effects of nitrogenous fertilizer on the growth, grain yield and grain protein concentration of...

Aust. J. Agric. Res., 1992, 43, 949-67

Effects of Nitrogenous Fertilizer on the Growth, Grain Yield azld Grain Protein Concentration of Wheat

G. K. McDonald

Department of Plant Science, The University of Adelaide, Waite Agricultural Research Institute, P.O. Glen Osmond, S.A. 5064.

Abstract

The responses of wheat to applications of nitrogenous fertilizer were examined between 1988 and 1990 at 10 sites in South Australia which were considered to be marginally deficient in N. Nitrogen rates ranged from 0 kg N/ha to 150 kg N/ha and the experiments were sown after a range of crops and pastures. Nitrogen often increased early crop vigour and subsequent vegetative growth but significant increases in grain yield occurred at three of the 10 sites only; at the remaining sites there was no significant response or there was a reduction in yield at the highest rates of N. Kernel weights fell and grain protein concentration increased at most sites as the rate of N increased. The total amount of N per kernel was relatively constant across the N treatments at each site and across the 10 sites it varied less than the starch content per kernel. Grain protein concentration therefore was affected more by the amount of starch deposited in the grain than by the total amount of nitrogen. The amount of dry matter remobilized post-anthesis, calculated from changes in dry weight, was high and at the majority of sites was increased with applications of nitrogenous fertilizer. Despite the generally large amount of dry matter remobilized, this appeared to be used inefficiently during grain filling and there was little evidence that it greatly contributed to grain growth and grain protein concentration. The relationship between starch content per kernel and N content per kernel varied between sites: in some cases starch and N were negatively correlated, while in other instances there was a positive correlation or no correlation. The data suggest that high grain protein concentration at high levels of N are not a direct consequence of increased mobilization of dry matter and greater translocation of N to the grain. Dry matter production at anthesis was correlated with the amount of growth after 10 weeks but generally this increased dry matter production was of no benefit to yield. It is concluded that in the medium rainfall areas of the state, there is no advantage to be gained from improved early vigour, except perhaps where poor early growth is due to inadequate management.

Keywords: wheat, nitrogen, fertilizer, yield, protein concentration.

Introduction

Legume-derived nitrogen (N) is still the major source of N for wheat crops in southern Australia; however, applications of nitrogenous fertilizer are becoming more important to maintain yields and grain protein levels, particularly in more intensively cropped systems. The rates used vary depending on the crop rotation, soil type, rainfall and expected yield, but generally in South Australia they are low, with application rates less than 25 kg N/ha being commonly used. Under

G. K. McDonald

rainfed conditions in South Australia, grain yield responses to additional N are variable because of the effects of residual soil N as well as the influence of moisture availability and temperature in spring (Russell 1968a, 19686). The efficient management of fertilizer N in these situations is difficult. Many wheat crops are not severely N deficient and the application of high rates of N can result in low or sometimes negative grain yield responses (haying off).

The problem of making most efficient use of N fertilizer is essentially one of achieving a balance between pre- and post-anthesis growth and water use. Applications of N fertilizer stimulate early vegetative growth. In low rainfall environments, increased early vigour, either through genetic improvement or better agronomy, has been promoted as being an effective means of improving water use efficiency and grain yield (Acevedo and Ceccarelli 1987; Shepherd et al. 1987; Richards 1991). However, greater vegetative growth also increases water use by the crop causing high levels of water stress late in the season which affects grain yield and protein concentration.

Carbon movement into the grain depends on the amount of carbon fixed post-anthesis and the remobilization of dry matter, while the N content of the grain relies on the post-anthesis uptake, which is usually small in dryland environments, and the remobilization of N from the vegetative tissues. In rainfed crops the amount of dry matter remobilized during the grain-filling period can be relatively large and can increase with greater levels of post-anthesis stress (Bidinger et al. 1977, Pheloung and Siddique 1991). Stress will therefore affect the remobilization of dry matter and of N and their allocation to the grain. Starch and protein deposition in the grain is asynchronous (Jenner et al. 1991) with the peak in protein deposition occurring prior to that of starch deposition (Blacklow and Incoll 1981), so the precise effects of post-anthesis stress on grain yield and grain protein concentration will be influenced by the timing and severity of the resultant water stress.

Therefore, the addition of nitrogenous fertilizer can increase vegetative growth and the amount of dry matter and N in the crop at anthesis, but perhaps more importantly, through its effects on crop growth and water use, it can affect the levels of water stress and the partitioning of dry matter and N during grain filling. As many of the cereal crops to which N is applied in South Australia can be considered to be marginally N deficient only, the rates of N applied may occasionally be too high and crops may hay off. There is, however, relatively little information from field experiments on the effect of N on the relationships between dry matter production, its remobilization post-anthesis, grain N yield and grain protein concentration. Experiments were therefore conducted to examine the responses to N in situations considered to be marginally deficient in N but which, nevertheless, were representative of sites where farmers are currently applying some N to wheat as a standard practice.

Materials and Methods Two series of experiments were conducted between 1988 and 1990 at 10 sites throughout

the cereal belt of South Australia. At six sites small plots were sown and at the remaining sites the fertilizer treatments were superimposed on commercial crops of wheat. Salient features of the sites are given in Table 1 and rainfall for each site in Table 2. Each experiment was a randomized complete block with between four and six replicates.

Table 1. Characteristics of the sites used between 1988 and 1990

Caltowie Mortlock Keith Wasleys Wasleys Charlick Mallala Owen Tarlee Kapunda Pea pasture

Ave. r/fall (mm)

Previous crop

Soil type

Soil depth ( 4

pHB 0-15 cm 15-30 cm 30-45 cm Mineral N~ (pg/g) 0-15 cm 15-30 cm 30-45 cm 47-70 cm

454

Oats-vetch hay crop

Red-brown Earth >loo

6.9 6.9 7.1

10.0 13.8 12.6 4-8

579

Faba bean

Rendzina

45-70

6.5 6.3 7.2

8.1 9.7

17.8 13-0

470

Vol. pasture (grassy

Solodiied Solonetz

30-60

7.0 7.4 7.8

18-4 5-6

11.5 -

468

Peas

Solonised Brown Soil

50-100

7.5 7.8

n.d.

14-4 22.1 n.d. n.d.

Vol. pasture Subterranean Medic (grassy) clover pasture seed crop

Solonised Red-brown Red-brown Brown Soil Earth Earth

50-100 >lo0 > 100

427

Peas

SoIonised Brown Soil

> 100

7.5 7 -7 8.0

7.4 9.5

11.8 n.d.

468

Faba bean

lled-brown Earth >loo

6.3 7.8 8.1

20.0 22.5 23.4 n.d.

501

Triticale

Brown Clay >loo

7.4 7.6 7.9

5.5 8.5 9.0 n.d.

A Long term average for Strathalbyn. 1 : 5 soil: water extract. n.d., not determined. Nitrate+Nitrit6+ammonium-N.

Table 2. Monthly rainfall for the 1988 and 1989 growing seasons and long-term averages at six sites

Caltowie Keith Mortlock Wasleys Charlick Mallala Owen Tarlee Kapunda Month 1988 Av. 1988 Av. 1988 Av. 1989 Av. 1989 AV* 1990 Av. 1990 Av. 1990 Av. 1990 Av.

(mmlmonth) Apr. 3 30 17 36 7 42 32 41 48 39 2 35 7 37 5 38 6 40 May 113 47 138 55 65 58 76 60 87 55 12 47 10 47 7 53 12 55 June 85 53 88 51 90 59 63 52 67 58 65 48 58 44 59 53 73 57 July 39 55 41 54 51 76 66 58 69 63 63 46 62 52 87 54 94 59 Aug. 22 57 40 58 26 79 64 53 60 60 60 46 50 50 99 62 44 59 Sept. 52 52 74 51 61 72 48 51 45 53 29 42 29 44 37 51 43 55 Oct. 20 42 22 45 23 59 24 44 37 45 20 37 27 41 36 45 23 47 Nov. 56 33 25 33 95 36 42 29 60 29 2 25 2 28 6 31 10 30 Dec. 13 27 13 25 8 18 44 24 8 24 22 21 21 21 26 24 60 25 AD^.-Oct. 334 336 420 350 323 403 373 359 413 373 251 266 243 278 275 315 350 372

A Long term average is for Strathalbyn.

Effect of Nitrogen Fertilizer on Wheat Production

Small Plot Experiments

Sites

Three sites were used in each of the years 1988 and 1989. In 1988, experiments were conducted at Caltowie, Mortlock Experiment Station, Mintaro and Keith, while in 1989 there was one site at the Charlick Experiment Station, Strathalbyn and two at Wasleys, one following a pea crop and the other after a grassy, volunteer pasture. The notable features of the weather are the high rainfall in May and June at each site and the low rainfall during October. In 1988 there was a period before anthesis and early grain-fill during which little rain fell. At Caltowie, for example, no rain was recorded during the 2 weeks before flowering and the 2 weeks after flowering. The crops at Keith and Caltowie became visually water stressed, particularly at Caltowie where the leaves where rolled and drought tipping was evident in many plots.

Varieties

The three varieties used were Osprey (late maturing), Spear (medium) and Schomburgk (early). The maturities of these three varieties encompass the range normally grown by South Australian farmers.

N treatments

Application rates of 0, 25, 50, 100 and 150 kg NJha were applied as low-biuret urea (46% N) at sowing. The fertilizer was applied first at a depth of 10-15 mm using a cone seeder and incorporated with harrows. Immediately after the N was applied, the wheat was oversown at a depth of about 30 rnrn with the superphosphate. Thus the N and the seed were physically separated in the soil to minimize any adverse effects of high rates of N on germination and establishment.

Sowing

The experiments were sown with a 10-row cone seeder (15 cm row spacing) in plots 10 m (1988) or 20 m (1989) long. A basal dressing of 18 kg P/ha as double superphosphate was applied with the seed at sowing. The experiments were sown on 1 June (Keith), 2 June (Mortlock), 3 June (Caltowie), 24 May (Wasleys-pea), 25 May (Wasleys-pasture) and 30 June (Charlick). Each year prior to sowing, 5 g of seed was sown into moist sand in the glasshouse and the number of seedlings that emerged was counted. The sowing rates of each variety were then adjusted to give 200 seedlings/m2. Subsequent measurements of seedling establishment showed that the mean plant population was generally greater than 170/m2 and was not affected by fertilizer rate.

Crop husbandry

In most experiments effective weed control was achieved by using post-emergent applications of Hoegrass @ and Ally @ at rates of 1.0 L/ha of Hoegrass and 7 g/ha of Ally. The exception was the 1989 experiment at Wasleys following a grassy pasture, where the crops were infested with barley grass. Handweeding of small areas of plots in selected treatments indicated that the presence of weeds reduced grain yield on average by about 1 t/ha

To reduce the effects of any root disease on the comparison of the three sites in 1988, superphosphate containing 2% triadimefon was used (Ballinger and Kollmorgen 1988). In 1989 the experiment was split so that half the plots received the fungicide-amended fertilizer while the remainder received double superphosphate only, but as there was no effect of the fungicide treatment on the growth and yield of wheat at any site in 1989 the results have been combined. In 1988 the three sites were free of foliar disease, but in 1989 the sites were sprayed once for stripe rust (Puccinia striiformis) using Bayleton @ at 0.75 L/ha. Some root knotting caused by cereal cyst nematode (Heterodera avenue) was observed at the two Wasleys sites

Following the analysis of the youngest emerged leaf blade in mid-August for a range of nutrients, the crops at Keith received a foliar application of zinc and those at Mortlock received both zinc and boron. The tissue analysis also indicated that adding nitrogen significantly increased the levels of zinc, phosphorus and sulfur in the leaves at Keith.

G. K. McDonald

Measurements

Dry matter production and grain yield

All quadrat measurements were based on a quadrat size of 4 rowsx0.5 m (0.30 m2). Dry matter samples were taken approximately 10 weeks after sowing, to assess early vigour, at anthesis and at maturity. In 1988, one quadrat sample per plot was taken, while in 1989 two samples were taken and bulked together. Tiller numbers were counted on the 1988 samples, but in 1989 only dry weight was measured. Grain yield, total dry matter, and yield components were measured in 1988 on the samples taken at maturity, whereas in 1989 only total dry matter, grain yield, ears/m2 and kernel weight were measured. Whole plot grain yield was estimated by harvesting the central eight rows of each plot with a small plot harvester after trimming 1-2 m from the ends of each plot.

Nitrogen analyses

Except at Charlick, the mineral N levels in the soil were assessed at each site 2-3 weeks after sowing. Soil samples were taken to a maximum depth of 60 cm in 1988, but to only 30 cm in 1989. The total mineral N content (nitrate+ammonium+nitrite N,,ug/g) was measured using steam distillation with CaO and Devarda's alloy (Keeney and Nelson 1982) after extracting 10 g of air dry soil in 50 mL of 2 M KCl. In both years the grain from the quadrat samples was ground and analysed for grain protein concentration using near-infrared reflectance. Grain protein concentration was adjusted to 11% moisture. Straw samples from Mortlock and Keith in 1988 were ground and analysed for N content using a Kjeldahl technique. Total N content (g ~ / m ~ ) was calculated from the N content of the grain and straw and the quadrat grain and straw yields and the nitrogen harvest index was derived.

Topdressed Experiments

In 1990 four experiments were established in commercial fields of wheat in the lower north of South Australia (Table 1). Seven rates of N, 0, 10, 20, 30, 40, 60 and 80 kg N/ha, were applied to the wheat crops 6 weeks after emergence. The rates differed from the earlier experiments so that the response to low rates of N could be described better. Apart from the fertilizer treatments, the husbandry of the crop was undertaken by the farmer as part of his normal management practice. The rates of phosphorus fertilizer varied from 11 to 14 kg P/ha, plant populations varied between 140 and 190/m2 and there was adequate weed and disease control in each paddock. The N was applied in early July to plots that were 50 mx4.5 m using a cone seeder. Sowing at the four sites occurred between 7 June and 29 June which were generally later than normal because of the late arrival of the opening rains.

Dry matter production was measured at anthesis by sampling from three, 0.3 m2 quadrats per plot. The plant material was dried, ground and analysed for total N using the Kjeldahl method. At maturity, three quadrat samples were taken and total dry matter, grain yield and yield components measured; harvest indices and kernels/m2 were derived. The grain yield was also estimated by harvesting 50 m from the centre of each plot using a small plot harvester. The grain and the straw were ground and analysed for total N concentration and the nitrogen harvest index derived.

Results

In the small plot experiments there were few significant interactions between variety and nitrogen rate at any site, and as the major interest of the experiments was the response to nitrogenous fertilizer, only the main effects of N will be discussed.

Dry Matter Production

Small plot experiments

Early growth of the crops at each site was significantly increased by the addition of N fertilizer (Table 3). In general, dry matter production increased


up to 100-150 kg N/ha. Measurements in 1989 of tiller numbers indicated that the increase in dry matter was associated with increased tillering at each site (data not presented) as well as an increase in the dry weight per tiller.

Table 3. Effect of applied N on dry matter production 10 weeks after sowing and at anthesis at six sites

N rate (kg N/ha) 0 25 50 100 150 1.s.d. Signif.A

( d m 2 ) 10 weeks

Caltowie 55 70 86 93 106 20.2 *** Keith 93 137 167 207 195 57.0 *** Mortlock 47 59 60 72 82 19.7 *** Wasleys-pea 74 102 120 132 142 18.7 *** Wasleys-pasture 37 45 50 52 52 10.1 *** Charlick 174 202 204 218 214 29.4 ***

Anthesis Caltowie 345 404 386 429 456 51.2 *** Keith 364 562 733 867 903 164.0 *** Mortlock 578 669 689 739 778 124.3 *** Wasleys-pea 717 759 782 831 876 71.5 *** Wasleys-pasture 454 506 504 507 526 50.3 *** Charlick 854 884 906 86 1 876 31.9 *

Significant increases in dry weight with applications of N were also measured at anthesis (Table 3); however, at each site except Keith, the relative responses were less than those measured at 10 weeks after sowing. Increases in tiller numbers were also measured in 1988 (data not presented), but in contrast to the measurement after 10 weeks, the dry weight per tiller declined at Caltowie and at Keith. At each site the dry weight at anthesis was significantly and positively correlated with the dry weight at 10 weeks.

Topdressed experiments

The only significant increase in dry matter production at anthesis occurred at Kapunda where dry weight increased up to the highest rate of N (Table 4).

Grain Yield and Yield Components


Unlike the effects of N fertilizer on vegetative growth, significant increases in grain yield were few and occurred only at Keith and Wasleys-pasture (Tables 5, 6). At the other sites there was no significant response to applied N up to 100 kg N/ha, but the yield at 150 kg N/ha was always significantly lower than that at 0 kg N/ha. Adding N increased the number of ears/m2 and the number of kernels/m2. The response of kernel number to added N tended to be greater than that of grain yield and to have a higher optimum N rate (Tables 5, 6). Kernel weight showed a consistent decline as the rate of N fertilizer increased at all sites except Charlick, although significant reductions did not occur until 100 kg N/ha or 150 kg N/ha. The fall in kernel weight was associated with an increase in kernel number but at 150 kg N/ha there was a decline in both kernel

G. K. McDonald

number and kernel weight. Detailed yield component analysis in 1989 showed that the number of spikelets per ear was little affected by N, but the number of kernels/spikelet was reduced at the highest rates of N (Table 5).


Significant responses to N occurred at two sites (Table 4). At Kapunda, grain yield increased up to 30 kg N/ha while at Mallala the grain yield was reduced at the highest rates of N. N treatment had no effect on ears/m2 except at Kapunda, and at high rates of N, kernel weights were reduced. Nitrogen treatment had little effect on kernel/m2 except at Kapunda (Table 4).

Table 4. Dry matter and yield responses to applied nitrogen at four sites in 1990

kgN/ha Dry matter Grain ~ a r s / r n ~ Kernel Grains/ HI at anthesis yield wt m2

(dm2) (t/ha) (md ('000)

Mallala 1.99 5 75 1.94 585 1.82 531 1.85 528 1.86 535 1.80 525 1.77 546 0.120 n.s.

Owen 2.84 310 2.95 352 2.99 330 2.76 320 2.88 310 2-60 316 2.70 311 ns . ns .

Tarlee 3.05 333 3-22 359 3.05 354 3-07 364 3.13 382 3.03 406 2.87 381 n.s. n.s.

Kapunda 2.12 200 2.45 230 2-40 239 2.78 236 2.76 260 2.75 259 2.76 307 0.421 49.6


Table 5. Effects of N rate on grain yield and yield components at three sites in 1988

N rate (kg N/ha) 0 25 50 100 150 1.s.d ~ i p i f . ~

Caltowie Keith Mortlock






Grain yield (t/ha) 1.47 1-42 1.36 2.98 3.27 2.92 4.69 4.81 4.60

~ a r s / m ~ 358 368 380 347 468 580 528 599 635

Spikeletslear 14.6 15.0 15.3 13.6 14.2 14.3 15.3 15.0 14.9

Kernels/spikelet 1.46 1.26 1.32 1.88 1.68 1.51 1.86 1.86 1.81

Kernel weight (mg) 17.1 17.1 17.1 30.5 26.8 20.0 30.9 29.3 27.4

~ e r n e l s / m ~ (x10-~) 7.52 6.89 7.36 8.79 10.69 11.96

14.93 16.63 16.88

Table 6. Effects of N fertilizer on grain yield and yield components in 1989

N rate (kg N/ha) 0 25 50 100 150 1.s.d. ~ i ~ n i f . ~

Grain yield (t/ha) Wasleys-pea 3.97 4.01 4.12 3.79 Wasleys-pasture 2.14 2.40 2.37 2.32 Charlick 3.36 3.32 3.23 3.15

Wasleys-pea Wasleys-pasture Charlick



~ a r s / m ~ 485 504 526 293 320 314 536 556 541

Kernel weight (mg) 37.5 34.5 31.3 36.4 34.1 31.9 27.8 26.7 27.8

~ e r n e l s / m ~ ( X

11.93 12.84 12.71 6.81 7.66 7.79

12.02 11.82 11.53

Remobilized Dry Matter

The amount of dry matter remobilized during the post-anthesis period was estimated from the difference in the dry matter at anthesis and the straw dry

G. K. McDonald

matter (straw dry matter = total dry matter-grain yield). This value is a crude estimate of the amount of remobilized dry matter as it takes no account of respiratory losses of dry matter. Other errors may arise from the loss of senesced leaf material, which at maturity may represent 10-15% of total dry matter, and which would cause the decline in dry matter post-anthesis to be overestimated. Also, being derived from three separate measurements of dry matter, the estimate of the amount of remobilized dry matter is subject to cumulative error. However, despite its variability, the data indicate some broad trends.

Small plot experiments , At all sites except Charlick, there was greater remobilization of dry matter

with the addition of N (Table 7) which, as a proportion of dry matter at anthesis, ranged over an average of 9% at Keith to 29% at Caltowie. The proportion of remobilized dry matter at individual sites varied little with N treatment.

Table 7. Effect of applied N on the remobilized dry matter at six sites

N rate (kg N/ha) 0 25 50 100 150 1.s.d. signif.*

Caltowie Keith -32 26 68 105 122 97.1 Mortlock 29 112 66 100 130 71.3

t Wasleys-pea 120 132 127 160 214 67.9 ** Wasleys-pasture 116 143 110 114 155 31.3 Charlick

t 187 206 223 175 208 n.s.


There was no significant effect of N on the amount of dry matter remobilized at any of the four sites. The mean amounts remobilized at each site were 221 g/m2 (Mallala), 148 g/m2 (Owen), 213 g/m2 (Tarlee) and 96 g/m2 (Kapunda), which represented 18-34% of the dry matter at anthesis.

In both series of experiments, the amount of dry matter remobilized was positively correlated with the dry matter at anthesis (Fig. 1)

Crop Nitrogen


Grain protein concentration was increased with applications of N fertilizer at all sites except Charlick (Table 8). The response in grain protein concentration partly reflected inversely the grain yield response at the site: where the grain yield increase was small there was a large increase in grain protein concentration and at Keith, where there was the greatest grain yield response, the increase in grain protein concentration was small. The N yield varied considerably between sites (Table 8) and the response to applications of N tended to reflect the trends in grain yield more so than grain protein concentration.

The N content per kernel was not significantly affected by N rate at three sites (Keith, Mortlock and Charlick) and was increased at the remaining sites


I I I I

300 400 500 600 700 800

Dry matter at anthesis g m+

Fig. 1. The relationships between dry matter at anthesis and the estimated amount of dry matter remobilized during the post-anthesis period. (a) Plot experiments: Caltowie ( a ) , Keith (W), Mortlock (A), Wasleys-pea (O) , Wasleys-pasture (0) and Charlick (A); ( b ) topdressed experiments: Mallala ( a ) , Owen (O) , Tarlee (W) and Kapunda(0).

(Table 8). These changes in N content per kernel could not be obviously related to the yield response at the site. Within each site, the amount of N per kernel was relatively stable despite significant changes in both kernel weight and grain protein concentration at most sites. The nitrogen harvest index at Keith and Mortlock (Table 9) declined with applications of N but the decrease was greater at Keith.

The starch content per kernel was estimated from the difference between the mean kernel weight and the protein content per kernel. There was no consistent relationship between the N content per kernel and starch content (Fig. 2a); at some sites (e.g. Caltowie, Wasleys-pea and Wasleys-pasture) there was a negative relationship while at others there was a weak positive relationship (Mortlock) or no apparent relationship (Keith, Charlick).


Addition of nitrogenous fertilizer increased the nitrogen uptake at anthesis at all sites (Table 10). The concentration of N at anthesis when no N was applied was 1.7, 1.6, 1 .4 and 1.1% at Mallala, Owen, Tarlee and Kapunda respectively and at each site nitrogenous fertilizer increased the N concentration at anthesis. There was no net increase in the N content of the shoot between anthesis and maturity. Nitrogen harvest index fell with applications of N (Table 10) although

G. K. McDonald

Table 8. Effect of applied N on grain protein concentration, N yield and N content per kernel at six sites

N rate (kg N/ha) 0 25 50 100 150 1.s.d. ~ i g n i f . ~

Caltowie Keith Mortlock Wasleys-pea Wasleys-pasture Charlick



Grain protein concentration (%) 15 .1 15.9 17.9 8.9 9.5 12.9

11.9 12.4 13.0 9.8 10.8 12-5 9.5 11.1 12.6

15.0 15.2 14.9 N yield (g/m2)

3.8 3.6 4.4 4 - 7 5.3 6 -0

10.8 11.7 11.8 7.6 8.3 8.6 4.1 5.1 5.5 8.8 8.4 8.3 N content per kernel (mg) 0.51 0.53 0.60 0.53 0.49 0.50 0.72 0 -71 0.70 0.64 0.65 0.69 0.60 0.66 0.70 0.73 0.70 0.72

Table 9. Effect of applied N on nitro~en harvest index

N rate (kg N/ha) 0 25 50 100 150 1.s.d. ~ i g n i f . ~

Keith 0.82 0.80 0.74 0.63 0.57 0.001 *** Mortlock 0.86 0.85 0.82 0.79 0.75 0.001 ***

both grain protein concentration and grain N yield increased, indicating that the amount of N taken up by the crop rather than its subsequent distribution was more important to grain N. The N concentration of the straw increased as the N rate increased (data not presented) and the average concentration at each site was 0.90% (Mallala), 0.40% (Owen), 0.42% (Tarlee) and 0.32% (Kapunda). As in the previous experiments, the average amount of N per kernel was relatively stable across all sites and N rates (Table 10) and the relationship between starch content and N content per kernel varied between sites (Fig. 2b).

Eficiency of N utilization

The average grain yield per unit of N in the crop at maturity, the N utilization efficiency, at each site was 41.6 kg/kg (Keith), 33.5 kg/kg (Mortlock), 16.8 kg/kg (Mallala ), 33.4 kg/kg (Owen), 31 .4 kg/kg (Tarlee) and 44 6 kg/kg (Kapunda) . The efficiency declined as the N rate increased, particularly at the responsive sites, Keith and Kapunda. The maximum value of 60-9 kg/kg was recorded


0.5 I I

1 0 2 0 3 0 4 0

Starch content per kernel (mg)

Fig. 2. The relationships between N content per kernel and starch content per kernel at 10 sites. (a) Plot experiments: Caltowie ( e ) , Keith ( I ) , Mortlock (A), Wasleys-pea (O), Wasleys-pasture (0) and Charlick (A); (b) topdressed experiments: Mallala ( e ) , Owen (O), Tarlee (B) and Kapunda(0).

at Keith at 0 kg N/ha and the minimum value of 15-0 kg/kg was measured at Mallala at 80 kg N/ha.

Discussion

Significant increases in grain yield were measured at only three of the 10 sites. The experiments were conducted in fields where the farmer would apply some N at sowing as part of his normal cropping program so the results highlight the problems often associated with efficiently managing N in this environment. The trends in yield components indicate that the lack of response was due to increased levels of stress around flowering and particularly during grain filling: vegetative growth was increased with N but there were reductions in kernel weight and grain set per spikelet at the highest rates of N.

The effects of high levels of soil N on vegetative growth, water use and yield of wheat crops are well documented (Barley and Naidu 1964; Storrier 1965; Fischer and Kohn 1966; Taylor et al. 1978; Mason and Rowland 1990). Large amounts of mineral N in the soil at the start of the season promote early growth and water use, but can also cause early depletion of soil moisture reserves resulting in crops haying off. This was observed in all years to varying degrees. Early in the season, water availability is not a major constraint and in the absence of weeds and disease, dry matter responses will largely reflect the availability of soil N during this time. The loss of N from the system by leaching and denitrification during winter, continued mineralization of organic N during spring increasing the

G. K. McDonald

Table 10. Nitrogen content and nitrogen concentration of the shoot and grain in responses to applied nitrogen at four sites in 1990

kg N/ha N content N content Grain N NHI Grain N per at anthesis at maturity yield protein kernel

( d m 2 ) ( d m 2 ) (dm2) conc. (%) (mg)

Mallala

10.5 7.2 11.4 7.5 10.0 6.4 11.5 7.2 11.1 7.3 11.3 6.8 11.8 7.1 1.54 n.s.

Owen 7.7 6.4 9.2 7.8 8.4 6.9 8.6 7.1 7.6 6.2 8.5 6 -7 9.4 7.5 1.51 1.19

Tar lee 8.7 7.1 9.0 7.1 9.2 7.4 9.9 7.7

10.4 8.0 10.5 7.9 11.3 8.3 1.49 1.12

Kapunda 4.4 3.4 4.9 3.8 5.1 8.3 5.3 4.2 6.5 5.1 7.1 5.2 8.3 6.1 1.55 1.24

supply of N, and increasing levels of water and heat as the season progresses could all have contributed to the subsequent decline in the response to N. The large responses in vegetative growth early in the season, although not reflected in grain yield, nevertheless affected the growth of the crop post-anthesis, the amount of dry matter remobilized and grain yield.

The N utilization efficiencies measured in the 10 experiments were comparable to those reported for high yielding irrigated wheat crops at Griffith ( c . 35 kg/kg; Stapper and Fischer 1990), but were generally lower than those measured in dryland wheat crops in southern New South Wales (32-66 kg/kg; Angus and Fischer 1991). In the latter experiments, grain yields were frequently higher than the yields from the South Australian experiments and in most cases additional


N increased grain yield; in a low yielding year the N utilization efficiency (c. 34 kg/kg) was similar to values from the South Australian experiments. The greater N efficiencies achieved by Angus and Fischer (1991) appear to reflect the greater responsiveness of their crops compared to dryland crops in South Australia. This could be due to the higher rainfall at the southern New South Wales sites, although the similar efficiencies of the irrigated crops and those from South Austrlia indicate that availability of moisture alone may not be the only factor contributing to high N utilization efficiencies.

There was considerable remobilization of dry matter during the post-anthesis period which as a proportion of dry matter at anthesis, was generally greater than the 14% reported for wheat crops in Western Australia, but similar to the range quoted for other environments (Siddique et al. 1989). When post-anthesis photosynthesis is reduced by stress, remobilization of dry matter increases, which may help sustain kernel growth (Gallagher et al. 1975; Pheloung and Siddique 1991). In the small plot experiments, kernel weight was generally lower at the high rates of N despite greater remobilization (Fig. 3 a ) and there was no consistent relationship between the amount of dry matter remobilized and grain yield. This suggests that the increase in remobilized dry matter at the higher rates of N made little direct contribution to kernel growth or consequently grain yield in these experiments, although measurements of the fate of the remobilized dry matter are necessary to verify this.

10 0 1 0 2 0 3 0

Remobilized dry matter per kernel (mg)

Fig. 3. The relationships between the kernel weight and the dry matter remobilized during the post-anthesis period at 10 sites. (a) Plot experiments: Caltowie (O), Keith (U), Mortlock (A), Wasleys-pea (O), Wasleys-pasture (0) and Charlick (A); ( b ) topdressed experiments: Mallala (O), Owen (O), Tarlee (U) and Kapunda(0).

G. K. McDonald

Increased remobilization of dry matter may not contribute proportionately to kernel growth and grain yield because of the utilization of part of the remobilized dry matter in dark respiration. Pearman et al. (1981), for example, found that adding N fertilizer increased dark respiration rates in winter wheat and further suggested that the increases in respiration could be compensated by greater use of stored, pre-anthesis assimilate &om the stems. They concluded that increases in dark respiration at high rates of N were a consequence of the greater maintenance requirement of the larger amount of biomass. The positive correlation observed in all 10 experiments between dry matter production at anthesis and the amount of dry matter remobilized post-anthesis (Fig. 1) is circumstantial evidence that similar effects may have been observed in these experiments.

Remobilized dry matter g m-2

Fig. 4. The relationships between the amount of dry matter remobilized during the post- anthesis period and grain protein concentration at 10 sites. (a, b) Plot experiments (1988 and 1989): Caltowie (e), Keith (H), Mortlock (A), Wasleys-pea (O), Wasleys-pasture (0) and Charlick (A), ( c ) topdressed experiments (1990): Mallala (e), Owen (O), Tarlee (m) and Kapunda(0).


There was little evidence to suggest that the greater remobilization of dry matter associated with the high N treatments increased the movement of N into the grain, notwithstanding the positive correlation between remobilized dry matter and grain protein concentration (Fig. 4). The addition of fertilizer reduced nitrogen harvest index (Tables 9, lo), and the experiments in 1990 showed that when there was no post-anthesis increase in N, the amount of N in the grain was related more to the amount taken up prior to anthesis than its subsequent distribution. The relationships observed in Fig. 4 therefore largely illustrate correlated events induced by the N treatments rather than a direct relationship between remobilization and grain N.

Despite the significant effect of N on kernel weight and grain protein concentration the amount of N per kernel within a site varied relatively little between N treatments (Tables 8, 10 ). The variation in N content across all sites and treatments (0.49-0.91 mg Nlkernel) was also less than the variation in the starch content (12-37 mg/kernel) (Fig. 3). The relative stability of N content per kernel compared with starch content suggests that the increase in grain protein concentration with added N was related more to a reduction in the supply of sucrose to the grain and/or a reduction in the synthesis of starch in the endosperm. Starch content in the grain was lower at high rates of N which was also reflected in lower kernel weights, despite often increased amounts of dry matter remobilized. Post-anthesis stress caused by high temperatures and plant water deficits will affect grain protein concentration by its effects on starch and protein deposition in the grain as well as protein metabolism in the leaves and stems (Bhullar and Jenner 1986; Jenner et al. 1991). Clearly, however, in the field environment the timing and severity of stress is highly variable and the inter-relationship between starch and N deposition in the grain reflects this variability (Fig. 2).

Large amounts of dry matter were remobilized post-anthesis in these experiments. This can increase with additions of nitrogeneous fertilizer but it may not directly contribute to greater grain protein concentrations or to the maintenance of kernel weight. Much of the dry matter remobilized appears to be utilized in other pathways. Siddique et al. (1989) have suggested that the high proportion of grain yield derived from remobilized dry matter [estimated from (remobilized dry matter /grain yield)] is one reason why dry matter production at anthesis should be maximized, but clearly this will only be true if the remobilized dry matter contributes significantly to grain filling. The results from the experiments reported here suggest this may not always be the case. Dry matter production, ears/m2 and grain yields from these South Australian experiments were on average greater than those reported by Siddique et al. (1989), which suggests that dry matter production is not limiting yield to the same extent in the two environments and that the value of increasing dry matter production to improve grain yield may differ.

The large early response to applied N was partly due to an increase in tiller numbers, many of which senesced as stress increased later in the season. Nevertheless, at maturity, the number of ears/m2 was increased with nitrogenous fertilizer and the number of kernels/m2 and ears/m2 were significantly correlated (r = +0-88, P < 0.01). Whether a more controlled pattern of tillering could result in more grain yield responses to N and what effects this would have on grain concentration should be investigated further.

G. K. McDonald

Vigorous early growth can improve water use efficiency and grain yield in low rainfall areas and greater seedling vigour has been promoted as a desirable breeding objective for such environments (Acevedo and Ceccarelli 1987; Shepherd et al. 1987). In the experiments conducted in 1988 and 1989, applications of nitrogenous fertilizer increased early growth but this was not important to grain yield. Greater early growth increased dry weight at anthesis, but subsequently grain yield was little affected or even reduced. Furthermore, the higher yielding sites did not necessarily have greater early dry matter production and within each site there was no benefit from more vigorous growth (Fig. 5). This may be generally true for the medium rainfall zone of South Australia because the levels of productivity in these experiments were representative of wheat crops grown over a larger area of the cereal belt. For example, over two successive seasons commercial wheat crops in the mid and lower north of the state produced on average, 140 g/m2 seven weeks after sowing which resulted in a mean grain yield of 3 - 2 t /ha (Wegener et al. 1989), similar to levels of productivity measured in these experiments. Thus, many of the commercial crops appear to have sufficient early growth if they are adequately managed and there may be only a marginal benefit to grain yield from trying to increase the vigour of crops in the medium rainfall zone of the cereal belt. Of course, there will be crops in the region that are not vigorous and as a consequence, will have low grain yields, but this is most likely to be due to deficiencies in management such as late sowing, poor weed control, poor nutrition or high levels of disease rather than to inherent environmental or genotypic factors. Improvements in vigour in these circumstances can be remedied by using current technologies and varieties.

0 1 0 0 2 0 0 3 0 0

Dry matter at 10 weeks g K2

Fig. 5. The relationship between dry matter at 10 weeks after sowing and grain yield at six sites: Caltowie (O), Keith (W), Mortlock (A), Wasleys-pea (O), Wasleys-pasture (0) and Charlick (A).

Acknowledgment

The work was supported by the Wheat Industry Research Committee for South Australia. The technical assistance, at various times, of I\dr P. Wurfel, Mr N. Bliescke and Mr A. Smith is gratefully acknowledged.

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Manuscript received 25 July 1991, accepted 25 February 1992

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