Euphytica 111: 1–8, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Genetic variation in sugarcane for ratooning after harvester damage inwet soil

Phillip Jackson1, Mike Braunack2, John Foreman1 & Thomas Peatey31CSIRO Tropical Agriculture, Davies Laboratory, Private Mail Bag, P.O. Aitkenvale, Qld, 4814, Australia;2Bureau of Sugar Experiment Stations, P.O. Box 566, Tully, Qld. 4854, Australia;3CSR Ltd., Macknade, Qld.4850, Australia

Received 17 February 1998; accepted 12 March 1999

Key words:mechanical harvesting, ratooning, selection, sugarcane, waterlogging

Summary

Genetic variation in ratoon growth and cane yield after mechanical harvesting under wet conditions was examinedamong 26 sugarcane clones of diverse genetic background, including materials closely related toSaccharumspontaneumand some commercial cultivars. The clones were evaluated under a control treatment (harvestingunder dry conditions and allowing the cane to ratoon without further disturbance), and a traffic treatment (sprayirrigation and then driving heavy field equipment over the cane stubble immediately after harvesting). Averagedover all clones, the traffic treatment had a large adverse effect on early growth and final ratoon yield. There wassignificant genotype× treatment interaction for early growth and canopy development, but not for final cane yield.There was also a high genetic correlation between genetic performance under the different treatments, indicatingthat selection for ratooning after dry harvesting conditions would be also effective for improving performance afterwet harvesting conditions. A sugarcane breeding program specifically targeting better ratoon performance underwet conditions would be difficult to justify.

Abbreviations:GE – genotype by environment; P – plant; 1R – first ratoon; 2R – second ratoon

Introduction

Following mechanical harvesting of sugarcane underwet conditions, growth and yield in the ratoon cropis often poor. The relative contribution of differentfactors are poorly quantified. However, direct mech-anical damage of the stool (Torres & Villegas, 1993)and soil compaction (Braunack et al. 1993; Swin-ford & Boevey, 1984) are probably major contributingfactors. Under very wet conditions, waterlogging ofthe cut stool may also be involved. Although harvest-ing under wet conditions often adversely affects theratoon crop, the benefits of maintaining a continuouscane supply to sugar mills often mean that a significantproportion of cane is harvested under wet conditions.This is obviously most common in regions such asnorth-east Australia that have wet climates.

Sugarcane growers often report that varieties dif-fer in response to harvesting under wet conditions.However, there are no published studies document-ing the magnitude of differences among sugarcaneclones. In addition to perceived differences amongpresently grown commercial varieties, the noble canes(Sacharum officinarum) grown commercially earlierthis century have been observed to ratoon more poorlyafter mechanical harvesting, especially after wet con-ditions, than current cultivars (which contain compon-ents ofS. officinarumand S. spontaneumgenomes).Further, someS. spontaneumclones are believed tohave very vigorous ratoon growth, including under ad-verse conditions (Berding & Roach, 1987). It has beensuggested that some of these genetic materials couldform the basis for breeding programs aimed specific-ally at developing sugarcane varieties with superiorratooning after wet harvesting.

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Table 1. List of clones used and their genetic background

Type/genetic background Clones

F1 (Erianthus sp.) LF65-3660, MQ88–1802

F1 (S. spontaneum) × F1 (S. spontaneum) MQ88-981, MQ88–1068, MQ88-833, MQ88-841, MQ88–1084,

MQ88-920, MQ88–1160, MQ88-859, MQ88-846, MQ88-808

Commercial× F1 (S.spontaneum) MQ84–29B, MQ84–19B, MQ79–141, BN78-8031

Commercial NCo-310, Triton, Q124, Q115, Q117, Q138

‘Commercial’ refers to a sugarcane clone that has been commercially grown; F1 refers to a cross between aS.officinarumclone and another species (specified) or between a commercially grown clone and another species(specified).

Given the economic importance of poor ratooningafter wet harvesting conditions in sugarcane indus-tries, and the possibility that large genetic variationto this constraint may exist, research was commencedto determine if selection and breeding for improv-ing performance under these conditions would be anattractive investment. The specific objectives of theresearch reported here were to:i. Determine the extent of genetic variation in re-

sponse for ratooning after wet harvesting condi-tions among sugarcane clones of diverse geneticorigins.

ii. Identify appropriate methods for selecting cloneswith superior ratooning under wet harvesting con-ditions, that may be used in sugarcane breedingprograms.

Materials and methods

Genetic material

Twenty-six clones (Table 1) were evaluated. Twentyof these were experimental clones derived from previ-ous introgression breeding programs in the sugarcanebreeding program conducted by CSR Ltd at Mack-nade, Australia (Jackson & Roach, 1992) and six werecommercially grown varieties. These clones were con-sidered typical of that which could be used as thebasis of a breeding program aimed at developing vari-eties with superior ratooning under adverse harvestingconditions. Features of such a sample include cloneswith a large component ofS. spontaneum, and severalcommercially grown varieties with favourable per-formance for important agronomic characters such assucrose content.

Experimental design

Two experiments (termed M1 and M2) were grownat Macknade experiment station, North Queensland,Australia (lat. 18.7◦S, long. 146◦E) on an alluvialsoil, classified locally as a Macknade series (Wilson& Baker, 1990). Each experiment was conducted ina split-plot design with two replications. The post-harvest treatments were applied to the whole plots andall 26 clones were grown as sub-plots within eachwhole plot. The subplots were arranged randomlywithin each whole plot. The subplots consisted ofthree 6m rows with a 1m gap between every secondplot.

Each experiment was grown up to the second ra-toon crop. Post harvest treatments were imposed afterharvest of the plant crop and the first ratoon crop inthe M1 experiment and after harvest of the plant cropin the M2 experiment. The times of planting and har-vest of each experiment are given in Table 2. Apartfrom the post-harvest treatments, cultivation and fer-tiliser application followed conventional commercialpractices carried out in the surrounding region.

Post-harvest treatments

The traffic treatment was designed with the aim ofevenly applying stool damage similar to that experi-enced during commercial mechanical harvesting underwet conditions. First, the crop was harvested un-der dry conditions with a commercial harvester andhauling out equipment. The trial was immediatelyspray irrigated with approximately 50mm water, withsprinklers arranged to achieve an even ground cover-age. Within 24 hours, a four wheel drive tractor andtrailer were driven along the rows to simulate damagefrom harvesting equipment under wet conditions. Forthe first ratoon crop, the trailer used was a single axle,dual wheel rollon-rolloff haulout bin with dual tyres

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Table 2. Planting and harvesting dates of each experiment and treatments imposed

Experiment Planting date Harvest date(s)1 post-harvest

treatments applied

M1 12 Aug 1993 4 Oct 1994, 13 Jul 1995, 18 Oct 1996 control, traffic, waterlogged

M2 10 Aug 1993 7 Sep 1994, 11 Jul 1995, 22 Aug 1996 control, traffic

1Dates are given for plant, first ratoon, and second ratoon crops, in order.

(0.85 m diameter) laden with a 1.5 tonne weight. In thesecond ratoon crop, high flotation equipment was usedwith a four tonne load. In imposing the impact, theleft hand wheel of the tractor and trailer were drivenalong the top of each row, with this being done foreach individual row. In this way, each row receiveda direct impact (from the left wheels) and an impactslightly to the side (from the right hand wheels).

Changes in soil physical properties in each trialbetween plant and ratoon crops following the harvest-ing treatments in each trial were detailed by Braunack& Peatey (1997). As expected, the traffic impactsresulted in significant increases in soil bulk densities.

The waterlogging treatment involved all actionsdescribed for the traffic treatment, plus continued ir-rigation following the impact from the tractor andtrailer. Irrigation was continued for one week follow-ing harvest, and about 20 mm was applied twice perday, such that free water was always visible in puddleson the ground surface. Prior to establishment of thetrial, the ground used was laser levelled to obtain azero slope so that waterlogging occurred evenly acrossall plots.

The control treatment involved harvesting underdry conditions and then irrigating, in exactly the sameway as applied for the traffic impact. However, unlikethe previous two conditions, the crop was then allowedto ratoon without further interference.

Measurements

Stalk number (m−2) was measured in each plot bycounts of stalks along a four metre section of each oftwo rows, including one middle row. Stalk counts weremade in the ratoon crops at approximately one monthand three months after harvest and then about fourmonths prior to harvesting. For the first two counts,all tillers were counted, regardless of size. For the lastcount, the aim was to obtain an estimate of final stalknumbers, and stalks less than 1 m high were excludedsince these would be either dead at harvest or wouldcontribute little to final yield.

Canopy light interception (%) was measured us-ing a Licor linear light interception probe (Licor inc.,Lincoln, Nebraska). Measurements were made in theratoon crops at approximately three months after har-vest. The relative canopy light interception in eachplot was estimated on a clear day between 10.00 amand 2.00 pm from the mean of six measurementsmade from ground level at six randomly chosen pointswithin two rows. These measurements were comparedwith readings taken in full sun within 10 minutes, andthe light intercepted by the canopy calculated fromthe mean of the six measurements divided by the fullsunlight measurement.

Stalk weight (kg/stalk) was determined fromweights of six or eight stalks, sampled at random fromtwo rows at final harvest.

Cane yield (t/ha) was calculated from the last countof stalk number× stalk weight at final harvest× 10.

Data analyses

Cane yield was firstly analysed within each of thetreatments in each experiment, in each ratoon crop,using a model partitioning variation due to blocks andgenotypes. Both genotypes and blocks were regardedas random effects. Heritability (in the broad sense) foreach treatment in each experiment was calculated fromthe ratio of genetic variance to phenotypic variance.Phenotypic variance (σp

2) was determined from:

σ 2p = σ 2

g + σ 2e /n

whereσ g2 = genetic variance,σ e

2 = error variance,and n = number of replicates within the trial. Broadsense heritability (H) was determined from:

H = σ 2g /σ

2p .

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Figure 1. Stalk numbers, canopy light interception, and cane yield in first ratoon (1R) and second ratoon (2R) crops in each treatment inexperiments M1 and M2. C, T and W refer to the control, traffic and waterlogged treatments, respectively.

Analyses of variance was done using the followingmodel:

yijm = µ+ tj + bmj + gi + (gt)ij + (gb)ijm

where yijm = observed yield of theith genotype inthe jth trial in themth block,µ = mean of all observa-tions, tj = effect of thejth post-harvest treatment, j = 1,2 or 3, bmj = effect of themth replicate within thejthpost-harvest treatment, m = 1, 2 (error 1), gi = effect ofthe ith genotype, i = 1 to 26, (gt)ij = interaction effectbetween theith genotype and thejth post-harvest treat-ment, and (gb)ijm = interaction effect between theith

genotype and themth block within thejth post-harvesttreatment (error 2).

In deriving expectations for mean squares, post-harvest treatments were defined as fixed effects, andgenotypes and post-harvest treatment replicates asrandom effects.

Genetic correlation (rg) between performance un-der two different treatments were determined from:

rg = rp/(H1.H2)0.5,

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Table 3. Genotypic, genotype× treatment, and error variance components (±standard errors), forstalk number (SN, per m2) and canopy light interception (LI)

Exp. Crop Attribute Genotypic G× treatment Error

variance variance variance

component component component

M1 1R SN – 1 month 192±52.9∗∗ 11.1±8.17∗ 15.8±2.54

SN – 3 months 195.1±54.1∗∗ 5.13±5.61∗ 23.2±3.73

LI – 3 months 0.0136±0.0041∗∗ 0.0041±0.0033∗∗ 0.0089±0.0014

2R SN – 1 month 19.9±5.82∗∗ –0.64±0.86 8.84±1.42

SN – 3 months 18.9±5.81∗∗ –1.02±1.44 14.6±2.35

LI – 3 months 0.0023±0.0013∗∗ –0.0002±0.0016 0.014±0.0023

M2 1R SN – 1 month 27.2±11.71∗∗ 4.7±5.10∗ 8.80±1.72

SN – 3 months 26.4±11.42∗∗ –0.81±1.09 9.32±1.82

LI – 3 months 0.0046±0.0022∗∗ 0.0010±0.0014 0.0044±0.0009

2R SN – 1 month 15.01±6.55∗∗ 2.59±3.01∗ 6.16±1.20

SN – 3 months 39.5±16.79∗∗ 1.08±2.40 9.75±1.91

LI – 3 months 0.0021±0.0013∗∗ 0.0000±0.0010 0.0058±0.0014

where rp= phenotypic correlation between geno-type means in the two treatments, and H1 and H2 arethe broad sense heritabilities in the two treatments.

Results

Post harvest effects

Mean stalk numbers for each post-harvest treatmentare shown in Figures 1a and 1b. In the first ratoon crop,there were more stalks in the M1 experiment than inthe M2 (Figure 1a). This would at least in part be dueto the later harvest date (and warmer weather) in M1experiment (Table 2). In both the M1 and M2 exper-iments, stalk numbers at about one month after theplant crop harvest were significantly less (p <0.05)in traffic treatment compared with control (Figure 1a).In the M1 experiment, the waterlogged treatment alsohad lower stalk numbers at one month after harvest.By 3 months after harvest, stalk numbers across thedifferent treatments in both experiments were sim-ilar (Figure 1a). However, canopy light interceptionat around this time remained significantly different,with the traffic and waterlogged treatments havingless developed canopies than the control treatments(Figure 1c). This difference was most marked in M2the experiment, where canopy light interception inthe control treatment was nearly double that of thetraffic treatment. These results indicate that while stalknumbers had become similar in the traffic and water-

logging treatments three months after harvest, stalkswere smaller, in terms of light intercepted per stalk.This smaller stalk size was also obvious from visualobservation.

Final cane yields in the first ratoon crop were alsosignificantly less in the traffic and waterlogged treat-ments in both experiments compared with the controltreatments (Figure 1e).

In the second ratoon crop, there was no effect of thetraffic treatment in the M1 experiment on stalk num-bers (Figure 1b) but a significant effect on canopy lightinterception was clear (Figure 1d). For the M2 experi-ment (where different post-harvest treatments were notimposed after harvest of the first ratoon crop) therewas no difference in stalk numbers (Figure 1b), buta small difference remained for canopy light intercep-tion (Figure 1d). For final cane yields in the secondratoon crop, the traffic and waterlogged treatmentswere significantly less (p <0.05) than the controltreatments for both experiments (Figure 1f). However,the difference in the M2 experiment was not as large asin the first ratoon crop, presumably because the differ-ent treatments had not been re-imposed after the firstratoon harvest.

Genetic variation

Within both experiments, there was significant vari-ation due to genotypes and genotype× treatmentinteraction for stalk numbers in the first ratoon cropat about 1 month after harvesting (Table 3). However,

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Table 4. Genetic variance component, error coefficient of vari-ation (cv,%), and broad sense heritability for cane yield (t/ha) inthe ratoon crops for each treatment within each experiment

Experiment & Genetic variance cv Heritability

treatment

1st ratoon crop

M1-C 172.0∗∗ 19.6 0.51

M1-T 98.0∗∗ 21.0 0.49

M1-W 150.5∗∗ 16.7 0.78

M2-C 107.1∗∗ 15.6 0.58

M2-T 77.2∗∗ 17.7 0.64

2nd ratoon crop

M1-C –14.1 31.9 0.0

M1-T 124.2∗ 30.3 0.35

M1-W 251.1∗∗ 21.3 0.71

M2-C 289.5∗∗ 18.3 0.69

M2-T 113.5∗∗ 25.9 0.42

the variance component for genotype× treatment in-teraction was much smaller than for genotypes. Atthree months, variation due to genotype× treatmentinteraction remained significant for stalk number forthe M1 experiment, but not for the M2 experiment. Atthis time, significant genotype× treatment interactionexisted for canopy light interaction in the M1 experi-ment but not for the M2 experiment, despite the largetreatment main effects in the latter (Figure 1c).

In the second ratoon crop, there was no significantgenotype× treatment interaction in the M1 experi-ment for stalk number or light interception (Table 3).Significant genotype× treatment interaction occurredfor stalk number at one month after harvest in theM2 experiment, but no such interaction was evidentfor stalk number or canopy light interception at threemonths.

Genetic variance components, heritabilities, anderror coefficients of variation for cane yield at harvestin the ratoon crops for each individual treatment×crop-year combination are shown in Table 4. Therewas significant (p <0.05) variation due to genotypesin all cases except for the control treatment in thesecond ratoon crop of the M1 experiment. For thelatter case, there was very high error variance in thesecond ratoon crop (Table 4) for some unknown reas-ons, and this possibly contributed to the statisticalnon-significance of the genetic effects. Within eachexperiment and crop year, there was no clear effect oftreatment on error variances. Thus it would appear thatthe traffic or waterlogging treatments did not introduce

an additional or problematic source of experimentalerror.

For both of the M1 and M2 experiments, andin both first and second ratoon crops, variationdue to genotype main effects was highly significant(p <0.01, Table 5), as would be expected for a di-verse set of sugarcane clones. However, variation dueto genotype× treatment interaction was not signific-ant in either experiment, in either the first or secondratoon crops. This indicates that the traffic impact andwaterlogging did not cause substantially different re-sponses among the varieties to those expressed underthe control conditions. Similar results were obtainedfor both the six commercial varieties and the other ex-perimental materials when considered separately (datanot shown). For example, in the M2 experiment, thesix commercial varieties had an average cane yieldfor the traffic treatment that was 64% of that for thecontrol treatment, while the comparative figure forthe other experimental material was 67%. Clearly,the significant genotype× treatment interactions ob-served for early growth in some cases (Table 3) didnot translate to significant effects for final cane yields.

Genetic correlations between cane yield in the con-trol treatment versus cane yield in other treatmentsfor each ratoon crop of each experiment are shownin Table 6. Correlations were not estimated for thesecond ratoon crop of the M1 experiment due to thenon-significant genotype effect in the control treat-ment (Table 4). For all cases in Table 6, the geneticcorrelations between cane yield in the different treat-ments are near to 1.0, which is consistent with the lackof significant genotype x treatment interaction withineach crop.

Discussion

The lack of genotype× treatment interaction and highgenetic correlations between the different treatmentsfor cane yield was a key and unexpected finding inthis study. This occurred despite the large detrimentaleffects of the traffic treatments on ratoon growth gen-erally. This result indicates that genetic variation forresponse to specific stresses of mechanical harvestingunder wet conditions is limited, compared with con-siderably larger genetic variation for growth and yieldmore generally. The results also indicate that indirectselection for ratooning performance after dry har-vesting conditions would be effective for improvingperformance after much wetter harvesting conditions

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Table 5. Genotypic and G× E interaction variance components (± standard errors) for caneyield (t/ha) within crops

Experiment Crop Genotypic variance G× treatment Error variance

component component component

M1 First ratoon 121.7±42.0∗∗ 19.3±33.6 190.0±30.6

Second ratoon 62.7±36.6∗∗ 40.6±70.4 396.4±63.8

M2 First ratoon 88.6±44.6∗∗ 7.25±24.9 118.6±23.1

Second ratoon 198.7±101.2∗∗ 14.5±57.3 282.0±55.3

Table 6. Genetic correlations (±standard errors) between cane yield in thecontrol treatment and other treatments

Experiment Crop Comparison Genetic correlation

M1 1R Control versus traffic 1.13±0.25

1R Control versus waterlogged 0.89±0.17

M2 1R Control versus traffic 0.98±0.17

2R Control versus traffic 1.04±0.23

at the same site. Given that genotype× site interac-tions in sugarcane are often important (eg. Jackson &Hogarth, 1992; Mirzawan et al., 1993) selection foradaptation to sitesper seis probably the most effectivepathway to improved ratoon yields under wet harvest-ing conditions, rather than attempting to select forfavourable responses specifically to the wet harvestingconditions.

At the commencement of this research we expec-ted that some of the material closely related toS.spontaneummay have exhibited superior performanceafter the adverse harvesting treatments. Further, it hadbeen suggested that such material could form the basisof a breeding program focused to develop varietiesmore suitable than existing cultivars for mechanicalharvesting under wet conditions. However based onresults obtained in this research it would appear thatprogress in a breeding program with this specific aimwould be difficult to achieve. There is no evidence ofyield responses among these materials that were su-perior to those observed in the commercial sugarcaneclones. Therefore there is no apparent benefit of us-ing theS. spontaneummaterial as parents to improveratoon yields compared with using commercial typeclones that have more favourable responses for otheragronomic attributes such as sugar content.

Despite the absence of significant G× E interac-tion between control and traffic treatments for finalyields, there was some G× E interaction for early

tillering and canopy development. The fact that theseearly differences did not persist until final yield sug-gests that some compensatory mechanisms operatedbetween early growth and final harvest. For example,clones that did not produce as many tillers under thetraffic treatment early in their growth may have lostfewer stalks than other more profuse tillering clones,or may have produced larger stalks by final harvest.

The absence of genetic variation in cane yield inresponse to wet harvesting conditions found in thisstudy does not preclude the possibility that differencesamong some individual sugarcane cultivars may ex-ist, as is sometimes reported by sugarcane growers.Differences among commercial cultivars, where theyexist, would be important to recognise; this informa-tion is useful in assisting optimal use and harvesting ofthose cultivars. Clearly, some degree of genetic vari-ation is feasible and likely considering the diversityof genotypes that may be generated from breedingprograms. The techniques developed and tested inthe present work demonstrate an effective method forscreening varieties in trials. The traffic and water-logging treatments did not generate increased errorvariances, which suggested that the stresses imposedwere able to generated in a uniform way across theexperimental plots using the techniques employed.The techniques used were also practical in that theywere relatively easy and cheap to apply. In sugarcanebreeding programs it may be useful to use such meth-

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ods in one trial in the last stage of selection beforerelease of varieties. This would allow ratooning per-formance of varieties under wet harvesting conditionsto be evaluated against existing cultivars, and identi-fication of varieties exhibiting exceptionally good orpoor performance.

Acknowledgments

This research was conducted with funding from theSugar Research and Development Corporation andCSR Ltd. We thank Tony Palmas, Steve Guazzo andAlf Girgenti for allowing us to conduct trials on theirfarms and assisting with harvesting and application oftraffic treatments. We also thank field staff in the CSRLtd Technical Field Department for conducting muchof the field work involved.

References

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