Comparing response of ryegrass-white clover pasture to gibberellic acid and nitrogen fertiliser applied in late winter and spring黑麦草–白三叶草混播草地对冬末和春季施用赤霉酸与氮肥的响应比较

ISSN: 0028-8233 (Print) 1175-8775 (Online) Journal homepage: http://www.tandfonline.com/loi/tnza20

RH Bryant, GR Edwards & B Robinson

To link to this article: http://dx.doi.org/10.1080/00288233.2015.1119164

Published online: 24 Feb 2016.

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RESEARCH ARTICLE研究论文

1Comparing response of ryegrass-white clover pasture to gibberellic acid and nitrogen fertiliser applied in late winter and spring黑麦草–白三叶草混播草地对冬末和春季施用赤霉酸与氮肥的响应比较

RH Bryant, GR Edwards and B RobinsonRH Bryant, GR Edwards and B Robinson

Faculty of Agriculture and Life Sciences, Lincoln University, Lincoln, Christchurch, New Zealand

林肯大学农业与生命科学学院，新西兰基督城林肯

2ABSTRACT摘要

The objective of this study was to compare the effect of one or two applications of gibberellic acid (GA) or nitrogen (N) fertiliser on dry matter (DM) yield and composition of perennial ryegrass-white clover pasture. Pasture was treated with 0 or 8 g a.i./ha of GA and 0 or 50 kg/ha N fertiliser in a 2 × 2 factorial design in late winter and spring. One application of N fertiliser increased DM yield by increasing leaf mass and tiller density of perennial ryegrass while one application of GA increased DM yield by increasing perennial ryegrass and white clover yield. There was no DM yield response to two applications of GA, though two applications of N did increase DM yield. A single GA application in late winter increased fibre and reduced protein concentration in ryegrass. The use of GA to increase DM yield with reduced herbage protein concentration may have reduced environmental impact through reducing N intake of livestock.

本研究旨在比较施用一次或两次赤霉酸（GA）或氮（N）肥对多年生黑麦草–白三叶草草地干物质（DM）产量及组成的影响。试验采用2 × 2析因设计，于冬末和春季对草地施用0或8 g a.i./ha的GA以及0或50 kg/ha的氮肥。施用一次氮肥通过增加多年生黑麦草的叶片质量和分蘖密度提高了DM产量，而施用一次GA则通过提高多年生黑麦草和白三叶草的产量提高了DM产量。施用两次GA对DM产量无显著影响，而施用两次氮肥则确实提高了DM产量。冬末单次施用GA提高了黑麦草的纤维含量并降低了其蛋白质浓度。利用GA在提高DM产量的同时降低牧草蛋白质浓度，可通过减少家畜的氮摄入量而降低对环境的影响。

ARTICLE HISTORY文章历史

Received 10 September 2015 Accepted 27 October 2015

KEYWORDS关键词

Lolium perenne; Trifolium repens; gibberellins; herbage quality; plant growth regulator; plant hormone

Lolium perenne；Trifolium repens；赤霉素；牧草品质；植物生长调节剂；植物激素

3Introduction引言

Maintaining an adequate feed supply for pastoral-based livestock systems requires careful management and an understanding of plant responses to changing environmental conditions. In many temperate regions throughout Europe, South America and Australasia, binary mixtures of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) are sown for livestock grazing due to their high pasture yield, ability to withstand a wide range of grazing conditions and high feeding value (Kemp et al. 1999). However, at critical times for livestock feeding, such as following calving or lambing in late winter and early spring, pasture growth rates can be insufficient to meet livestock requirements.

为以放牧为基础的家畜生产系统维持充足的饲草供应，需要精细的管理以及对植物响应环境条件变化的认识。在欧洲、南美洲和大洋洲的许多温带地区，由于多年生黑麦草（Lolium perenne）与白三叶草（Trifolium repens）二元混播草地产量高、能够耐受多种放牧条件且饲用价值高，因而被广泛种植用于家畜放牧（Kemp et al. 1999）。然而，在家畜饲喂的关键时期，如冬末和早春产犊或产羔之后，草地生长速率可能不足以满足家畜的需求。

Despite low temperatures at this time, it has been shown that the application of exogenous gibberellic acid (GA) can stimulate grass growth (Morgan & Mees 1958; Finn & Nielsen 1959; Zaman et al. 2014) and increase feed supply, as can application of nitrogen (N) fertiliser (Harris et al. 1996; Parsons et al. 2013). However, the associated increase in herbage N content with N fertiliser may come at an environmental cost if N fertiliser results in increased N intake. High N intake of intensively grazed animals leads to increased excretion of N in the urine (Higgs et al. 2012) and increased risk of nitrate leaching under urine patches (Li et al. 2012). Recently, use of exogenous GA has been investigated as a means of reducing environmental N losses (Whitehead & Edwards 2015) as it has been associated with reduction in herbage N content in a number of studies (Morgan & Mees 1958; Finn & Nielsen 1959; Brown et al. 1963; McGrath & Murphy 1976; Zaman et al. 2014), and has potential to reduce N intake and lower urinary N excretion.

尽管此时气温较低，但已有研究表明，施用外源赤霉酸（GA）能够刺激禾草生长（Morgan & Mees 1958; Finn & Nielsen 1959; Zaman et al. 2014）并增加饲草供应，施用氮（N）肥亦有同样效果（Harris et al. 1996; Parsons et al. 2013）。然而，若施用氮肥导致氮摄入量增加，则随之而来的牧草氮含量升高可能带来环境代价。集约放牧家畜的高氮摄入会导致尿液中氮排泄量增加（Higgs et al. 2012），并提高尿斑下硝酸盐淋溶的风险（Li et al. 2012）。近年来，外源GA已被作为减少环境氮损失的一种手段加以研究（Whitehead & Edwards 2015），因为多项研究表明其与牧草氮含量降低相关（Morgan & Mees 1958; Finn & Nielsen 1959; Brown et al. 1963; McGrath & Murphy 1976; Zaman et al. 2014），并具有降低氮摄入量和减少尿氮排泄的潜力。

© 2016 The Royal Society of New Zealand

© 2016 新西兰皇家学会

In spite of evidence for improved herbage growth following GA application, adoption by farmers has been limited. This is thought to be due to previous high cost of the product at high rates of GA (63–600 g a.i./ha), and the risk of longer term reductions in DM yield (Matthew et al. 2009). However, manufacture of GA has become more economical and rates required to elicit a response are lower (5–10 g a.i./ha) than those previously studied (Matthew et al. 2009). Indeed, it was recognised that the high rates of GA may have been responsible for observed yield reductions. A study using low GA rates (8 g a.i./ha) showed increased pasture height and improved DM yields above mowing height in spring (Matthew et al. 2009). Although in the study of Matthew et al. (2009) residual effects of GA on subsequent regrowth were not statistically significant, the authors observed a tendency for lower DM yields and reduced tiller numbers in plots where GA was used without N fertiliser. The evidence that an interaction between GA and N existed led the authors to highlight the need for future consideration of these interactions and, in particular, how N may reduce negative effects of GA on subsequent regrowth.

尽管已有证据表明施用GA后牧草生长得到改善，但农户的采用仍然有限。这被认为是由于以往高施用量GA（63–600 g a.i./ha）下产品成本较高，以及存在长期DM产量下降的风险（Matthew et al. 2009）。然而，GA的生产已变得更加经济，且引发响应所需的施用量（5–10 g a.i./ha）低于以往研究所用的施用量（Matthew et al. 2009）。事实上，已有人认识到高施用量GA可能是所观察到的产量下降的原因。一项采用低施用量GA（8 g a.i./ha）的研究表明，春季草地高度增加，刈割高度以上的DM产量提高（Matthew et al. 2009）。虽然在Matthew et al.（2009）的研究中，GA对后续再生的残留效应在统计上不显著，但作者观察到，在未配施氮肥而单独施用GA的小区中，DM产量和分蘖数有降低的趋势。GA与氮之间存在交互作用的证据促使作者强调，今后需要考虑这些交互作用，特别是氮如何减轻GA对后续再生的负面影响。

The purpose of this study was to compare pasture growth response to GA, N fertiliser and their interaction following one or two applications in late winter and early spring. Effects on botanical composition, grass morphology and nutrient composition were included to quantify treatment effects on DM yield parameters.

本研究的目的是比较冬末和早春施用一次或两次GA、氮肥及其交互作用后草地的生长响应。研究还纳入了对植物学组成、禾草形态和养分组成的影响，以量化各处理对DM产量参数的效应。

4Materials and methods材料与方法

Experimental site and pastures试验地点与草地

The experiment was conducted at Iversen Field, Lincoln University, Canterbury, New Zealand, on a Wakanui silt loam soil (43°38^′S. 172°27^′E; 10 m above sea level) between August and November 2009. The experimental area was situated within a 0.9 ha pasture that had been sown, following cultivation, with a mixture of an early maturing perennial ryegrass (cv. Meridian, heading date –17 days, AR1 endophyte) and white clover (cv. Denmark) at 15 and 3 kg/ha, respectively, on 7 November 2008. Soil tests prior to commencement of the trial showed: pH = 5.6; Olsen P = 13 mg/L; and sulfate sulfur = 6 mg/kg. Pastures had previously been managed by rotational grazing with sheep to maintain herbage mass in the range 1200 to 1500 kg DM/ha.

试验于2009年8月至11月在新西兰坎特伯雷林肯大学Iversen Field进行，土壤为Wakanui粉砂壤土（43°38^′S. 172°27^′E；海拔10 m）。试验区位于一块0.9 ha的草地内，该草地于2008年11月7日经耕作后播种，混播早熟多年生黑麦草（品种Meridian，抽穗期–17天，AR1内生菌）和白三叶草（品种Denmark），播种量分别为15和3 kg/ha。试验开始前的土壤测试结果为：pH = 5.6；Olsen P = 13 mg/L；硫酸盐硫 = 6 mg/kg。此前草地采用绵羊轮牧管理，使牧草量维持在1200至1500 kg DM/ha范围内。

Experimental design试验设计

The experiment consisted of a 2 × 2 factorial design with two regrowth cycles laid out in four randomised blocks each 8 × 8 m. The two treatments used were: 1, N fertiliser at two rates of 0 or 50 kg N/ha as urea (46% N); and 2, GA at rates of 0 or 8 g a.i./ha as ProGibb- SG containing 40% GA3 (Nufarm Ltd). Each block was divided into four 2 × 8 m plots which were randomly allocated one of the four treatments. All plots were mown with a rotary lawnmower to a uniform height of 4.5 cm 24 hours prior to treatment application.

试验采用2 × 2析因设计，包括两个再生周期，布置于4个随机区组中，每个区组面积为8 × 8 m。所用的两个处理为：1，氮肥，两个施用水平为0或50 kg N/ha，以尿素（含N 46%）形式施用；2，GA，施用量为0或8 g a.i./ha，以含40% GA3的ProGibb- SG（Nufarm Ltd）形式施用。每个区组划分为4个2 × 8 m的小区，随机分配4个处理之一。所有小区在施用处理前24小时用旋转式割草机统一刈割至4.5 cm高度。

ProGibb-SG and non-ionic surfactant Contact Xcel (Nufarm Ltd) were mixed with water at rates of 20 g and 25 ml per hectare, respectively, and applied to experimental areas with a knapsack sprayer on 5 August 2009. Urea was applied by hand-spreading within 48 hours of mowing. Measurements were carried out between 33 and 41 days after GA and N treatment application.

将ProGibb-SG和非离子表面活性剂Contact Xcel（Nufarm Ltd）分别按每公顷20 g和25 ml的用量与水混合，于2009年8月5日用背负式喷雾器喷施于试验区。尿素在刈割后48小时内人工撒施。测定在施用GA和氮处理后第33至41天之间进行。

On completion of measurements following one application, the experimental site was set-stocked with sheep from 18 September to 2 October 2009. On 3 October 2009, sheep were removed and plots were mown with a rotary lawnmower to a uniform height of 4.5 cm and the GA and N treatment applications were repeated on 5 October 2009. A similar regrowth interval was used following the second application of GA and N treatments. Daily climate data were sourced from the NIWA database for the Lincoln Broadfields site (Station H32645) located 1 km from the experimental area.

在第一次施用后的测定完成后，于2009年9月18日至10月2日对试验地进行绵羊定牧。2009年10月3日移走绵羊，用旋转式割草机将各小区统一刈割至4.5 cm高度，并于2009年10月5日重复施用GA和氮处理。第二次施用GA和氮处理后采用了相似的再生间隔。逐日气候数据来源于NIWA数据库中距试验区1 km的Lincoln Broadfields站点（站号H32645）。

5Herbage measurements牧草测定

Dry matter yield干物质产量

Accumulated DM yield to ground level and above mowing height (4.5 cm) was determined at the end of each regrowth period. Dry matter yield to ground level was determined on 7 September and 11 November 2009 by harvesting two 0.2 m² quadrats per plot to < 1 cm stubble height using hand shears. After harvesting, each sample was thoroughly mixed, and soil and faecal matter removed. A subsample of 50 g fresh weight was removed to determine botanical composition. The remaining bulk sample was oven dried at 65 °C until it reached a constant weight. The sum of the subsample and bulk sample was used to calculate DM yield.

在每个再生期结束时，测定地面以上和刈割高度（4.5 cm）以上的累积DM产量。于2009年9月7日和11月11日测定地面以上DM产量，方法为每个小区用手剪收获两个0.2 m²样方，留茬高度 < 1 cm。收获后将每个样品充分混匀，并去除土壤和粪便。取50 g鲜重的子样品用于测定植物学组成。其余大样品在65 °C烘箱中烘至恒重。以子样品与大样品之和计算DM产量。

Dry matter yield above mowing height was determined at the end of each regrowth by mowing (4.5 cm height) a 3.2 m² strip within each plot. Total fresh matter (FM) of the mown herbage was recorded and a subsample of approximately 50 g of fresh herbage was removed, weighed and dried at 65 °C to a constant weight for DM% determination. Compressed pasture height was determined from 20 readings per plot using a rising plate meter (Jenquip F150 Electronic Pasture Meter) prior to mowing.

在每次再生结束时，通过在每个小区内刈割（高度4.5 cm）一条3.2 m²的条带测定刈割高度以上的DM产量。记录刈割牧草的总鲜物质（FM）重量，取约50 g鲜牧草子样品，称重后在65 °C下烘至恒重，以测定DM%。刈割前使用升降盘式测量仪（Jenquip F150 Electronic Pasture Meter）每小区读取20个读数，测定压缩草层高度。

Sward characteristics草层特征

The subsamples from quadrat cuts were manually separated into vegetative and flowering ryegrass tillers, white clover, weed and dead material. The number of vegetative and flowering tillers were recorded. All components were then oven dried at 65 °C until they reached a constant weight and dry weights recorded. Botanical composition on DM basis was then determined. Mean tiller weight of vegetative and flowering ryegrass was calculated as the dry weight of each category divided by the number of tillers in the sample. Tiller density was calculated by multiplying average tiller weight (g DM/tiller) by the content of ryegrass in the pasture mass.

将样方刈割所得的子样品人工分拣为营养型和开花型黑麦草分蘖、白三叶草、杂草及枯死物质。记录营养型和开花型分蘖的数量。随后将所有组分在65 °C烘箱中烘至恒重并记录干重，进而确定基于DM的植物学组成。营养型和开花型黑麦草的平均分蘖重以各类别干重除以样品中分蘖数计算。分蘖密度以平均分蘖重（g DM/分蘖）乘以草地牧草量中黑麦草的含量计算。

Tiller morphology分蘖形态

Individual plants were sampled by extracting 10 whole plants from each plot by cutting below ground level using a knife. From each plant, two primary tillers with intact crowns were carefully separated. Each tiller was dissected into leaf, pseudostem and truestem (crown and internodes), and length recorded. For each tiller, the length of the youngest fully emerged leaf from ligule to leaf tip and the width at the widest point were measured with a ruler. The leaf, pseudostem and truestem components of tillers were then oven dried at 65 °C, weighed and mass per tiller of each component determined.

每个小区用刀在地面以下切割，取10株完整植株作为单株样品。从每株植株上小心分离出两个带有完整分蘖节的初级分蘖。将每个分蘖解剖为叶片、假茎和真茎（分蘖节和节间），并记录长度。对于每个分蘖，用直尺测量最幼完全展开叶从叶舌至叶尖的长度及其最宽处的宽度。随后将分蘖的叶片、假茎和真茎组分在65 °C下烘干、称重，并确定每个分蘖各组分的质量。

Nutritive and mineral composition营养成分与矿质组成

Leaf and sheath material above 4.5 cm was sampled at 15:30 hr. Herbage clipped with hand shears was immediately placed on ice and transferred to a –20 °C freezer within 30 minutes of sampling. Frozen herbage was freeze dried and any clover and weed material was carefully removed from the dried sample before grinding through a 1 mm sieve (Retsch ZM200 rotor mill, Retsch GmbH). Ground perennial ryegrass was scanned for crude protein (CP), soluble sugars and starch (SSS), crude fibre (NDF), acid detergent fibre (ADF) organic matter (OM) and in vitro digestibility of organic matter in the DM (DOMD) using near infrared spectrophotometry (NIRS, Model: FOSS NIRSystems 5000) as described by Bryant et al. (2012). Nitrogen content was calculated by dividing CP by 6.25.

于15:30采集4.5 cm以上的叶片和叶鞘材料。用手剪剪取的牧草立即置于冰上，并在采样后30分钟内转移至–20 °C冰箱。冷冻牧草经冷冻干燥后，将干燥样品中的三叶草和杂草小心剔除，再用1 mm筛（Retsch ZM200转子粉碎机，Retsch GmbH）粉碎。按照Bryant et al.（2012）所述方法，采用近红外光谱法（NIRS，型号：FOSS NIRSystems 5000）对粉碎的多年生黑麦草进行扫描，测定粗蛋白（CP）、可溶性糖和淀粉（SSS）、粗纤维（NDF）、酸性洗涤纤维（ADF）、有机质（OM）以及DM中有机质体外消化率（DOMD）。氮含量以CP除以6.25计算。

Separate herbage samples were collected for mineral analysis by harvesting a 30 × 30 cm quadrat of pasture to ground level on 15 September. Clover and weeds were removed from perennial ryegrass and discarded. The ryegrass samples were washed thoroughly with deionised water and placed in a drying cabinet at 105 ^oC until a constant weight was obtained. Herbage was dissolved using microwave digestion in 8 mL of Aristar nitric acid (± 69%), filtered using Whatman 52 filter paper (pore size 7 µm), and diluted with milliQ water to a volume of 25 mL. Concentrations of boron (B), calcium (Ca), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), phosphours (P) and zinc (Zn) were determined using inductively coupled plasma optical emission spectrometry (ICP-OES Varian 720 ES). Wageningen reference soil (ISE 921) and plant (IPE 100) material were analysed for quality assurance (Houba et al. 1998). Recoverable concentrations were 91%–108% of the certified values. The molar equivalents of Ca, Mg and K were used to calculate the ratio of K/(Mg + Ca).

于9月15日收获30 × 30 cm样方内地面以上的草地植物，另行采集用于矿质分析的牧草样品。将三叶草和杂草从多年生黑麦草中剔除并弃去。黑麦草样品用去离子水彻底清洗后置于干燥箱中，在105 ^oC下烘至恒重。牧草样品采用微波消解法在8 mL Aristar硝酸（± 69%）中消解，用Whatman 52滤纸（孔径7 µm）过滤，并用milliQ水定容至25 mL。采用电感耦合等离子体发射光谱仪（ICP-OES Varian 720 ES）测定硼（B）、钙（Ca）、镉（Cd）、铬（Cr）、铜（Cu）、铁（Fe）、钾（K）、镁（Mg）、锰（Mn）、磷（P）和锌（Zn）的浓度。分析瓦赫宁根参考土壤（ISE 921）和植物（IPE 100）标准物质以进行质量保证（Houba et al. 1998）。回收浓度为认证值的91%–108%。以Ca、Mg和K的摩尔当量计算K/(Mg + Ca)比值。

6Statistical analysis统计分析

For each regrowth, differences in DM yield, nutrient composition and botanical composition were analysed for variance using Genstat (v. 12.2 VSN international) with GA and N fertiliser and their interaction as fixed terms and blocks as the random term. Where interactions were significant, means were separated using Fisher’s protected LSD test.

对于每次再生，采用Genstat（v. 12.2 VSN international）对DM产量、养分组成和植物学组成的差异进行方差分析，以GA、氮肥及其交互作用为固定项，区组为随机项。当交互作用显著时，采用Fisher保护性LSD检验进行均值比较。

7Results结果

Climate conditions for the experiment are presented in Table 1. Rainfall was greater following the second application than after the single application. Day length increased from 9.50 to 11.50 hr/day during the first regrowth and from 12.40 to 14.46 hr/day during the second regrowth period. Growing degree days (GDD) between mowing date and harvest date for the first and second application of GA or N treatments were 175 °C and 237 °C, respectively (assuming a base temperature of 4 °C; Hutchinson et al. 2000).

试验期间的气候条件见表1。第二次施用后的降雨量高于单次施用后。第一次再生期间日长由9.50 h/d增至11.50 h/d，第二次再生期间由12.40 h/d增至14.46 h/d。第一次和第二次施用GA或氮处理后，刈割日至收获日之间的生长度日（GDD）分别为175 °C和237 °C（假定基础温度为4 °C；Hutchinson et al. 2000）。

Dry matter yield干物质产量

Dry matter yield to ground level in the first regrowth cycle was increased by 407 kg DM/ha and by 217 kg DM/ha after one application of GA or N respectively (P < 0.05, Table 2). At rates of 8 g a.i. GA and 50 kg N/ha this equated to DM responses of 50.8 kg DM/g GA and 4.3 kg DM/kg N. Above mowing height the DM response to GA was still apparent, increasing DM yield by 360 kg DM/ha (P < 0.05). However, the response to N above 4.5 cm (138 kg DM/ha) was no longer significant. Dry matter yield for the second regrowth was not affected by GA at either mowing height or ground level. By contrast, DM yield was increased (P < 0.05) by two applications of N regardless of whether measurements were to ground level or mowing height, giving respective advantages of 1119 and 756 kg DM/ha over plots without N. Response to GA or N above ground level in the second regrowth cycle after two applications were −5.9 kg DM/g GA and 22.4 kg DM/kg N, respectively. There was an interaction between N and GA above mowing height for DM yield in the second regrowth, indicating that response to N was reduced when applied with GA.

在第一个再生周期中，施用一次GA或氮后，地面以上DM产量分别增加了407 kg DM/ha和217 kg DM/ha（P < 0.05，表2）。在GA 8 g a.i.和N 50 kg/ha的施用量下，相当于DM响应分别为50.8 kg DM/g GA和4.3 kg DM/kg N。在刈割高度以上，对GA的DM响应仍然明显，DM产量增加了360 kg DM/ha（P < 0.05）。然而，4.5 cm以上对氮的响应（138 kg DM/ha）不再显著。第二次再生的DM产量无论在刈割高度还是地面水平均未受GA影响。相比之下，无论测定至地面还是刈割高度，施用两次氮均提高了DM产量（P < 0.05），较未施氮小区分别增加1119和756 kg DM/ha。施用两次后第二个再生周期中地面以上对GA或氮的响应分别为−5.9 kg DM/g GA和22.4 kg DM/kg N。第二次再生中刈割高度以上DM产量存在氮与GA的交互作用，表明氮与GA配施时对氮的响应降低。

Sward characteristics草层特征

Within a week of the first GA application, treated plots could be visibly differentiated from untreated plots. Treated plots visually appeared more yellow and taller than plots without

第一次施用GA后一周内，处理小区即可与未处理小区在外观上明显区分。与未施用

^{1SEM, standard error of} the mean for the interaction.

^{1SEM，交互作用的平均值}标准误。

GA. Although the colour differences eventually disappeared, the height effect of GA was still evident 33 days later when sward surface height was measured (Table 3). The botanical composition of the swards was dominated by perennial ryegrass which accounted for 830 ± 25 g/kg DM. There was a positive effect of GA on total ryegrass and white clover yields (P < 0.05), although, because perennial ryegrass DM yield also increased, there was no GA effect on the proportion of white clover (Table 3). Nitrogen fertiliser application reduced white clover proportion (P < 0.05) while increasing perennial ryegrass proportion (+68 g/kg DM, P < 0.05). Weed and dead material contributed to less than 50 g/kg DM and were not affected by treatment.

GA的小区相比，处理小区在视觉上显得更黄、更高。尽管颜色差异最终消失，但33天后测定草层表面高度时，GA对高度的影响仍然明显（表3）。草层的植物学组成以多年生黑麦草为主，占830 ± 25 g/kg DM。GA对黑麦草总产量和白三叶草产量有正效应（P < 0.05），但由于多年生黑麦草DM产量也同时增加，GA对白三叶草比例无影响（表3）。施用氮肥降低了白三叶草比例（P < 0.05），同时提高了多年生黑麦草比例（+68 g/kg DM，P < 0.05）。杂草和枯死物质所占比例低于50 g/kg DM，且不受处理影响。

Apart from a 1.6 cm increase (P < 0.05) in sward height, two applications of GA had no effect on sward characteristics. On the other hand, two N fertiliser applications increased sward height by 3 cm, reduced white clover content and yield, and doubled the density of perennial ryegrass seed head compared to plots which didn’t receive N (Table 3; P < 0.05). Tiller density was greater in N fertilised plots after two applications. There was no effect of GA on tiller density after two applications.

除草层高度增加1.6 cm（P < 0.05）外，施用两次GA对草层特征无影响。另一方面，与未施氮小区相比，施用两次氮肥使草层高度增加3 cm，降低了白三叶草含量和产量，并使多年生黑麦草穗密度增加一倍（表3；P < 0.05）。施用两次后，施氮小区的分蘖密度更高。施用两次后GA对分蘖密度无影响。

Tiller morphology分蘖形态

An interaction between GA and N for tiller density showed that after one application tiller density was not affected by GA application but N fertiliser application increased tiller density in the absence of GA. Tillers from one GA application were 11.2 mg DM heavier, and had longer leaves, longer pseudostem and truestem length (P < 0.05; Table 4). Nitrogen fertiliser increased leaf width by 0.32 mm compared with plots without N fertiliser. After two applications, the effect of GA on leaf and pseudostem length was no longer apparent (Table 4). There was an interaction between GA and N treatment for the leaf to stem ratio of vegetative tillers, but in this instance both GA and N improved leaf to stem ratio compared with plants that received no N or GA (Table 4). Plots that received N fertiliser had longer seed heads (P < 0.05) and tended to have larger flowering tillers than non N treated plots (105 vs 86 mg DM/tiller, P < 0.10).

GA与氮对分蘖密度的交互作用表明，施用一次后，分蘖密度不受GA施用的影响，但在不施GA的情况下，施用氮肥提高了分蘖密度。施用一次GA的分蘖重量增加11.2 mg DM，且叶片更长、假茎和真茎更长（P < 0.05；表4）。与未施氮肥小区相比，氮肥使叶片宽度增加0.32 mm。施用两次后，GA对叶片和假茎长度的影响不再明显（表4）。营养型分蘖的叶茎比存在GA与氮处理的交互作用，但在此情况下，与未施氮或GA的植株相比，GA和氮均提高了叶茎比（表4）。施用氮肥的小区穗更长（P < 0.05），且开花型分蘖有大于未施氮小区的趋势（105 vs 86 mg DM/分蘖，P < 0.10）。

^{1SEM, standard error of} the mean for the interaction. 2Total vegetative and flowering ryegrass tillers. Values in the same row with differing superscripts are significantly different (P < 0.05).

^{1SEM，交互作用的平均值}标准误。2营养型和开花型黑麦草分蘖总数。同一行中上标不同的数值差异显著（P < 0.05）。

^{1SEM, standard error of} the mean for the interaction. ^{2Leaf to} stem ratios are for vegetative tillers. Values in the same row with differing superscripts are significantly different (P < 0.05). Measurements determined from tillers to ground level.

^{1SEM，交互作用的平均值}标准误。^2叶茎比为营养型分蘖的数值。同一行中上标不同的数值差异显著（P < 0.05）。测定值基于地面以上的分蘖。

Nutritive composition营养成分

Gibberellic acid altered nutrient composition of perennial ryegrass after one application by increasing fibre content (both NDF and ADF fractions) and reducing crude protein content (Table 5). This was accompanied by a small but significant reduction in digestibility of 1.2% (P < 0.05) in GA plots compared with non-GA plots. Average CP content was relatively high at 225 g CP/kg DM in the first harvest. Nitrogen fertiliser increased the CP by 19% (P < 0.05), reduced NDF by 4% and reduced SSS by 10% compared to perennial ryegrass with no N fertiliser (P < 0.05). Following two applications of GA, fibre content was reduced (P < 0.05) but minor changes in CP and SSS were not significant and there was no effect of GA on DOMD.

施用一次赤霉酸后，通过提高纤维含量（NDF和ADF组分）和降低粗蛋白含量改变了多年生黑麦草的养分组成（表5）。与此同时，与非GA小区相比，GA小区消化率小幅但显著降低了1.2%（P < 0.05）。第一次收获时平均CP含量较高，为225 g CP/kg DM。与未施氮肥的多年生黑麦草相比，氮肥使CP提高了19%（P < 0.05），NDF降低了4%，SSS降低了10%（P < 0.05）。施用两次GA后，纤维含量降低（P < 0.05），但CP和SSS的微小变化不显著，GA对DOMD亦无影响。

Nitrogen fertiliser increased Cu, K and Zn concentrations and reduced the Cd concentration in perennial ryegrass (Table 6). An interaction between GA and N for B showed the B concentration was increased by N fertiliser but not when GA was applied. Gibberellic acid had little affect on mineral concentration. Sodium concentration was increased by 47% by GA application. The milliequivalent ratio of K/Ca + Mg was 1.34 ± 0.07 and it was unaffected by one GA or N application.

氮肥提高了多年生黑麦草中Cu、K和Zn的浓度，并降低了Cd的浓度（表6）。GA与氮对B的交互作用表明，氮肥提高了B浓度，但在施用GA时则不然。赤霉酸对矿质元素浓度影响甚微。施用GA使钠浓度增加了47%。K/Ca + Mg毫当量比为1.34 ± 0.07，且不受施用一次GA或氮的影响。

8Discussion讨论

DM yield reponse after one application施用一次后的DM产量响应

Irrespective of whether N fertiliser was applied, the immediate response to an application of 8 g a.i./ha of GA in late winter was a 40% increase in DM yield above ground level in early spring. This is equivalent to an additional 51 kg DM/g GA (Table 2). Recent studies comparing low rates of GA (between 8 and 30 g a.i./ha) on perennial ryegrass based pastures have also reported positive responses to GA both above mowing height (32% increase, 20 kg DM/g GA; Zaman et al. 2014) and above ground level (17% increase, 24 kg DM/g GA; Matthew et al. 2009). In an independent assessment of a commercial product containing GA, Edmeades & McBride (2012) found response to ProGibb at 8 g a.i./ha ranged between 13% and 63% with a mean DM yield increase of 36%. Similarly, in their study of pasture responses to fertiliser products Edmeades & McBride (2012) noted that responses to N were always positive, with a mean DM yield increase of 57% and a response range of 10% to 145%.

无论是否施用氮肥，冬末施用8 g a.i./ha的GA后，早春地面以上DM产量的即时响应为增加40%。这相当于每克GA额外增加51 kg DM（表2）。近期比较低施用量GA（8至30 g a.i./ha）对以多年生黑麦草为主的草地影响的研究，也报道了在刈割高度以上（增加32%，20 kg DM/g GA；Zaman et al. 2014）和地面以上（增加17%，24 kg DM/g GA；Matthew et al. 2009）对GA的正响应。在对一种含GA商业产品的独立评估中，Edmeades & McBride（2012）发现，对8 g a.i./ha ProGibb的响应介于13%至63%之间，平均DM产量增加36%。同样，Edmeades & McBride（2012）在其关于草地对肥料产品响应的研究中指出，对氮的响应始终为正，平均DM产量增加57%，响应范围为10%至145%。

^{1SEM, standard error of} the mean for the interaction. ^{2DOMD, digestible} organic matter in the dry matter (g/kg).

^{1SEM，交互作用的平均值}标准误。^{2DOMD，干物质中可消化}有机质（g/kg）。

^{1SEM, standard error of} the mean for the interaction.

^{1SEM，交互作用的平均值}标准误。

In the current study, the DM yield response of pasture to one application of N fertiliser was low at 4.3 kg DM/kg N. In grasses such as perennial ryegrass DM yield is a function of both tiller number and size, and grasses will often compensate one or the other to achieve maximum leaf area index (Bircham & Hodgson 1983; Matthew et al. 1995). Response of pastures to N fertiliser in spring typically follows this growth strategy, often exhibiting increased tillering and tiller density (Gislum & Griffith 2004), wider, longer leaves and lower weight per unit area (Wilman & Pearse 1984). Over time the regrowth cycle switches to one of self-thinning due to canopy closure and shading of the tiller base if defoliation does not occur. In the current study, the low thermal accumulation in the first regrowth cycle (175 GDD) supports observations for short term N response of increased leaf width and tiller density. Because leaf tissue expansion is driven by temperature and N when water is not limiting (Gastal et al. 1992), it is likely that temperature, rather than N, limited the response to N fertiliser in the first regrowth cycle.

本研究中，草地对施用一次氮肥的DM产量响应较低，为4.3 kg DM/kg N。在多年生黑麦草等禾草中，DM产量是分蘖数和分蘖大小的函数，禾草常通过补偿其中之一以达到最大叶面积指数（Bircham & Hodgson 1983; Matthew et al. 1995）。春季草地对氮肥的响应通常遵循这一生长策略，常表现为分蘖增加和分蘖密度提高（Gislum & Griffith 2004）、叶片更宽更长以及单位面积重量更低（Wilman & Pearse 1984）。随着时间推移，若不进行刈牧，由于冠层郁闭和分蘖基部遮荫，再生周期将转为自疏阶段。本研究中，第一个再生周期积温较低（175 GDD），支持了短期氮响应表现为叶宽和分蘖密度增加的观察结果。由于在水分不受限制时叶片组织扩展受温度和氮驱动（Gastal et al. 1992），第一个再生周期中限制氮肥响应的可能是温度而非氮。

Nitrogen fertiliser is also often responsible for the suppression in clover content in mixed swards (Harris et al. 1996), generally due to competition. The increased tiller density of perennial ryegrass and subsequent dominance of botanical content indicate that the positive DM yield response to N at ground level was due to the perennial ryegrass rather than the white clover. This could explain the lack of N effect on total yield above mowing height as the recent initiation of tillers were not sufficiently large to contribute to yield above 4.5 cm (Table 2).

氮肥也常导致混播草层中三叶草含量受到抑制（Harris et al. 1996），这通常是竞争所致。多年生黑麦草分蘖密度的增加及其随后在植物学组成中的优势表明，地面以上DM产量对氮的正响应源于多年生黑麦草而非白三叶草。这可以解释为何氮对刈割高度以上总产量无影响，因为新近萌发的分蘖尚不足以大到对4.5 cm以上的产量产生贡献（表2）。

On the other hand, the positive pasture DM yield response to GA was attributed to both the increase in white clover yield in addition to perennial ryegrass yield. The increased white clover yield in mixed swards in response to GA observed in this study agrees with previous observations (Percival 1980; van Rossum et al. 2013). Matthew et al. (2009), in their review on gibberellins, highlighted the range of functions gibberellins play in plant growth and that the size of the response to exogenous GA is often influenced by the plant species to which it is applied. Moreover, greater sensitivity of white clover compared with grass was demonstrated in a pot study by Finn & Nielsen (1959) who found that legumes, including white clover, were more responsive to GA than grasses as clover yields continued to increase with increasing rate of GA application.

另一方面，草地DM产量对GA的正响应既归因于多年生黑麦草产量的增加，也归因于白三叶草产量的增加。本研究中观察到的混播草层中白三叶草产量对GA的增产响应与以往的观察结果一致（Percival 1980; van Rossum et al. 2013）。Matthew et al.（2009）在其关于赤霉素的综述中强调了赤霉素在植物生长中发挥的多种功能，并指出对外源GA响应的大小常受施用植物种类的影响。此外，Finn & Nielsen（1959）的一项盆栽试验证明了白三叶草较禾草具有更高的敏感性，他们发现包括白三叶草在内的豆科植物对GA的响应强于禾草，三叶草产量随GA施用量的增加而持续增加。

The presence of ryegrass yield response to GA was intuitive as there were visible differences in height of tillers between plots which had or had not received this treatment. Typically, grasses respond to GA by elongation of leaf and sheath tissues, manifesting in increased height (Matthew et al. 2009) which is consistent with our own observations of increased leaf and stem and greater compressed height (Tables 3 and 4). Earlier studies have also reported significant ryegrass response to GA (Morgan & Mees 1958; Finn & Nielsen 1959; Zaman et al. 2014). However, these experiments used much higher rates of between 20 and 600 g a.i./ha compared to the 8 g a.i./ha used in the current study.

黑麦草产量对GA存在响应是可以预见的，因为施用与未施用该处理的小区之间分蘖高度存在明显差异。通常，禾草对GA的响应表现为叶片和叶鞘组织伸长，从而表现为高度增加（Matthew et al. 2009），这与我们观察到的叶和茎增加以及压缩高度增加一致（表3和表4）。早期研究也报道了黑麦草对GA的显著响应（Morgan & Mees 1958; Finn & Nielsen 1959; Zaman et al. 2014）。然而，与本研究所用的8 g a.i./ha相比，这些试验所用施用量高得多，为20至600 g a.i./ha。

An interaction between GA and N in the present study showed that both tiller size and density were improved when GA-treated plants also received N. A recent controlled environment pot study by Parsons et al. (2013) examined the DM response of winterderived perennial ryegrass after 2, 3 or 4 weeks regrowth using low rates of GA (8 g a. i./ha) comparable to that used in the current study. In comparison, the yield results of Parsons et al. (2013) after 3 weeks’ regrowth, which are equivalent in thermal time units (178 GDD) to those of the current study (175 GDD), showed little aboveground response of GA-treated ryegrass under low N supply. Under high N supply, however, Parsons et al. (2013) showed significant increases in shoot DM yield. Further field studies which include root measurements would aid understanding of biomass allocation as the aforementioned pot study indicates some important growth responses to GA under nutrient-limiting conditions.

本研究中GA与氮的交互作用表明，当经GA处理的植株同时施用氮时，分蘖大小和密度均得到改善。Parsons et al.（2013）近期在可控环境下进行的一项盆栽试验，采用与本研究相当的低施用量GA（8 g a. i./ha），考察了冬季来源的多年生黑麦草在再生2、3或4周后的DM响应。相比之下，Parsons et al.（2013）再生3周后的产量结果（其热时间单位为178 GDD，与本研究的175 GDD相当）显示，在低氮供应下，经GA处理的黑麦草地上部响应甚微。然而，在高氮供应下，Parsons et al.（2013）的结果显示地上部DM产量显著增加。鉴于上述盆栽试验表明在养分受限条件下GA会引起一些重要的生长响应，今后开展包括根系测定在内的田间研究将有助于理解生物量分配。

By shifting the distribution of tiller mass to leaf, sheath and stem, the resulting GA-treated pasture would have had a bulk density which favoured the upper sward horizon. While this may not have given way to increased perennial ryegrass DM yield at ground level, the significant effect of GA relative to N above mowing height suggests that perennial ryegrass is likely to have been an important contributor to total DM yield, when determined above mowing height (Table 3). The practical implications of assessing DM yield to ground level or grazing height are complex, as on the one hand increased bulk density at the base of the sward through increased tillering has positive correlations with persistence and long term pasture yield (Edwards & Chapman 2011), while on the other hand it is favourable to increase sward height and bulk density to promote DM intake (Laca et al. 1992). Tools which measure herbage mass—such as the rising plate meter—for feed planning and animal allocation purposes, are sensitive to variation in sward bulk density and alternative calibrations are likely to be required when used to determine pasture mass of GA-treated pastures.

通过将分蘖质量的分配转向叶片、叶鞘和茎，经GA处理的草地的容重分布会偏向草层上部。虽然这可能并未导致地面以上多年生黑麦草DM产量增加，但与氮相比，GA在刈割高度以上的显著效应表明，在测定刈割高度以上产量时，多年生黑麦草很可能是总DM产量的重要贡献者（表3）。以地面或放牧高度评估DM产量的实际意义较为复杂：一方面，分蘖增加导致草层基部容重提高，与草地持久性和长期产量呈正相关（Edwards & Chapman 2011）；另一方面，提高草层高度和容重有利于促进DM采食量（Laca et al. 1992）。用于饲草规划和家畜分配的牧草量测定工具（如升降盘式测量仪）对草层容重的变化较为敏感，用于测定经GA处理草地的牧草量时可能需要采用替代校准。

DM yield response after two applications施用两次后的DM产量响应

There was no improvement in pasture DM yield when two consecutive GA applications occurred (Table 2), and evidence that yield reductions may be possible was shown by the negative DM yield response evident after the second application (–6 kg DM/g GA). There is little reported data on repeat low rate applications of GA, although several studies note DM yield depressions in subsequent harvests when no GA or N is applied consecutively (Morgan & Mees 1958; Leben et al. 1959). Those authors attributed the yield depression to reduced access to soil nutrients through depletion of root stocks. Morgan & Mees (1958), however, tested this by replacing N and other minerals following the first harvest and found this had little effect on subsequent DM yield depression as a result of GA application. Morgan (1968) later postulated that elongation of stem in GA plants led to removal of a larger proportion of leaf area after cutting and slower relative growth rate. Loss of leaf area after cutting and insufficient regrowth interval before the following harvest, which would allow plants to replenish depleted reserves, may both contribute to long term reductions in growth from GA-treated plants. In the current study lower application rates (8 g a.i./ha) and lengthy regrowth intervals of 5 weeks may have allowed pastures to recover sufficiently from loss of leaf area and any loss of stubble or root reserves—a response which appears evident in the initial stages of growth (first 2 weeks) as shown by Parsons et al. (2013).

连续施用两次GA后，草地DM产量未见提高（表2），且第二次施用后出现的负DM产量响应（–6 kg DM/g GA）表明可能出现减产。关于重复低施用量施用GA的报道数据很少，但若干研究指出，在未连续施用GA或氮的情况下，后续收获中会出现DM产量下降（Morgan & Mees 1958; Leben et al. 1959）。这些作者将产量下降归因于根系贮藏物质耗竭导致获取土壤养分的能力降低。然而，Morgan & Mees（1958）通过在第一次收获后补充氮和其他矿质元素对此进行了检验，发现这对施用GA导致的后续DM产量下降影响甚微。Morgan（1968）后来推测，GA处理植株的茎伸长导致刈割后被移除的叶面积比例更大，相对生长速率更慢。刈割后叶面积的损失以及下次收获前再生间隔不足（再生间隔足够可使植株补充耗竭的贮藏物质），都可能导致经GA处理植株生长的长期下降。在本研究中，较低的施用量（8 g a.i./ha）和5周的较长再生间隔，可能使草地从叶面积损失以及留茬或根系贮藏物质的损失中得到充分恢复——Parsons et al.（2013）的研究表明，这一响应在生长初期（前2周）即较为明显。

In this study the cost of applying GA, and the DM response obtained, demonstrate that cost is not expected to prevent farmer adoption. At current retail prices of $188/250 g ProGibb and $575/tonne urea the cost of extra DM grown in this experiment was equivalent to 4 c/kg DM or 29 c/kg DM following one application of GA or N respectively or 7 c/kg DM or 9 c/kg DM for net DM yield following two applications of GA or N respectively. Economically, use of GA to increase pasture production following one or two consecutive applications is considerably lower than estimated break-even feed costs of < 20c/kg DM reported by Laborde et al. (1998).

本研究中施用GA的成本及所获得的DM响应表明，成本预计不会阻碍农户采用。按目前零售价ProGibb $188/250 g和尿素$575/吨计算，施用一次GA或氮后，本试验中额外生产的DM成本分别相当于4 c/kg DM和29 c/kg DM；施用两次GA或氮后，以净DM产量计分别为7 c/kg DM和9 c/kg DM。从经济角度看，施用一次或连续两次GA以提高草地产量的成本，远低于Laborde et al.（1998）报道的估计盈亏平衡饲料成本（< 20c/kg DM）。

Nutritive composition营养成分

Any improvement in DM yield in response to GA should not occur at the expense of the nutritional value of the herbage. As shown above, treatment of pastures with GA resulted in elongation of leaf, sheath and increased stem of ryegrass. This led to increased fibre and numerically lower digestibility (DOMD) of ryegrass, though it could also be argued that digestibility values of over 80% reflect potential for high energy supply. There was little effect of N fertiliser on organic matter digestibility, which supports observations of Peyraud & Astigarraga (1998) who reviewed the effects of N fertiliser on nutritive value and found only a small positive effect (0.02 units) of N on digestibility. Those authors noted that the variation in DOMD was related to changes in cell wall content which was likely to decline following N fertilisation in early regrowth when growth was succulent, but the effects of N on cell wall disappeared after 6 weeks. In the present study, considering the low thermal time and reduced fibre content due to N treatment, our results are consistent with the effects of N on cell wall in early regrowth.

GA引起的任何DM产量提高都不应以牺牲牧草营养价值为代价。如上所述，用GA处理草地导致黑麦草叶片、叶鞘伸长以及茎增加。这导致黑麦草纤维含量增加，消化率（DOMD）在数值上降低，但也可以认为，超过80%的消化率值反映了其具有高能量供应的潜力。氮肥对有机质消化率影响甚微，这支持了Peyraud & Astigarraga（1998）的观察结果，他们综述了氮肥对营养价值的影响，发现氮对消化率仅有很小的正效应（0.02单位）。这些作者指出，DOMD的变化与细胞壁含量的变化有关：在再生早期植株多汁时，施氮后细胞壁含量可能下降，但氮对细胞壁的影响在6周后消失。在本研究中，考虑到热时间较低以及氮处理导致的纤维含量降低，我们的结果与再生早期氮对细胞壁的影响一致。

An important finding as a result of GA treatments was the negative effect on crude protein (CP) concentration of the ryegrass. With one application of GA, increased DM yield was achieved at a lower herbage N content than the control, whereas with one application of N fertiliser, increased DM yield was achieved with a higher herbage N content than the control. In earlier studies, which included measurements of herbage N or protein concentrations, reductions in N concentration in GA treated plants were observed in some (Morgan & Mees 1958; Finn & Nielsen 1959; Percival 1980), but not all (Scott 1959; Matthew et al. 2009; Parsons et al. 2013) experiments. Explanation for the variation in protein response of plants treated with GA is unclear, but may be linked to a combination of influences due to the environment and regrowth duration. These results may have important implications for management of N losses (eg. nitrate and nitrous oxide) from animals in livestock grazing systems as mineral N concentration in the forage and dietary N intake are closely linked to drivers of N loss (Higgs et al. 2012; Whitehead & Edwards 2015). However, it is important to note here that GA application did result in increased white clover content which may offset reductions in ryegrass N concentration, due to white clover which typically has a herbage N concentration of 40–48 g/kg DM (Ayres et al. 1998; Black et al. 2007).

GA处理的一项重要发现是其对黑麦草粗蛋白（CP）浓度的负面影响。施用一次GA时，DM产量的提高是在牧草氮含量低于对照的情况下实现的；而施用一次氮肥时，DM产量的提高则是在牧草氮含量高于对照的情况下实现的。在早期包括牧草氮或蛋白质浓度测定的研究中，部分试验（Morgan & Mees 1958; Finn & Nielsen 1959; Percival 1980）观察到GA处理植株氮浓度降低，但并非所有试验均如此（Scott 1959; Matthew et al. 2009; Parsons et al. 2013）。GA处理植株蛋白质响应存在差异的原因尚不清楚，但可能与环境和再生时长的综合影响有关。这些结果可能对家畜放牧系统中动物源氮损失（如硝酸盐和氧化亚氮）的管理具有重要意义，因为饲草中矿质氮浓度和日粮氮摄入量与氮损失的驱动因素密切相关（Higgs et al. 2012; Whitehead & Edwards 2015）。然而，需要指出的是，施用GA确实导致白三叶草含量增加，而白三叶草的牧草氮浓度通常为40–48 g/kg DM（Ayres et al. 1998; Black et al. 2007），这可能抵消黑麦草氮浓度的降低。

Gibberellic acid had little impact on the concentration of minerals in ryegrass, with the one exception being Na uptake which was increased by over 1.4 g Na/kg DM compared to non-GA treatments. The reasons for this are not clear, though increased Na in GA-treated pasture could be associated with increased water demand from stimulated growth, increase in cell expansion and cell volume, and changes in sugar transport, which are all regulated in some way by GA (Paleg 1965). The limited effect of GA on minerals in the present study support those of Morgan & Mees (1958) who observed no changes in mineral uptake of perennial ryegrass pastures as a result of GA treatment. The increased K concentration of herbage due to N fertiliser supports earlier observations (Kemp 1983; Mckenzie & Jacobs 2002). The ratio of K/(Mg + Ca) was not affected by GA application and was below the recommended maximum of 2.2 for reducing the risk of hypomagnesemia (Kemp & t’Hart 1957; Grunes & Welch 1989).

赤霉酸对黑麦草中矿质元素浓度影响甚微，唯一的例外是Na吸收，与非GA处理相比增加了1.4 g Na/kg DM以上。其原因尚不清楚，但经GA处理的草地中Na增加可能与生长受刺激导致的需水量增加、细胞扩展和细胞体积增加以及糖运输的变化有关，而这些过程均在某种程度上受GA调控（Paleg 1965）。本研究中GA对矿质元素影响有限，这支持了Morgan & Mees（1958）的结果，他们观察到GA处理并未改变多年生黑麦草草地的矿质元素吸收。氮肥导致的牧草K浓度升高支持了早期的观察结果（Kemp 1983; Mckenzie & Jacobs 2002）。K/(Mg + Ca)比值不受GA施用的影响，且低于降低低镁血症风险的推荐最大值2.2（Kemp & t’Hart 1957; Grunes & Welch 1989）。

9Conclusion结论

In late winter, one application of GA, at a rate of 8 g a.i./ha, resulted in increased shoot DM yield, although there was no apparent benefit in applying two applications of GA. Nitrogen fertiliser also increased DM yield with ongoing DM yield advantages following two applications. In binary pasture mixtures, such as that of perennial ryegrass and white clover, DM yield is determined by the relative growth of each species and by the size and density of the plant units. While the effect of one application of GA or N on DM yield were positive, the factors determining the yield differed between treatments and this has practical implications for determination of DM yield and nutritive value. One application of GA in late winter could be used to reduce herbage CP content and reduce the risk of urinary N losses; however, these hypotheses require testing under grazing as differences in clover may maintain high N intake. The use of GA in conjunction with N fertiliser to promote maximum pasture production in early spring is recommended as a tool for farming systems to reduce early season feed shortages.

冬末以8 g a.i./ha的施用量施用一次GA可提高地上部DM产量，但施用两次GA未显示出明显益处。氮肥也提高了DM产量，且施用两次后仍具有持续的DM产量优势。在多年生黑麦草与白三叶草等二元混播草地中，DM产量取决于各物种的相对生长以及植株单元的大小和密度。虽然施用一次GA或氮对DM产量的影响均为正效应，但决定产量的因素在不同处理间存在差异，这对DM产量和营养价值的测定具有实际意义。冬末施用一次GA可用于降低牧草CP含量并降低尿氮损失的风险；然而，由于三叶草的差异可能使氮摄入量保持较高水平，这些假设需要在放牧条件下加以检验。建议将GA与氮肥配合施用以促进早春草地产量最大化，作为农业生产系统缓解早季饲草短缺的一种手段。

10Disclosure statement利益冲突声明

No potential conflict of interest was reported by the authors.

作者声明不存在潜在的利益冲突。

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