napus L.) Varieties Differing in Nitrogen Efficiency

Abstract

The relationship between nitrogen efficiency (NE), defined as seed yield per unit nitrogen (N) application, and seed quality was examined in two oilseed rape (Brassica napus L.) varieties at 5 N application levels, 0.6, 3, 6, 12, 15 mmol L?¹, N₁, N₂, N₃, N₄ and N₅, respectively. Seed yield, oil yield and protein content were increased with the increase in N application level, but NE and oil content were decreased, and the fatty acid composition in seed was hardly changed. Analysis of seven fatty acids revealed aslight decrease in the contents of erucic acid and arachidonic acid with the increase in N application level, but no obvious change in the contents of palmitic acid, stearic acid, oleic acid, linoleic acid and linolenic acid. Compared with the low NE variety H29, the seed yield and contents of erucic acid and arachidonic acid in the high NE variety bin270 were more markedly increased with the increase in N application level, and the oil content was hardly changed. The seed yield, oilcontent and oil yield were higher in the high NE variety than in the low NE variety at all 5 N application levels. There were no significant differences in protein, palmitic acid, stearic acid and oleic acid contents between the varieties at any of the 5 N application levels, but there were slight differences in the linoleic acid and linolenic acid contents between the two varieties. In brief, N application improved oil yield more greatly in the high NE variety than in the low NE variety, but hardly affected the fatty acid composition. Therefore, the seed quality and oil content of oilseed rape may not be decreased by breeding of a high NE variety with a high N absorption efficiency and high N use efficiency.     

Response of Russet Burbank and Shepody potatoes to nitrogen fertilizer in two cropping systems

Russet Burbank and Shepody potatoes were grown with at-planting N fertilizer rates ranging from 0 to 270 kg ha^?1 during 1986 through 1989. Experiments were conducted each year following small grains and red clover. Total yields and tuber size were strongly increased by N on most sites where potatoes followed small grains. Specific gravities declined with increasing N rate. Total yields of Russet Burbank and Shepody were optimized at an average of 196 and 211 kg ha^?1 of N, respectively, following small grains. The effect of N fertilizer on yields was much less dramatic following red clover. Total yields averaged 88% of maximum with only 45 kg ha^?1 of N applied, compared to 77% of maximum for this N rate following small grains. Total yields for the two varieties were optimized at 126 and 136 kg ha^?1, respectively. U.S. #1 yields were generally not increased at N rates above 45 to 90 kg ha^?1 following red clover and tuber size was not increased at rates above 90 to 135 kg ha^?1. Based on these studies, the N fertilizer credit for red clover grown prior to potatoes can be up to 75 kg ha^?1. Maintenance of tuber quality necessitates conservative use of N fertilizer when potatoes are grown following legumes. The highest N rates tested suppressed total yields of Russet Burbank, a late-season, indeterminate variety, by approximately 9% averaged over cropping systems.     

长江中游不同品质类型小麦产量形成及氮素吸收利用对氮肥的响应

为探究长江中游不同品质类型小麦产量形成和氮素吸收利用对氮肥的响应,选用强筋品种郑麦9023、中筋品种鄂麦596和弱筋品种扬麦13为供试材料,设置0、60、180和360 kg·hm^-2共4个施氮水平,分析了不同施氮水平下各小麦品种的产量、干物质积累分配、氮素吸收分配及利用率的差异。结果表明,与不施氮处理相比,施氮处理下郑麦9023、鄂麦596和扬麦13的籽粒产量均显著提高,增幅分别为34.8%～152.0%、30.8%～160.5%和34.6%～137.3%,穗粒数增加是施氮增产的主要原因。增施氮肥显著增加了小麦开花前和成熟期干物质积累量,促进了花前干物质向籽粒的转运。施氮也显著提高了三个品种的籽粒氮素积累量,但显著降低了氮肥偏生产力。品种间比较,鄂麦596的平均产量比郑麦9023和扬麦13分别高14.0%和21.8%,主要得益于其较高的穗粒数、千粒重以及干物质积累量;鄂麦596的花前氮素转运量及转运效率、氮肥偏生产力、氮肥农学利用率均高于其余两个品种,但花后氮素吸收量和氮肥回收率较低。这说明,施氮对不同品质类型小麦产量形成、干物质和氮素积累分配以及氮素利用效率的...     

施氮量对强筋小麦物质积累与籽粒产量的影响

为探究冀东地区强筋小麦适宜施氮量,选用2个强筋小麦品种津农7号(氮高效型)和中麦998(氮低效型)为试验材料,设置0 kg·hm^-2(N₀)、180 kg·hm^-2(N₁)、210 kg·hm^-2(N₂)和240 kg·hm^-2(N₃)4个施氮量处理,研究施氮量对强筋小麦籽粒产量和干物质与氮素积累转运的影响。结果表明,在N₀～N₂处理范围内,2个强筋小麦品种的籽粒产量、花前干物质转运量和氮素转运量随施肥量增加均显著增加,N₃处理下2品种的籽粒产量和花前氮素转运量较N₂处理无显著变化,说明适量施氮可促进强筋小麦氮素转运与籽粒产量的提高。增加施氮量可促进花前干物质向籽粒的转运,津农7号的干物质转运率高于中麦998,2个品种的花后干物质对籽粒的贡献率为60.59%～77.16%,说明小麦籽粒产量主要来源于花后干物质积累。2个品种开花...     

Differences in the nitrogen use efficiency of perennial ryegrass varieties under simulated rotational grazing and their effects on nitrogen recovery and herbage nitrogen content

Eight varieties of perennial ryegrass (six new varieties and two old ones) grown at five levels of applied fertilizer (100, 200, 300, 400 and 500 kg of N ha^–1) were cut monthly during two growing seasons (March to October in 1997 and 1998) and their herbage dry‐matter (DM) yield and nitrogen (N) content were determined. Herbage leaf content and the N content of young fully expanded leaves were also measured in 1997, and monthly recovery of applied N was determined in both the first and second harvest years by using ¹⁵N. The rank order of varieties was similar for annual yield of DM and N at all five fertilizer levels. Proportional differences between varieties in DM yield were greatest in the first cut of each year, the late‐heading candidate variety Ba12151 out‐yielding the old late‐heading variety S23 by more than 70%. However, differences in annual DM yield were much more modest than in early spring yield, up to 10% in 1997 and up to 21% in 1998. The relatively small differences in total annual DM yield were attributed to only a small proportion of the applied N being recovered during a single regrowth period, most of the remainder becoming available for uptake in subsequent regrowth periods. There were small but highly statistically significant differences among varieties in the N content of their leaves, leaf N content being inversely related to yield of DM and N. This lends further support to the hypothesis that the metabolic cost of protein synthesis and turnover is a key factor controlling genetic variation both in leaf yield and in annual DM and N yield under frequent harvesting. Seasonal variation in herbage N content was much greater than differences among varieties in mean N content over all harvests. In May of both years at all applied fertilizer levels, herbage N content fell below the 20 g N kg^–1 DM level required by productive grazing animals.     

Fertilizer application methods for Irish potatoes compared

This study was conducted in eastern Virginia on a Sassafras fine sandy loam soil to determine whether fertilizer for Irish potatoes could be broadcast before planting instead of band-placed at planting. Band placement is considered the most efficient method; however, use of the broadcast method involves less labor and speeds up the planting process. Two rates of fertilizer were used, 100 and 150 pounds per acre (112 and 169 Kg/Ha) of N, P₂O₅ and K₂O (43.7 and 65.6 pounds of P and 84.0 and 124.5 pounds of K) in a commercial material. The two rates were applied broadcast after plowing and disced in, on the rye cover before plowing, on the rye cover in January and band-placed at planting. Pungo and Superior varieties were compared, and responses varied from year to year and between varieties. The yields of Pungo potatoes were not significantly influenced by the two fertilizer rates or methods and time of application. Thus, for the Pungo variety, 100 pounds per acre (112 Kg/Ha) each of N, P₂O₅ (43.7 pounds P) and K₂O (83 pounds K) were adequate, either broadcast or band-placed. The Superior variety was less tolerant of adverse growing conditions than the Pungo and when stunted produced higher yields with band-placed fertilizer. The Superior variety under more optimum conditions (1968) produced higher yields with the broadcast fertilizer application.     

不同基因型小麦产量和氮利用效率的差异及其相互关系

为给氮高效利用小麦品种的选育提供科学依据,通过土培盆栽试验,以河南省大面积种植的20个小麦品种为材料,比较分析了不施氮(N0)、施纯氮180 kg·hm-2 (N180)、360 kg·hm-2 (N360)三种施氮水平下不同基因型小麦产量、氮素积累量、氮利用效率和氮响应度的差异及其相互关系.结果表明,在同一施氮水平下,不同基因型间、相同基因型不同施氮水平间小麦产量、氮积累总量、氮利用效率和氮响应度存在显著差异.与N0相比,施氮处理的籽粒产量、生物学产量、籽粒氮素积累量、氮素积累总量均显著提高,大多数品种的收获指数表现为施氮处理大于不施氮处理,但多数小麦品种的氮素收获指数却随施氮水平增加而降低.相关分析表明,籽粒产量与氮素积累量呈极显著正相关(rN0=0.944**,rN180＝0.919**,rNs60＝0.981**),氮利用效率与氮素积累量之间亦呈极显著正相关(rN0=0.944*,rN180 =0.919*,rN360＝0.982*).以氮利用效率和氮响应度为指标进行系统聚类分析,将供试基因型划分为高效弱响应、高效强响应、低效弱响应和低效强响应4种类型.由此说明,在现代高产小麦的育种过程中,高肥育种兼顾低肥水平下的表现,在一定范围内可同时对小麦氮高效利用和氮强响应型进行遗传选择.     

不同施肥水平对杂交水稻氮、磷、钾吸收积累的影响

为探索不同施肥量下2个杂交水稻植株氮、磷、钾的吸收积累特性,采用田间小区试验研究了两个杂交稻品种抽穗期和成熟期的植株氮、磷、钾吸收积累。结果表明,2个杂交水稻品种在不同施肥水平下,抽穗期和成熟期茎＋叶鞘、叶、穗各器官内的氮、磷、钾的含量无显著差异;抽穗期,叶的氮含量最高,茎＋叶鞘的磷、钾含量最高;成熟期,叶的氮含量、茎＋叶鞘的磷含量、穗的钾含量均下降。抽穗期,氮、钾吸收量及干物质量红泰优996高于粤优589,磷吸收量粤优589高于红泰优996;成熟期,氮、钾吸收量及干物质量粤优589高于红泰优996,磷吸收量2品种差异不明显。2个杂交水稻品种地上部植株对氮、磷、钾积累量比例的变幅范围均在高产水稻积累变幅范围内。     

FERTILE TILLER PRODUCTION AND SEED YIELD IN MEADOW FESCUE (FESTUCA PRATENSIS L.)

The effect of defoliation and date of nitrogen application in the spring to seed crops of meadow fescue, S215 and S53, were compared in two harvest years, in drills spaced at 18 in. S215 tillers were generally more robust than S53 tillers, and S215 produced a higher number of fertile shoots as the result of a higher conversion rate of over wintering to fertile tillers. N applied early and absence of defoliation gave the highest ratio of seed heads to over wintering tillers.
Seed- and straw-yields were depressed by April and May defoliations; May cutting had a particularly drastic effect on both varieties. S53 yields in all treatments were generally inferior to those of the hay variety, S215.
Head lengths and spikelet numbers were higher and 1000-seed weight lower in S53 than in S215.     

陕南地区冬油菜种植体系中的氮肥优化施用量

本研究旨在为陕南地区科学施肥提供依据，于2019-2020年进行了田间试验，以明确该地区甘蓝型冬油菜绿色高效生产的氮肥适宜用量。采用裂区设计，主处理选用当地栽培的早熟品种大地199和常规熟期品种秦优28，副处理是5种施氮量（N₀、N₉₀、N₁₈₀、N₂₇₀、N₃₆₀）。结果发现：常规熟期品种秦优28在越冬期的氮素营养状况强于早熟品种大地199，收获时地上部干物质积累量和氮素累积量也均高于后者。线性+平台模型分析发现，常规熟期品种秦优28和早熟品种大地199达到最大产量的氮肥投入阈值分别是188.9 kg/hm²和238.1 kg/hm²，获得最高产量分别是3774.4 kg/hm²和3678.2 kg/hm²。此外，两种不同熟期的品种产油量均在氮肥投入量为180 kg/hm²时达到最大，分别为1823.4 kg/hm²和1735.1 kg/hm²，且两个品种间差异不显著。油菜籽粒中硫苷含量有随着施氮量增加而增加的趋势，在两种施氮条件下，含量均小于国标规定的45.00 μmol/g。对于早熟品种大地199和常规成熟品种秦优28来说，当理论上施氮量分别不超过183.6 kg/hm²和188.5 kg/hm²时，与播前土壤根层持有的硝态氮储量（65.4 kg/hm²）相比，油菜收获后的土壤根层（0～100 cm）硝态氮储量没有额外的累积。而土壤非根层（100～200 cm）如果没有硝态氮的额外累积，两个品种要求的施氮量分别不超过263.5 kg/hm²（秦优28）和379.7 kg/hm²（大地199）。综上，在陕南地区综合考虑品种、产量、产油量、品质和土壤剖面中残留硝态氮储量等因素，该地区冬油菜绿色高效生产的氮肥适宜用量为180 kg/hm²。     

Effects of rates and times of application of starter dressings of nitrogen fertilizer to surface sown perennial ryegrass-white clover on hill peat

The effects on clover and grass growth of five levels of ammonium nitrogen (NH₄-N) applied before sowing, at sowing and after nodule initiation have been investigated. A pot experiment in which S184 white clover was grown in a peat soil showed that NH₄-N up to 688 mg N per pot (approx. equivalent to a field rate of 120 kg ha^-1 N) applied before sowing and at sowing did not affect clover growth. N-fixing activity (C₂H₂-reduction), on the other hand, was reduced progressively up to the highest level (688 mg N per pot). Application after nodule initiation increased growth relative to the zero-N treatment at all levels of application. Maximum growth and N fixation occurred at 516 mg N per pot where the DM yield was 70% higher than in the absence of added N. A field trial in which S184 clover and S24 perennial ryegrass were surface sown on to a peat soil showed an increase in grass and clover growth in the first year in response to 120 kg ha^-1 N applied at sowing. Grass growth alone was increased at 120 kg ha ^-1 N applied 40 d before sowing. Lower rates of application before sowing and at sowing did not affect clover or grass growth. The effect of the delayed application of NH₄-N on legume growth was less marked than that in the pot experiment, 90 kg ha^-1 N stimulating clover growth by 40% in the first year. The effect was however different from that in the pot experiment, in that, whilst 30kg ha^-1 N increased N fixation relative to the zero-N treatment, plants exposed to higher levels showed a depression in N-fixing capacity. N-fixation was correlated with nodule numbers in the delayed NH₄-N application, the closest correlation being with the number of multilobed nodules which was highest at 30 kg ha^-1 N and lowest at 120 kg ha^-1 N. It is suggested that circumstances exist when the use of a relatively low starter N dressing (20–60 kg ha^-1 N) at sowing would not increase clover or grass growth in the early stages of the establishment of a hill reseed. Under such circumstances higher rates of application (100 kg ha ^-1 N), preferably delayed until the seedlings are in a position to take up the nitrogen rapidly, would have the greatest effect.     

Animal production evaluation of herbage varieties.

Perennial ryegrass/white clover (Lolium perennel Trifolium repens) pastures of three white clover varieties were compared at UK lowland (Plas Gogerddan, PG) and upland (Bronydd Mawr, BM) sites over three harvest years (1989–91) under continuous variable stocking (ewes and lambs until weaning and lambs thereafter). Mean annual lamb output from small-leaved S 184 (1179 kg ha^?1) was 29% greater than that from prostrate small-leaved AberEndura at PG with medium-leaved Huia also giving 19% more output than AberEndura. At BM, output from S 184 (863 kg ha^?1) was 19 and 14% greater than that from AberEndura and Huia respectively. The differences in lamb output between the small-leaved varieties owed to a combined effect of higher individual lamb growth rates and greater stock-carrying capacity, both of which were more pronounced in the post-weaning period. The higher output from S 184 relative to Huia at BM was attributed to higher stocking rate, particularly after weaning. Clover productivity and persistence were also studied under three cutting-only managements at PG. Performance under a treatment cut at 2–3 cm every 10 d (T1), which is synonymous with assessment of persistence in UK National List testing, was poor with clover growing point number of only 498 m^?2 compared with 4906 m^?2 on the grazed sward in autumn 1991. Although productivity and clover content under T3 (cut at 3–4 cm every 42 d — similar to National List yield regime) gave the same varietal ranking as lamb production, there was an under-estimation of the small-leaved varieties, particularly AberEndura, relative to Huia. An intermediate treatment (T2), cut at 3–4 cm every 21 d, also over-estimated the performance of the medium-leaved variety. Herbage data from ground level sampling every 21 d using exclosure cages on the grazed swards were also poorly related to lamb performance. These results highlight the effect of clover variety on lamb production, which appeared to be independent of leaf size, and also confirm the existing problems associated with assessment of white clover varieties using a cutting regime.     

长期水氮互作下不同年代冬小麦的产量和光合特性

为了解不同水氮互作下不同年代推广冬小麦品种（现代品种冀麦325和20世纪70年代品种冀麦7）籽粒产量及光合特性的变化，于2017-2018年度（丰水年）和2018-2019年度（枯水年）冬小麦生长季，在长期水氮定位试验田进行了灌水量、施氮量和品种三因子裂裂区试验。主区为灌水，设生育期灌拔节水750 m·hm^-2(灌1水)和灌拔节水750 m·hm^-2 +开花水750 m·hm^-2(灌2水)；副区为氮肥，设不施氮(N0)及施氮总量为60(N1)、120(N2)、180(N3)、240(N4)和300 kg·hm^-2(N5)6个水平。结果表明，2017-2018年度，相同灌溉水平下，小麦开花期旗叶光合速率、蒸腾速率随施氮量增加均先增后减，冀麦325旗叶光合速率、蒸腾速率最大值对应的氮肥处理均为N2，冀麦7均为N3；冀麦325的产量随施氮量增加总体均呈逐渐增加的趋势，N2处理产量基本达到稳定值，再多施氮时灌1水有减产趋势，灌2水的产量增加缓慢；冀麦7的产量随施氮量增加均先增后减，灌1水时N2处理产量较高（仅低于N5），灌2水时N2处理产量最大。2018-2019年度，相同灌溉水平下，两个小麦品种籽粒产量随施氮量增加均先增后减，灌1水和灌2水时产量最大值对应的氮肥处理分别为N3和N2。在丰水年，不同年代冬小麦灌1水、灌2水的最佳施氮量均为120 kg·hm^-2；在枯水年，灌1水和灌2水的最佳施氮量分别为180 kg·hm^-2和120 kg·hm^-2。     

The effects of contrasting cutting regimes on the components of clover and grass growth in microswards

Microswards of white clover and perennial ryegrass were subjected to one of four treatments: weekly cutting to 3·5 cm, weekly cutting incorporating a period of no cutting for 6 weeks starting 27 April (early rest), weekly cutting incorporating a period of no cutting for 6 weeks starting 8 June (late rest), or cutting every 3 weeks. Two sward types were used: a mixture of white clover cv. Milkanova with perennial ryegrass cv. Melle, and white clover cv. Kent with perennial ryegrass cv. Melle. Growth measurements (leaf appearance, branching/tillering and stolon internode length) were confined to the first three treatments with records collected during contiguous 21-d measurement periods. Vertical height increments of clover and grass and red:far-red light ratios at the sward bases were also recorded at frequent intervals. At the end of the experiment population densities and unit weights were recorded for all treatments. Significant treatment effects on the rate processes were largely confined to the 21-d period immediately after weekly cutting of rested swards had resumed. On previously rested compared with weekly cut swards, clover leaf appearance rates were increased by 40% and branching rates by 164%. During the same period, grass leaf appearance rates were reduced by 50% and net tillering changed from positive to negative values. Though the rate responses were transient, effects were still apparent at harvest in September, when population density and content (proportion by population density and weight) of clover were significantly higher in the late rest treatment. The variety Kent showed a consistent, though usually nonsignificant, higher leaf appearance and branching rate compared with Milkanova, and in September was characterized by a higher population (7400 m^?2 compared to 3200 m^?2) of smaller units (27 compared to 46 mg/apical meristem) than Milkanova. The results are discussed in relation to defoliation effects and the role of light quantity and quality as they influence the component growth processes. Attention is drawn to the importance of canopy structure and the climatic and/or phenological differences in the relative seasonal behaviour of clover and grass, together with varietal variation within species in influencing responses to management manipulations.     

不同小麦品种氮素利用效率特征差异的研究

为了给氮高效优质小麦品种的选育和栽培提供理论依据,采用盆栽试验,研究了郑麦0943、郑麦0856和郑麦7698三个中强筋小麦品种的氮素利用效率特征以及茎叶中氮组分、氮代谢酶活性的差异。结果显示,施氮可显著促进小麦氮素的吸收和累积,不同小麦品种氮素利用效率特征存在明显差异。3个小麦品种中,郑麦0943氮素吸收量最大,氮素吸收效率和氮素利用效率均较高,氮素收获指数也显著高于其他两个品种,且籽粒中氮素来源以后期根系氮素的吸收和转移为主,前期地上部营养体的氮素转移能力也明显强于其他两个品种;不施氮条件下,郑麦0943开花期茎叶中具有较高的营养性氮含量和较低的结构性氮和功能性氮含量,且施氮后功能性氮含量增幅最大,硝酸还原酶和谷氨酰胺合成酶活性也最高;不施氮条件下,郑麦7698氮素干物质生产效率和氮素收获指数均最低,郑麦0856氮素干物质生产效率最高,但氮素收获指数低于郑麦0943。综合而言,郑麦0943的氮肥生产效率最高,而郑麦0856的氮素农学利用效率最高,二者的氮素吸收效率和氮素表观回收率均高于郑麦7698。     

Grain yield responses of lowland rice varieties to increased amount of nitrogen fertilizer under tropical highland conditions in central Kenya

Tropical highland conditions in Mwea Kenya, ensure the high radiation and the large day–night temperature differences. Such conditions are generally believed to promote rice growth and yield, but the current grain yield is lower than the expectation. In the current standard N fertilizer practice in Mwea, 75 kg nitrogen (N) ha^?1 is applied in three splits at fixed timing. The effects of increases in N fertilizer amount (125, 175, and 225 kg N ha^?1) on rice growth and yield were evaluated to test the hypothesis that unachieved high rice grain yield in Mwea is due to insufficient amount of N fertilizer. Two popular lowland varieties in Mwea (Basmati 370 and BW196) and two varieties reported as high yielding in other countries (Takanari and IR72) were used. Shoot dry weight (DW) increased with increases in the amount of N fertilizer applied in three splits at fixed timing, irrespective of variety. It reached approximately 20 t ha^?1 under increased N conditions (>75 kg N ha^?1) in several cases, indicating that high biomass production could be achieved by increasing N application rate. However, the increased biomass did not increase grain yield, due to decreased grain filling under high N conditions in all varieties. Thus, N amounts above 75 kg ha^?1 were ineffective for increasing grain yields in Mwea, where N fertilizer was applied in three splits at fixed timing. Increasing influence of low temperature under high N conditions may be one of the reasons for the decreased grain filling in Mwea.     

Yield potential and radiation use efficiency of “super” hybrid rice grown under subtropical conditions

China's “super” hybrid rice breeding project has developed many new varieties using a combination of the ideotype approach and intersubspecific heterosis. It is controversial whether these “super” hybrid varieties have increased the yield potential of irrigated rice. This study was conducted to compare grain yield and yield attributes among “super” hybrid, ordinary hybrid, and inbred varieties. Field experiments were done in Liuyang (moderate-yielding site) and Guidong (high-yielding site) counties, Hunan Province, China, in 2007 and 2008. Two varieties from each varietal group were grown in each field experiment under moderate and high N rates. Grain yield, yield components, aboveground total dry weight, harvest index, total N uptake, and crop radiation use efficiency (RUE) were measured for each variety. A significant difference in grain yield was observed among the varieties and varietal groups but not between the two N rates. “Super” hybrid varieties have increased rice yield potential by 12% compared with ordinary hybrid and inbred varieties. The higher grain yield of “super” hybrid varieties was attributed to improvement in both source and sink. “Super” hybrid varieties produced more biomass than ordinary hybrid and inbred varieties. Long growth duration and high accumulated incident radiation were partially responsible for high biomass production for the “super” hybrid varieties. “Super” hybrid varieties had significantly larger panicle size (spikelets per panicle) than ordinary hybrid and inbred varieties, which resulted in larger sink size (spikelets per m²). Crop RUE did not explain the yield superiority of “super” hybrid rice. Our study suggests that “super” hybrid rice varieties do not necessarily require more N fertilizer to produce high grain yield.