Field Crops Research, 17 (1987) 245-258 245 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Lodging Effects on High-Yielding Crops of Irrigated Semidwarf Wheat

R.A. FISCHER ~ and M. STAPPER 2

ICSIRO Division o/Plant Industry, G.P.O. Box 1600, Canberra, A.C.T. 2601 (Australia) 2CSIRO Centre for Irrigation Research, Private Bag, Griffith, N.S. W. 2680 (Australia)

(Accepted 19 June 1987)

ABSTRACT

Fischer, R.A. and Stapper, M., 1987. Lodging effects on high-yielding crops of irrigated semidwarf wheat. Field Crops Res., 17: 245-258.

The effect of lodging caused by overturning in saturated soil was studied in high-yielding crops of irrigated semidwarf wheat (Triticum aestivum L. and T. turgidum. L. vat durum) during two years in northwest Mexico and one in southeast Australia. Plots lodged artificially or lodged nat- urally were compared to plots which never lodged, usually because of protection provided by sup- portive mesh.

Culm lodging to an almost horizontal position caused grain yield to be reduced by 7-35%, with the greatest effect when lodging occurred in the first 20 days after anthesis. In one crop the yield reduction was clearly much less than in others with the same cultivar. Lodging just before anthesis was less deleterious, perhaps because the crop was able to right itself quickly by node bending. Kernel number per unit area tended to be reduced by early lodging, and kernel weight by later lodging accompanied by small increases in grain nitrogen percentage. Lodging during rainy rip- ening conditions increased grain sprouting.

Lodging after anthesis reduced crop growth rate, and the adverse effect of lodging on grain yield is ascribed to this reduction in photoassimilate supply. The crop in which lodging had least effect on grain yield was characterized by a reduced degree of source limitation during grain filling.

INTRODUCTION

Improve d lodging res is tance, ar is ing f rom sho r t e r and st i ffer straw, is one major reason why m o d e r n win te r cereal var ie t ies have h igher grain yield (Vo- gel e t al., 1963; P i n t h u s 1973; F ischer and Wall, 1976; W y c h an d Rasmusson , 1983). Improved lodging resis tance increases yield largely by pe rmi t t ing greater doses of n i t rogen ferti l izer. However , unde r ce r ta in agronomic condi t ions , even m o d e r n shor t cul t ivars will lodge. Thus , spr ing whea t var ie t ies car ry ing o n e o r even two major N o r i n 10 dwarf ing genes ( R h t l , Rh t2 ) have been observed to lodge in Mexico and Austral ia . Th i s is mos t c o m m o n wi th we l l -g rown crops nea r or a f te r f lowering and subject to wind dur ing flood i r r igat ion or du r ing

0378-4290/87/$03.50 © 1987 Elsevier Science Publishers B.V.

246

intense rainstorms. Such lodging occurs because of movement of the plant crown in the saturated surface soil, rather than because of buckling failure of the wheat stems. This has been termed 'root lodging' by Pinthus {1973) and 'overturning lodging' by Neenan and Spencer-Smith (1975). Less commonly, lodging due to stem bending has been observed during the vegetative stage in very leafy densely-tillered crops.

To quantify accurately the effect of lodging on grain yield requires artificial lodging and/or artificial lodging-protection treatments (Pinthus, 1973 ). Early studies of artificial lodging pointed to substantial yield reductions ( 30-40% ) with lodging of wheat and barley crops around heading time, and to somewhat lower reductions with later lodging (reviewed by Pinthus, 1973 ); these reduc- tions did not include losses due to interference with mechanical harvesting. The cause of these effects on grain yield was never investigated although De Wit ( cited in Van Dobben, 1966) calculated that the less-favourable light dis- tribution in lodged crops would reduce crop photosynthesis.

In the studies referred to by Pinthus (1973) the yields of unlodged crops were low, old cultivars being used. In modern, high-yielding semidwarf culti- vars, with higher kernel numbers and a tendency towards greater source lim- tiation during grain filling (Fischer, 1983), yield could be more sensitive to post-anthesis lodging. Alternatively, some believe that the advent of shorter semidwarf wheats has reduced the importance of lodging as a problem. We needed to know the magnitude of lodging effects on yield currently, in order to evaluate agronomic and breeding research priorities. Therefore it was desirable to repeat lodging experiments with modern high-yielding cultivars grown at high levels of fertility. We now report four such experiments under low-latitude irrigated conditions, with crops lodged artificially at different stages of devel- opment or protected against natural lodging. In one experiment, treatments with growth regulators and early mowing were included. Crop growth sam- plings were used in two of the experiments in order to elucidate the cause of lodging effects on yield.

METHODS

Experiments 1 and 2 were done in 1973-75 at CIANO (Centro de Investiga- ciones Agricolas del Noroeste) near Ciudad Obregon in northwest Mexico (iat. 27 °N). Experiments 3 and 4 were conducted in 1985 at CIR (CSIRO Centre for Irrigation Research), Griffith, N.S.W. ( lat. 34 ° S ). Weather conditions during the winter-spring growing season (Table 1) were close to normal: CIANO is somewhat warmer than Griffith; taken together the sites are representative of much of the world's low-latitude regions in which irrigated wheat is grown.

T h e soil type at CIANO was a luvic xerosol (50% clay at the surface ); at Griffith itwas a Hanwood loam (28% clay; Butler, 1979 ). Flood irrigation is the com-

monest method of watering wheat in each region.

247

TABLE 1

Mean temperature and solar radiation at CIANO in northwest Mexico (lat. 27 ° N ) and at Griffith in New South Wales (lat. 34 ° S) for the wheat seasons studied

CIANO Griffith

Month Solar radiation Mean temp. Month Solar radiation Mean temp. MJ m-2 day -1 (°C) MJ m-2 day -1 (°C)

1973-74 1974-75 1973-74 1974-75 1985 1985

Nov. 14.9 13.2 20.0 19.6 May 11.2 12.6 Dec. 13.3 12.2 16.0 14.4 June 9.7 8.4 Jan. 13.1 13.1 14.8 14.4 July 10.3 8.1 Feb 17.5 16.9 14.7 14.7 Aug. 11.9 9.9 Mar. 20.4 20.7 17.6 16.7 Sep. 18.2 11.2 Apr. 24.6 23.3 20.9 17.7 Oct. 21.2 16.4

Nov. 25.6 19.3

Exper iment 1, sown on 28 November 1973, comprised three varieties (Ye- cora 70, Cajeme 71 and Cocorit 71 ) as main t rea tments and five artificial- lodging subtrea tments (including control, see Table 2) in a split plot design; Cocorit 71 is a durum wheat (Trit icum turgidum L. var. durum), the others bread wheats ( T. aestivum L.). Experiment 2 was sown on 10 December 1974 with one variety (Yecora 70) and six lodging t rea tments (see Table 2) in a randomized block design. Experiment 3, sown on 12 June 1985, comprised nine t rea tments in a randomized block design; seven lodging t rea tments applied to the variety Yecora 70 and two to the variety Egret (see Table 3). In Experi- ment 4, sown on 22 April 1985 with the semiwinter variety Osprey, the crop was subjected to four t rea tments in a randomized block arrangement. One t rea tment (1) involved protection against lodging throughout the life of l:he crop; a second (2) involved natural lodging. In the third t rea tment (3) the crop was sprayed with growth regulators as recommended ( Cycocel at 3 1 h a - 1 on 24 July; Ethrel at 1 1 ha-1 on 6 September) , and in the fourth (4) it was mowed to 7-10 cm above the ground level on 3 July in order to simulate grazing, a common management practice for such early-sown varieties. Trea tments 3 and 4 were not protected from natural lodging. There were three or four rep- lications in each experiment. In Experiments 1 and 2, plot size was 2 mX 1.8 m; in Experiment 3 it was 5 m × 1.7 m, and was 2.5 mX6.0 m (mesh) and 4.8 mX10.0 m (no mesh) in Experiment 4. All wheat varieties contained the dwarfing gene Rh t l , while Yecora and Cajeme contained Rht2 as well.

Lodging protection was given to control plots at CIANO in 1973-74, and at Griffith, where lodging tends to be more common, to plots to be lodged artifi- cially after anthesis as well as to control plots. This was done by growing the crop through light rope (ClANO) or steel (Griffi th) mesh (openings 20 cm X 20

248

TABLE 2

The effect of artificial lodging on lodging score and grain yield and its components during two seasons at ClANO in northwest Mexico

Variety/ Date of Lodging Grain Kernel Kernel Total Harvest Initial lodging" score @ dry wt. number dry wt. dry wt. index lodging maturity (g m -2) (100 m -2) (rag) (g m -2) (%) angle

Experiment 1 Yecora 70 nil - - 0 699 183 38.2 1546 45.2 45 ° A - 2 d 6 607* 177 34.6 1450 41.9" 80 ° A - 2 d 33* 573* 162" 35.5 1326" 43.2 80* A+ 14d 67* 504* 162" 31.5" 1283" 39.2* 80 ° A+26d 67* 613" 172 36.0 1431 42.9 C~eme 71 nil - - 0 556 168 33.3 1473 37.7 45* A+3d 32* 536 159 33.6 1409 37.9 80 ° A+3d 69* 467* 157 29.7 1315" 35.5 80* A+ 15d 67* 527 165 32.0 1386 38.0 Cocorit 71 nil - - 0 700 150 46.8 1574 44.5 45 ° A - 2 d 75* 561" 137 40.8* 1421" 39.5* 80 ° A - 2 d 89* 536* 126" 42.9 1374" 39.0* 80* A+14d 94* 512" 139 36.8* 1432" 35.8* 80 ° A+26d 92* 587* 147 39.8* 1471 39.8*

SE (within 3.5 23 6.2 1.5 44 1.0 variety)

Experiment 2 Yecora 70 nil - - 0 691 148 46.7 1511 45.8 45* A+5d 15" 701 148 47.5 1536 45.8 45 ° A + 18d 50* 706 153 46.1 1491 47.4 80 ° A + 5d 50* 645 141 45.8 1394 46.3 80 ° A + 18d 67* 627 137 45.6 1419 44.2 80 ° A+29d 76* 675 145 46.8 1442 46.9 SE 3 21 5.2 0.8 55 0.8

aDays from 50% anthesis (A). *Significantly different from nil lodging.

c m ) s u s p e n d e d i n i t i a l l y 15 c m above t h e g r o u n d a n d t h e n m o v e d to r e m a i n a t

a b o u t 30 c m be low t h e t op of t h e crop. T h e m e s h was eas i ly r e m o v e d , e i t h e r

j u s t p r i o r to a r t i f i c i a l lodg ing , or a t m a t u r i t y o n t o t a l l y p r o t e c t e d t r e a t m e n t s .

Al l a r t i f i c i a l l odg ing was b r o u g h t a b o u t b y f l o o d i n g t h e e x p e r i m e n t , t h e n g e n t l y

p u s h i n g al l c u l m s i n t h e p l o t to t h e s a m e spec i f i ed a n g l e ( to t h e v e r t i c a l ) w i t h

249

TABLE 3

The effect of lodging on final lodging score, grain yield and its components a t Griff i th in 1985

Variety/ Lodging date Lodging Grain Kernel Kernel Total dry Harvest Initial score @ dry wt. number dry wt. index lodging maturity (g m -2 ) (100 m -2 ) wt. (g m -2 ) (To) angle (mg)

Experiment 3" Yecora nil - - 0 786 188 41.9 1776 44.1 natural A + 14d 35* 670* 187 35.9* 1612 39.7 45* A + 6 d 50* 611 ' 175 34.7* 1567 36.1" 80 ° A - 9 d 38* 700* 181 38.8 1759 41.4 80 ° A + 6 d 66* 558* 177 31.8" 1512" 35.4* 80 ° A + 2 1 d 94* 573* 182 31.5" 1497" 37.9* 80 ° A + 3 5 d 89* 717 185 38.8 1691 42.4 Egret nil - - 0 740 225 32.9 1962 37.0 80 ° A + l d 57* 489* 151" 32.5 1643" 30.8* SE 4 27 13 1.3 88 1.7

Experiment 4 Osprey 1. nil - - 5 891 260 34.3 2392 37.3 2. na tura l A - 8 0 d 88* 565* 178" 31.7 2070* 26.9* 3. na tura l A - 80d 80* 558* 203* 27.3* 1975" 28.8* 4. natural A + l d 93* 485* 174" 28.4* 1816" 27.0* SE 3 67 20 2.0 103 2.8

aGrain yield, kernel number and kernel weight are means from the hand and machine harvests. *Significantly different from nil lodging.

a wooden stake dragged horizontally at mid crop height down the length of the plot.

A high level of crop management was adopted in each experiment in order to reach maximum yields. Depending on experiment, seeding rate was 100-150 kg ha - 1 and row spacing 18-30 cm; plant populations were high, ranging from 150 to 210 m-2. From 25 to 35 kg/ha P was applied at sowing. The total amount of nitrogen applied, mostly at sowing, was 200 kg N ha -1 (CIANO) and 150 kg N ha - 1 ( Griffith ); the CIANO crops followed wheat the previous year, those at Griffith followed an 18-month period of cultivated fallow. Irrigations were scheduled as recommended and weed and disease control was satisfactory, with the exception of Experiment 1 when moderate levels of stripe rust (Puccinia striiformis) arose on Yecora 70 and Cajeme 71.

Lodging was scored as follows:

lodging score = % of plot area lodged X angle of lodging/90

250

The lodging angle refers to the angle from the vertical of a line from the plant base to the spike and was included in the score in order to represent, albeit imperfectly, the degree to which the crop canopy was compressed by lodging. Lodging score therefore has a range from 0 to 100 (whole plot completely flat). Since some plots partially recovered from artificial lodging and others lodged further due to natural lodging, all plots were scored repeatedly and at maturity. Other measurements in all experiments included estimation of the date of 50% anthesis, and quadrat harvests (1.2 m 2 plot-1, avoiding edge rows) at physio- logical maturity for the determination of grain yield, biomass, kernel weight and numerical components of yield; kernel number and harvest index were calculated. Mature plant height (to tip of median spike) was also measured. In Experiments 3 and 4, quadrat harvests (0.6 m 2 plot- 1 ) were taken during the season to monitor the effect of lodging on crop growth rate in some treat- ments. All weights refer to oven-dry (70 °C ) unless otherwise mentioned. Ex- periment 3 was harvested by machine on 9 December, five days after the final hand-harvest. Because substantial rain fell between physiological maturity and machine harvest of plots at Griffith (51 mm between 25 November and 7 De- cember), and effects of the degree of lodging on grain sprouting might be ex- pected, this was assessed in grain samples from Experiment 3. Sprouted grains (radicle emerged) were counted and a Falling Number detemination (Inter- national Cereal Chemists Specification # 107, 15% moisture basis) was per- formed on ground grain as an indicator of amylase activity. In all experiments, either grain nitrogen content was measured by Kjeldahl analysis or grain pro- tein by near-infra-red reflectance. Where appropriate, stage of development on the decimal scale of Zadoks et al. (1974) is given.

RESULTS

CIANO experiments

In Experiment 1 at CIANO, 50% anthesis (A) was reached on 22 February ( Yecora 70 and Cocorit 71 ) and on 5 March ( Cajeme 71 ). Although the stripe- rust score ( % foliage affected) at four weeks after anthesis averaged 47% (Ye- cora 70) and 34% (Cajeme 71) the score was not significantly affected by lodging treatment, and rust probably only decreased grain yield slightly (co- variance analysis suggested 2 g m-2/% ). Therefore stripe rust is assumed not to confound results for these varieties.

Most lodging treatments significantly reduced yield in Experiment 1 (Table 2 ), the latest lodging (at 26 days after anthesis) and less severe early lodging ( 45 ° ) having least effect. There is some evidence that yield of Cajeme 71 was less affected by lodging. Lodging reduced numbers and weights of kernels, total dry weight and harvest index. Not shown in Table 2 is spike number per m 2, which was not significantly affected by lodging (overall mean, 441 m-2) , chaff

251

dry weight which was significantly reduced by lodging at anthesis, and plant height which was 80, 90 and 100 cm for Yecora 70, Cajeme 71 and Cocorit 71, respectively, in unlodged plots. Also, grain N% was significantly increased by 80 ° lodging at anthesis and 2 weeks later (2.36% and 2.31%, respectively, ver- sus 2.15% for the control).

In contrast to the previous year, lodging treatments in Experiment 2 had little effect ( < 10% ) on grain yield of Yecora 70 (Table 2 ) ; no individual treat- ment had a statistically significant effect although the mean grain yield of the three severe (80 ° ) lodging treatments was 7% lower than that of the other three treatments. Other components of yield were unaffected by lodging (mean spike number, 419 m-2) . The unlodged plant height was 70 cm, somewhat lower than the previous season; control kernel number was also clearly less and kernel weight greater. Fifty percent anthesis was reached on 14 March.

Griffith experiments

In Experiment 3, anthesis occurred on 10 October (Yecora 70) and 15 Oc- tober (Egret). Lodging 1 to 21 days after anthesis caused large reductions in grain yield (Table 3 ) ; in Yecora 70 natural lodging (which commenced at A + 14 days) and lodging at 9 days before anthesis had smaller but statistically sig- nificant effects, while lodging at 35 days after anthesis had a smaller (9%), non-significant, effect. With Yecora 70, yield effects largely arose from changes in kernel weight, whereas with Egret kernel number was affected. Both total dry weight and harvest index were depressed by lodging but spike number (mean 623 m -2 for Yecora) was unaffected. The lodging score at maturity for pre- anthesis lodging was lower than for later lodging, primarily because this crop was standing vertical (but about 10 cm shorter than the control) within 4 days of lodging, due to culm bending at lower stem nodes; it lodged again naturally at 20-30 days after anthesis. The mature plant height of unlodged crops was 89 cm (Yecora 70) and 105 cm (Egret).

The percentage of sprouted grain, normally zero, was high in all samples from Experiment 3 (Table 4), especially in lodged crops. Also, lodging reduced Falling Number in Yecora 70 (Table 4), but all samples were well below the lower limit of 300 set by the trade for good bread-making quality. Post-anthesis lodging significantly increased grain protein content ( Table 4 ).

Crop sampling in Experiment 3 showed that 80 ° lodging at 6 days after an- thesis significantly reduced crop growth rate over the ensuing 14 days (9.1 vs 22.5 g m -2 day- l ) , but lodging at 9 days before anthesis had no effect, and lodging to the same degree at 34 days had a less-marked effect (Fig. 1 ). Kernel weight with A + 6 days lodging was little affected at 14 days after lodging (14.4 versus 16.8 mg, Fig. 1 ) but more markedly reduced at maturity.

In Experiment 4, the early planting date and semiwinter habit of the variety Osprey led to a large amount of vegetative growth, heavy tillering, and early

252

TABLE 4

The effect of lodging on grain quality characteristics, Experiment 3, Griffith, 1985

Variety Lodging % sprouted Falling Protein grains ~ number ( % )

angle date

Yecora

Egret

nil 3.8 83 13.9 natural A + 14d 10.8" - - - - 45 A + 6d 14.3" - - 80 A - 9d 12.5" 70* 13.8 80 A + 6d 17.0* 69* 14.4 80 A + 2 1 d 20.5* 62* 14.9" 80 A + 35d 14.5* 63* 13.7 SE 1.2 0.2

nil 4.3 292 11.2 80 A + ld 2.5 271 12.3" SE 11 0.2

*Significantly different from nil lodging at P < 0.05. ~Transformed before statistical analysis.

natural lodging from bending of the pseudostem (Fig. 2). With treatment 2 this arose on July 15, just before Zadoks stage 30, and some 80 days before 50% anthesis which was reached on 4-5 October (6 October for treatment 4). After July 15 there was a slight recovery from lodging in treatment 2, until a rain- storm on September 11 when lodging became severe again. Growth regulators ( treatment 3 ), which were not applied until after the onset of lodging, had a

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Zadok s s c o r e

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3 1 . 4 3 9 6 5 72 I i i I I I i A U G SEPT OCT

8 0 83 8 9 I I

NOV

Fig. 1. Above-ground total dry weight and kernel weight changes for: the unlodged crop ( • ) ; for crops lodged artificially to an angle of 80 ° to the vertical, at 9 days before anthesis ([]) ; 6 days after ( ~ ); and 35 days after ( v ); Yecora 70, Griffith 1985, Experiment 3.

Fig. 2. Lodging score changes with time for: lodging-protected ( • ); unprotected crops ( o control; D plus growth regulator (S); and ~ mown (M) ); development score (Zadoks scale) for protected crop shown along axis; Osprey, Griffith 1985, Experiment 4.

253

2500

E 2000

v

1500

lo00

~_ $00

0

I

June July Aug Sept Oct Nov

Fig. 3. Above-ground total dry-weight changes with time for the same crops shown in Fig. 2; Osprey, Griffith 1985, Experiment 4; lines fitted by hand, standard errors shown.

small ameliorating effect on lodging, and mowing delayed but did not eliminate lodging (Fig. 2). Despite the earliness of natural lodging, steel mesh (treat- ment 1 ) again effectively protected the crop against lodging. Mature plant height for unlodged crop was 107 cm (treatment 1 ), 91 cm (treatment 3) and 99 cm (treatment 4).

Grain yield was reduced 37% by natural lodging in Experiment 4 (Table 3); growth-regulator application did not affect this reduction, while mowing led to a somewhat larger reduction in yield (45%). Yield reductions were almost entirely due to reduced kernel number per m 2. Spike number was not signifi- cantly affected (mean, 609 m -2) ; the number of kernels per spike was signif- icantly lower with treatment 4 (mowing). Total dry matter, harvest index and chaff dry weight were depressed by treatments 2, 3 and 4. Grain nitrogen was significantly higher in lodged treatments (mean, 2.73% ) than in the control (2.39%).

From visual observations, it was estimated that all crops in Experiment 4 reached full ground cover ( > 90% ) by about June 20. Mowing of treatment 4 on July 3 removed 180 g m -2 of dry material (Fig. 3); ground cover dropped to 30%, but by August 1 full ground cover had again been attained. Growth analysis did not reveal large effects of treatment on crop growth rate at the early October sampling, but significant reductions with all lodged crops did arise in the final growth interval (Fig. 3 ).

DISCUSSION

One reason for our studies was that previous reports had only dealt with moderate or low-yielding cultivars and environments. There is no doubt that our yields in the absence of lodging were high and representative of modern spring-type cultivars and high-input irrigated agriculture: the average yield of unlodged crops across all experiments was 8.2 t ha-1 (at 10% grain moisture).

254

c 100 O o "o

90 K~ -0 _o "- 80

N 70

N 60 • -~ - 2 0 >-

~, Experiment 2 /

I I I I I I

A + 20 + 4 0

Time of lodging (days f rom anthesis)

1.3

Fig. 4. The yield of artificially (80 ° ) lodged crops of Yecora 70 as a percentage of unlodged yield and a function of t iming of lodging: Experiment 1 ( o ) and 2 ( ~ ) at CIANO, Mexico; Experiment 3 ( v ) at Griffith, Australia; lines fitted by hand.

Grain yield responses to lodging

Yield reductions almost always accompanied lodging, with the magnitude of the reduction depending on timing and, to a lesser extent, season, variety and degree of lodging. Artificial 80 ° lodging for Yecora 70 led to greatest yield re- ductions of 20-30% (10% in 1974-75) in the period from anthesis to approx- imately A + 20 (Fig. 4), as reported earlier (Day, 1957; Weibel and Pendleton, 1964). The smaller yield reductions with lodging before anthesis may be re- lated to the ability for the lodged crop to stand up again. Smaller yield reduc- tions with late lodging presumably reflect partial escape of the grain-filling process from any deleterious effects of lodging. Nevertheless, even the latest artificial lodging (A + 35 at Griffith, Experiment 3), when the grain was at Zadoks stage 84 (85--soft dough) and was 83% of its final size in the absence of lodging, caused yield and kernel weight reductions of 9 and 7%, respectively, in both cases significant at the 10% level. Grain-filling duration depends on temperature; it lasted until about 40 days after anthesis for Yecora 70 at Grif- fith (Fig. 1 ) when the mean temperature was 17.5 ° C. A similar duration would be expected in Experiments 1 (mean temp. 17.4 ° C ) and 2 (mean temp. 16.8 ° C ) so that the reduced kernel weight responses to late lodging with Yecora 70 in these experiments could not have been due to a faster completion of grain filling.

The markedly reduced effect seen with artificial lodging in 1974-75 (Exper- iment 2, Fig. 4) is interesting because the weather in this season was little different from that in 1973-74 (Table 1 ). However, the control crop was un- usual relative to crops of Yecora 70 in adjacent experiments, and in the pre- vious season (Table 2 ), in that kernel number was low and kernel weight very high, the latter suggestive of an ample post-anthesis assimilate supply per grain - - a point which will be reexamined later. It is suspected that an inadvertent delay in irrigation just prior to anthesis caused this reduction in kernel number.

255

The initial degree of lodging is another factor which clearly affected the yield response to lodging: across the six paired comparisons, 45 ° lodging reduced yield by 9% while 80 ° lodging reduced it by 17%, significantly more. The ma- turity lodging score confirmed that the 45 ° artificial lodging treatment re- mained less lodged.

The yield responses to natural lodging (studied in 1985 only) are difficult to interpret because the lodging in any one plot often occurred on several oc- casions with steadily increasing degree of severity. Nevertheless, yield reduc- tions were substantial, i.e. 15% in Yecora 70, a variety with two major dwarfing genes, and 37% in Osprey, a single-gene dwarf, and amounted to the loss of 1.3 and 3.7 t ha -1 of grain (10% moisture), respectively. Also, the loss of yield is likely to be greater when normal field-harvesting procedures are used, for our harvesting gathered all lodged spikes. Since such natural lodging has com- monly been observed in farmer fields, it clearly represents a major inefficiency.

Amelioration of lodging was not a major objective of this study, but the rel- evant treatments in Experiment 4 deserve some comment. Growth regulator, while applied at the recommended stages and being effective in reducing plant height, did not sufficiently reduce lodging nor lead to independent advantages so as to lessen the yield loss; lodging in fact commenced before the first appli- cation of regulator. Even under less-extreme conditions than those of Experi- ment 4, we have found growth regulator applications similar to those recommended in Europe to be of little value in preventing lodging ( M. Stapper, unpublished data, 1987). Mowing in Experiment 4, by delaying the onset of lodging until anthesis (Fig. 2 }, may have caused the final lodging to be in fact more severe. The somewhat greater yield reduction with this treatment could simply reflect this, or may be due to other deleterious effects of mowing. Mow- ing did not, however, directly damage shoot apices, delay anthesis date, or markedly reduce crop growth rate at the critical time just before anthesis. Clearly other approaches to lodging prevention, both agronomic and genetic, need to be pursued.

Cause of the yield response to lodging

It was earlier suggested that the physiological explanation of lodging effects on yield lie in its effects on crop growth rate. De Wit (cited in Van Dobben, 1966) calculated that the less-favourable distribution of light amongst leaves following lodging could reduce post-heading growth by 200 g m -2 in the Neth- erlands; Hanson et al. (1985) measured such reductions in spring barley. This is not contradicted by the fact that lodging at 9 days before anthesis in Exper- iment 3 showed no reduction in crop growth rate since this crop righted itself quickly. Similarly, growth before anthesis was not affected in severely early- lodged treatments 2 and 3 of Experiment 4 (Fig. 3), as there was a degree of re-erection of the upper part of culms with such early lodging, so that even

256

4 0 0

~ E .= % 300

.E '5 200

g ]

0

1:1 O /

~Y

:1

A/% X

i i = i 1 0 1 O0 2 0 0 3 0 0 4 0 0

Reduction in total dr, /weight with l o d g i n g ( g / m z)

Fig. 5. The absolute decrease in grain yield as a function of the decrease in final total dry weight resulting from all lodging treatments: Experiment 1 ([] Yecora 70, • Cajeme 71, ~ Cocorit 71); Experiment 2 (• Yecora 70 ); Experiment 3 ( <> Yecora 70, v Egret); and Experiment 4 ( o Osprey).

though the lodging score was high (Fig. 2) and lower leaves heavily shaded, the more important upper leaves could have been displayed more normally. Crop growth analysis here (Figs. 1 and 3) confirms that permanent lodging, at least after anthesis, had a clear depressive effect on crop growth rate (as distinct from effects on duration of growth, which could conceivably have caused differences in final total dry weight).

In our studies, lodging almost always reduced final total dry weight, these reductions being equal to or more than the reductions in grain yield (Fig. 5 ). The points for Experiment 2 and for Cajeme 71 in Experiment 1, both crops with reduced-yield response to lodging, fall well below those of other experi- ments in Figure 5. This supports our earlier suggestion that yield in Experi- ment 2 in particular was not greatly limited by photoassimilate production; final total dry weight was not so markedly affected either, perhaps because of the greater degree of sink limitation. The ratio of the change in grain yield to that in total dry weight with post-anthesis treatments, such as shading, af- fecting assimilation, has been used as an index of source limitation for grain filling (Fischer, 1975). That this ratio was about 0.45 for Yecora in Experi- ment 2 is further evidence of low source limitation ( = high sink limitation) as suggested earlier for that experiment; more normally the ratio is 0.8 for Yecora 70 (Fischer, 1975), as seen in Experiments 1 and 3 (Fig. 5).

It has been argued that kernel number per m 2 within a cultivar is largely determined by assimilate supply up to anthesis (Fischer, 1985) whereas in well-watered crops kernel weight is largely the product of post-anthesis photo- assimilate production (e.g. Bidinger et al., 1977). Kernel-number reductions were greater with lodging commencing at or before anthesis, while kernel-weight reductions were greater with later lodging (Tables 2 and 3). This pattern sup- ports the idea that the reductions in kernel number and weight with lodging

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were due to reductions in photoassimilate supply. The consistent increase in grain nitrogen percentage whenever kernel weight was decreased by lodging is also suggestive of overall photoassimilate shortage, and is against the notion of direct impairment of translocation, which also seems unlikely because stems did not buckle.

Whilst a reduction in the overall photoassimilate supply with lodging ap- pears to offer a sensible explanation of our results, two other mechanisms of yield depression are possibly involved. An uneven distribution of light in an almost horizontally lodged canopy between uppermost culms, probably having too much potential assimilation, and lowermost ones having far too little, may aggravate the situation; certainly the latter culms take on an abnormal senes- cent appearance after several days under such conditions. Also, kernel number could be directly reduced by the microclimate of the lodged canopy, as well as indirectly via photoassimilate supply. In such a case, Fischer (1985) has sug- gested that there are depressions in kernel number per unit of spike weight present at anthesis, or, less precisely, kernels per unit of chaff weight at ma- turity. Some evidence for this is seen in those of our crops in which kernel number per m 2 was significantly reduced by lodging before or at anthesis (Egret in Experiment 3; all lodged treatments in experiment 4): for these two cases kernels per unit chaff weight fell 21%, while chaffweight was reduced only 9%. This is suggestive of a reduction in grain set, which could have been due to elevated humidity in the lodged canopy since high humidity is known to reduce grain set in wheat (I.A. Dawson, CSIRO Division of Plant Industry, unpub- lished data, 1986). Increased grain sprouting with Yecora 70 at Griffith also presumably reflects higher humidity around the lodged spikes after the fre- quent rains at maturity.

ACKNOWLEDGEMENTS

We wish to thank Dr R. Maurer and Messrs G. Howe and J. Cook for able technical assistance, and Mr A. Ross of the CSIRO Wheat Research Unit for doing the Falling Number and grain protein determinations in Experiment 3.

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