Short communication
Effect of defoliation on garlic yield
Julio Muro
a,*, Ignacio Irigoyen
a, Carmen Lamsfus
b,
Ana FernaÂndez Militino
caDepartamento de ProduccioÂn Agraria, Universidad PuÂblica de Navarra, E-31006 Pamplona, Spain
bDepartamento de Ciencias del Medio Natural, Universidad PuÂblica de Navarra, E-31006 Pamplona, Spain
cDepartamento de EstadõÂstica e IinvestigacioÂn Operativa, Universidad PuÂblica de Navarra, E-31006 Pamplona, Spain
Accepted 8 February 2000
Abstract
The results of the four trials to determine the effects of different defoliation treatments on garlic yield, carried out in the Central Ebro Valley (Spain), are presented. Four defoliation levels of 0 (control), 33, 66 and 100% were applied at seven different developmental stages. The results demonstrate a close relationship between yield reduction and the defoliation treatment in¯icted. The higher the defoliation level, the higher the yield reduction. Defoliation imposed at the onset of bulb formation resulted in maximum yield reduction. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Allium sativum L; Hail; Yield loss; Leaf defoliation
1. Introduction
In Spain, garlic crops cover some 31 000 ha, amounting to 75% of the total area for this crop in Europe, and 4% in the world (FAOSTAT, 1998). Garlic crops are exposed to different types of defoliation, and this can be effected by natural causes such as hailstorms, wind, insect attack or diseases or by accidental causes such as the incorrect administration of herbicides or damage by farm machinery.
*Corresponding author. Tel.:34-948-169110; fax:34-948-169169. E-mail address: [email protected] (J. Muro).
These defoliation treatments may reduce the commercial yield, as has been shown with other similar crops such as onion (Muro et al., 1998a), sugar beet (Muro et al., 1998b) or potato (Cranshaw and Radcliffe, 1980). The range of these diminishing levels depends on the extent to which defoliation takes place and on the developmental stage at which the crop ®nds itself when defoliation occurs. Bisht and Agrawal (1994) developed a system, to assess the reduction in yield brought about by defoliation, during the last third of the crop cycle.
The aim of this study is to quantify the commercial yield reduction (expressed as a percentage of potential commercial yield) attributable to different defoliation levels undergone throughout the garlic crop cycle.
2. Materials and methods
Four trials were carried out in two localities (San AdriaÂn and Calahorra) in the Central Ebro Valley (Spain) during the years 1991 and 1992, the farthest distance between trials was 4 km. The climate in the region is Temperate Mediterranean. The soils wereXeric torri¯uvent, which are young, deep calcareous soils with a pH from 7.5 to 8.0 and a loamy clay texture (in the case of the San AdriaÂn trials) or a loamy sandy one (as in the case of the Calahorra trials). A Spanish cultivar ``Morado de PedronÄeras'' was used in the trials. For the 1991 trials, bulbs were planted on 31 January, while they were planted on 22 December 1991 for the 1992 trials. Density was 45 cm between rows and 10 cm between plants. Crop husbandry (irrigation, fertilisation, etc.) followed the methods normally employed for this crop in the region.
Defoliation was imposed by using shears at four levels of defoliation: 0 (control), 33, 66, and 100% of the leaf surface area. The defoliation method used was described by Muro et al. (1998a). Seven defoliation stages regularly distributed throughout the crop growth cycle were applied. These stages are described in Table 1, and appear arranged according to the number of days from planting, total number of leaves, number of green leaves, and the ratio of bulb diameter/neck diameter.
A randomly selected split-plot experimental design was established with four replications. In the main plot defoliation treatments were applied at seven different stages, and the sub-plots (four) were made up of 4 m rows with 40 plants each. Each of the four defoliation levels previously de®ned as 0 (control), 33, 66 and 100% were applied in the sub-plots.
2.1. Crop yield reduction
To obtain percentage yield reduction, i.e. the percentage by which the ®nal commercial yield of a defoliated sub-plot appears diminished in comparison with the ®nal commercial yield of a non-defoliated (control) sub-plot. For each defoliation treatment the following expression was used for each trial and developmental stage:
% yield reduction100 control yieldÿsub-plot yield control yield
For each trial and defoliation stage, yield reduction equations were calculated regressing percentage of defoliation applied (x) on percentage of yield reduction (y).
3. Results
3.1. Effect of defoliation treatments on yield
Commercial yield for each treatment is shown in Table 2. In all the trials, control yields differed by a signi®cant 66% (P<0.05) from those involving defolia-tion, while these, in their turn, differed from those involving defoliation by 100%.
Defoliation at stage 6, when relevant bulb formation begins to take place, proved to be the most sensitive, showing signi®cant differences, as compared with the initial and ®nal stages. Defoliation at stages 4 and 5 were those that followed in terms of sensitivity to defoliation according to the trials, albeit signi®cant
Table 1
Defoliation treatment stages (7): days from planting, total number of leaves, number of green leaves (non-senescent) and bulb diameter/neck diameter ratio for the 1991 and 1992 trials
Stages Trials, 1991 Trials, 1992
Days
Table 2
Commercial yield (g mÿ2) for garlic crop subjected to four defoliation levels (control, 33, 66 and
100% of leaf surface removed) at seven developmental stages (values are means of four replicates)
Stage Defoliation
Control 33% 66% 100%
Mean S.D. Mean S.D. Mean S.D. Mean S.D.
San AdriaÂn 1991
differences with these defoliation stages varied with the different trials. There were no signi®cant differences between defoliation stages 1, 2, 3 and 7 in any of the trials. One hundred percent defoliation applied at developmental stage 6 caused the greatest yield losses. In this connection, it is important to point out that the yield reduction was worked out on the basis of the reduction in the weight of the bulbs, and not on that of the number of bulbs, since plant death brought about by defoliation did not produce signi®cant differences between plots.
3.2. Yield reduction equations
For each defoliation stage and trial, yield reduction equations were calculated regressing the percentage of defoliation (x) on the percentage yield reduction (y). Linear regression equations of yax have been chosen since they have shown higher signi®cance than other (quadratic, cubic, etc.) regression equations. The value of coef®cienta, its intervals with a con®dence of 95%,r2and signi®cance of the adjustment are summarised in Table 3.
4. Discussion
Judging from these results, it should be noted that the response obtained for the crops in 1991 and 1992 was slightly different, for the 1992 trials showed more sensitivity to defoliation. The year 1992 was characterised by a cold winter, with a mean temperature 1.48C lower than in a standard winter, and by a cold month of
Table 3
Values of the coef®cienta, and its intervals (a) with 95% con®dence in the linear regression equationsyax(xpercentage of defoliation;ypercentage yield reduction) and the signi®cance of the adjustment (r2coef®cient of determination) for each defoliation stage in each of the four trials (n16)
Stage San AdriaÂn 1991 Calahorra 1991 San AdriaÂn 1992 Calahorra 1992
a aa r2 a a r2 a a r2 a a r2
1 0.24 0.07 0.77b 0.14 0.12 0.26d 0.43 0.10 0.85b 0.41 0.09 0.84b 2 0.21 0.04 0.85b 0.19 0.07 0.68b 0.47 0.08 0.91b 0.47 0.13 0.79b
3 0.27 0.06 0.86b 0.29 0.04 0.93b 0.71 0.09 0.94b 0.61 0.11 0.89b 4 0.33 0.06 0.89b 0.30 0.07 0.85b 0.79 0.12 0.92b 0.67 0.11 0.92b 5 0.41 0.07 0.91b 0.37 0.14 0.67c 0.89 0.08 0.97b 0.64 0.11 0.90b 6 0.73 0.05 0.98b 0.68 0.07 0.96b 0.83 0.09 0.95b 0.77 0.08 0.96b 7 0.25 0.07 0.75b 0.23 0.08 0.68b 0.31 0.10 0.74b 0.37 0.10 0.79b
aInterval of coef®cient
awith a 95% con®dence.
bP<0.001. c
P<0.01.
June that same year (with a mean temperature 4.48C colder than in a standard year), and this would have had a notable in¯uence on the development of the crop and on the response to the defoliation treatments. Therefore, there arose two additional problems during the 1992 trials, namely, a weak crop implantation due to the cold winter and a weaker ripening subsequent to the cold days following the defoliation treatments. Both, could have caused the yield reductions to be higher than those occurring in 1991, which was a year characterised by the normal development of the crop and by a winter mean temperature only 0.18C lower than in a standard year.
Thus the results obtained render it impossible to obtain average data for the four trials as was the case for similar trials carried out on onion (Muro et al., 1998a), in which results from ®ve trials were averaged out.
A comparison with the results obtained by Bisht and Agrawal (1994) is here in order. They obtained an equation for maximum yield reduction of y0.69x (redesigned equation based on their data) in the seventh week prior to harvesting. This period corresponds approximately to the sixth defoliation stage in the 1991 trials and the ®fth in the 1992 trials. According to the data shown in Table 3, this yield reduction is included inside our predictor intervals, de®ned with a con®dence of 95% except for the case of the trial carried out in San AdriaÂn in 1992 (y(0.890.08)x). However, in the results of both authors, a gradual loss of sensitivity to defoliation treatments occurs between the seventh and third weeks prior to harvesting, while in our trials a sharp fall in sensitivity occurs between stages 6 and 7 (see Table 3), equivalent to ®fth and fourth week prior to harvesting. This period coincides with the beginning of June when high summer temperatures cause a rapid ripening of bulbs and plant senescence.
Although garlic has a similar response to onion (Muro et al., 1998a), it differs from others such as sugar beet (Muro et al., 1998b), potatoes (Wille and Kleinkopf, 1992), strawberries (Seipp, 1989), sun¯ower (Schneiter and Johnson, 1994) or rice (Counce et al., 1994).
These data could be used for modelling the yield reduction due to defoliation treatments produced by different biotic and abiotic agents.
Acknowledgements
The authors thank AGROSEGURO S.A. for funding the trials.
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