Developing a Strategy for minimising levels and variability of maleic hydrazide residues in treated bulbs
Project Number / FV 227Project Leader / S. L. Reynolds
Status / Final Report 2002
Date / 30 October 2002
Key worker / S. L. Reynolds (project leader, author)
S. Nawaz (senior analyst, author)
G. Dunsford, K. Kitching (analyst)
Project Location / Central Science Laboratory
York, YO41 1LZ
Project co-ordinator / A. Findlay
Lakes Farm Office
Cardington Lakes Farm
Bedford MK44 3SH
Collaborating Organisation / Allium and Brasica Centre
Wash Road, Kirton
Boston, Lincolnshire PE20 1QQ
Contact: D. O'Connor
Date project Commenced / 1 August 1999
Date completion due / 31 July 2002
Key Word / onions, sprout control, maleic hydrazide, FazorÒ, residue levels, residue variability, storage
Whilst reports issued under the auspices of the HDC are prepared from the best available information, neither the authors nor the HDC can accept any responsibility for inaccuracy or liability for loss, damage or injury from application of any concept or procedure discussed.
CONTENTS
Section / PagePRACTICAL SECTION FOR GROWERS / 1
Science Section / 2
Introduction / 2
Materials and methods / 4
Analytical Quality Control / 7
RESULTS AND DISCUSSION / 8
Conclusions / 19
Recommendations / 19
ABBREVIATIONS / 20
Glossary of terms / 20
References / 20
ã2002 Horticultural Development Council
PRACTICAL SECTION FOR GROWERS
Action points for growers
· Adequate sprout control will be provided in bulbs with a single growing point stored in a refrigerated facility for up to 8 months with a minimum MH residue level of 1mg/kg. A lower level of MH will offer similar sprout control in bulbs stored in CA stores. The corresponding minimum levels of MH in bulbs with a more normal multiple growing point is expected to be higher.
· To achieve a 1mg/kg minimum level in all bulbs requires an average MH level of 2 mg/kg in composite samples. This average level can be achieved with a 50% rate of Fazor® or Royal MH®, as long as the crop is treated after necking has commenced.
· The adjuvant HOW004 will aid the uptake of MH when crops are treated at a later stage of plant development i.e. after the onset of necking and especially when applied with Royal MH®. The adjuvant LI-700 improved MH uptake when the MH was applied early (before necking).
· The use of CA storage was shown to provide a very significant extra means of sprout control, when residues of MH in the bulbs were low (around 2mg/kg). Financial considerations will determine whether or not this approach is commercially viable.
· Assessments showed that approx. 87% of MH was taken up within 3 hours of application.
Expanded practical and financial benefits
· Growers wish to minimise MH residues in stored onions without compromising bulb quality. Residue variability is the main obstacle in achieving this aim. Reduced variability would allow growers to use lower application rates of MH, resulting in lower residue levels in the resultant cured bulbs. Lower mean residue levels would not only minimise the risk of exceeding the MRL, but also be more acceptable to retailers. However, due to uncontrollable factors identified in these trials (number of growing points in bulb, plant shape and spray distribution), it was not possible to significantly reduce bulb-to-bulb residue variability.
· The only practical way to acceptably lower MH levels in bulbs intended for long term storage is to use C.A. storage.
Science Section
Introduction
Maleic hydrazide (MH) is a pyridazone plant growth regulator, which is widely used to prevent sprouting in stored onions. It is applied to onions in the field when the bulbs are mature. The crop is harvested 7-14 days later and cured before storage. In addition to sprout suppression, an indirect benefit of MH treatment is the elimination of volunteers, which may carry disease, in the following season.
Figure 1. Chemical structure of maleic hydrazide
The distribution of radio-labelled MH in onion bulbs has been studied. The work showed that approximately 70% of the mass of MH was transferred to the bulb, of which 48% was found in the outer part, 16% in the inner shoot leaves and 4% in the basal root plate. The concentration in the root plate was 6 times greater than in the outer bulb layers (ANON 1984).
An investigation into the fate of MH in onions showed that 78% of labelled MH applied to onion tops was absorbed within 1 hour of application. At harvest, 2 weeks after treatment, 85-90% of the label survived as MH, the remainder converted to maleate, fumarate and maleimide. MH is stable at pH 5 and 7 to photolysis in water at 25°C but less stable at pH 9 (ANON 1999).
Residues in the range of 2 to 7 mg/kg occurred in onions when MH was applied at different stages of plant maturity. Experience has shown that the amount required for total sprout inhibition in onions during storage is approximately 5 mg/kg (ANON 1984).
Work carried out to assess the variability in a number of important crops (PSD 1997) showed that residue levels in individual units can be extremely variable. An investigation of MH residue levels in onions after normal commercial treatment showed that individual unit variability was relatively high with a typical CV of 40% (Nawaz et al. 1997). A recent WPPR survey of retail samples of onions showed that a significant proportion of UK produced onions contained MH residues at levels in the range of 1-6 mg/kg (Reynolds et al 2000). Although, these levels are relatively high compared with other pesticide residues in food, the JMPR have concluded that the intake of residues of MH resulting from its approved uses is unlikely to present a public health concern (ANON 1999). Even though there are no likely health hazards, comparatively high residue levels, combined with the issue of residue variability, have placed the growers under increased pressure from retailers and consumers to try to reduce the residue levels of MH in stored bulb onions.
The current work was undertaken to investigate possible means of minimising both variability and residue levels in bulb onions without compromising sprout control. The main findings of the first two years work were that a mean residue level of 3 mg/kg (from a composite sample), coupled with cold storage, would provide adequate sprout suppression for up to 8 months. However, this minimum level varied between the two years trials, and was thought to be influenced by bulb morphology. Following treatment, the main route of MH translocation was found to be via the leaves. About 50% of the MH deposited on the leaves was translocated to, and remained in, the bulbs as intact MH at harvest. The use of an adjuvant LI-700 facilitated the translocation/uptake process when treatments were undertaken relatively early (before the onset of necking). Typically, the residue levels in the bulbs decreased by 30% over the cold storage period. The timing of treatment had a significant influence on the resulting residue levels in the bulbs.
The main factors leading to residue variability (30-60%) were thought to be the spray application technique, variations in the surface area of the spray target (leaves), variation in the mass of the bulbs (i.e. dilution effect), differences in the rates of uptake, translocation and metabolism. An early application of MH (HI = 3 weeks) led to apparently lower than expected deposition on the leaves and thus resulted in lower bulb residues. The plant morphology varied between the seasons and was thought to have a significant effect on the resulting residue levels and subsequent sprout control.
The objectives of the work undertaken in the third year were to assess the efficacy of, and residues associated with, MH treatments (2 different formulations) in conjunction with selected adjuvants. The effects of timing of the application were to be further assessed during the latter stages of plant maturity (5% necking and 50% necking). Further storage experiments were to be conducted to compare the effectiveness of controlled atmosphere (CA) storage and cold storage.
MATERIALS AND METHODS
Figure 2. Details of
plot treatments
A commercial crop of brown F1 hybrid onions cultivated on sandy soil at Cardington, Bedfordshire was used for the experiments. Plots were treated as shown in Figure 2, on two separate days. The first set of treatments was carried out on the 24 August and the second set on the 28 August. One plot was left untreated to act as a control. Each plot was 20m long and 10m wide, comprising of five beds. Buffer zones (5 x 10 m) were used for starting and stopping the tractor and to avoid any possible spray drift onto adjacent plots. The outer beds on either side of each plot were not sampled, to avoid possible edge effects. Selected plants in the control plot were manually treated with 300 µl of a FazorÒ dilution prepared in the lab (8g/L FazorÒ º 5.3 g/L of MH) on the 20th, 24th and 28th August 2001.Weather conditions
On all three days the weather was sunny with a slight breeze at an average speed of 2-4 m/sec. All the treatments were completed by 12.30 hr on each occasion.
/ plot 12, 100% Royal MH + HOW004, day 1
plot 11, 100% Royal MH, day 1
plot 10, 50% Fazor + LI-700 and HOW004, day 1
plot 9, 50% Fazor + LI-700, day 1
plot 8, 50% Fazor, day 1
plot 7, 100% Fazor, day 1
plot 6, 100% Royal MH + HOW004, day 1
plot 5, 100% Royal MH, day 1
plot 4, 50% Fazor + LI-700 and HOW004, day 1
plot 3, 50% Fazor + LI-700, day 1
plot 2, 50% Fazor, day 1
plot 1, 100% Fazor, day 1
Control
Adjuvant
LI-700
This adjuvant is an emulsifiable concentrate containing lecithin, propionic acid and alkylphenyl hydroxypolyoxyethylene. It is approved for use on a wide range of edible crops as a penetrating, translocating and acidifying agent.
HOW004
There was no information available for this adjuvant, further information is available from Certis, Amesbury, Wilts, SP4 7RX.
Sprayer Details
sprayer Berthound pressure 2.0 bar
nozzle type Lurmark twins boom height 1 m
nozzle spacing 0.5 m number of nozzles 20/10 m
average crop height 30 cm tractor speed 8.5 km/hour
The actual deposition rate was measured by random placement of 9 wads (containing 3 layers each) of uniform filter paper disks (60 cm²) in plastic dishes in each plot. After spraying, the filter papers were collected and analysed as 3 composites of 3 wads.
Sample collection
Random samples were taken for analysis at various stages during onion production and storage. The timings of various stages are given in Table 1.
Table 1: Timings of sample collection
Sampling Stage / DateManual treatments / 20/08/01
24/08/01
28/08/01
Application / 24/08/01
28/08/01
Harvest / 05/09/00
End of cold storage / 08/05/02
Manual treatments
Leaves of selected plants (20) were treated with 300 µl of a 5.3 mg/ml FazorÒ solution on three separate days (Appendix 2). Ten of the plants were collected immediately for analysis. The remaining plants were labelled with relevant information and harvested on 5th September 2001.
Application
Tank dilutions were prepared to provide the MH levels given in Table 3. Two samples were taken from the tank (prior to each application) for laboratory analysis to check the actual levels of MH that was about to be applied.
Immediately after the spray had dried, within 1 hour of each application, 30 random plants were sampled from each treatment. Three composite samples (10 units each) were placed in polythene bags and transported, on the same day, to the laboratory. The samples were cryogenically milled and details of the samples and the results of MH analyses are given in Appendix 1.
Washes
On the day of treatment with the commercial sprayer, 10 plants were collected 3 hours after application. The leaves from each plant were dipped into 1 L water to estimate the amount of MH that remained unabsorbed.
Harvest
Two hundred and fifty plants were taken from each plot on 5th September. The leaves and bulbs were separated. The leaves were discarded and 30 bulbs from each plot were placed in individual labelled polythene bags and taken to the laboratory. The individual bulbs were weighed and analysed as 3 composite samples each comprising of 10 bulbs. The sample details and results of analyses are given in Appendix 1. The remaining bulbs were cured for 4 weeks at 28°C with a relative humidity of 55%. The temperature was then gradually reduced to 0.5°C over a 2 week period and half of the bulbs were allowed to remain at this temperature (cold storage) for next seven months, and the other half were stored under controlled atmosphere conditions. The controlled atmosphere storage used the same conditions as the cold storage, except that an elevated level of CO2 was maintained throughout the storage period.
Simulated shelf-storage
The samples were removed from storage on the 8th May 2002 and then held at room temperature for 7 weeks. Bulbs from each population were subjected to visual examination throughout this period to assess the degree of sprouting. Bulbs were examined using the method employed by retailers (Nawaz et al 2002). The results of these assessments are given in Appendix 3.
Sample preparation
Blank samples
Untreated samples from the control plot were collected at the time of application and at harvest. These samples were frozen and cryogenically milled in a Stephan food processor in presence of dry ice. Sub-samples were stored at -20°C and were used as analytical blanks, and to check for any possible cross contamination from spray drift.