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Rural Industries Research & Development Corporation
The Impact of Double Harvesting on Victorian Peppermint Crop Productivity
by Fred BienvenuOctober 2002
RIRDC Publication No 02/131 RIRDC Projects No VMP-1A and VMP-2A
Background
The practice of double harvesting peppermint oil crops is employed in the USA where special circumstances exist. Higher yields can be achieved but care needs to be taken to ensure that rootstock depletion does not affect the ensuing year’s oil potential. Most oil producers in the USA, except in parts of the state of Washington, employ double harvesting only in the final year of a rotation.
The Victorian Peppermint Industry Strategy Plan, adopted by growers in May 1995, focused on maximising yield of peppermint oil. This project was based on the assumption that under Victorian conditions double harvesting would significantly increase sustainable yields. Further it was considered that the products from double harvest could be blended to produce a similar unit value to normal harvested peppermint.
This project aimed to establish the agronomic production conditions and harvest times which maximise “yield and productivity” of peppermint crops produced under Victorian conditions.
Further, the project sought to establish market acceptability of blended oils and hence determine the feasibility of double harvesting a strategy to maximise returns.
In the presence of adequate water, nitrogen is the key nutrient that can be used to modify growth characteristics, time to physiological maturity, oil yield and composition. As a consequence this project focussed on the potential to use nitrogen to manipulate crop growth, oil yield and oil composition under a number of harvest scenarios comparing double harvest regimes with “normal” harvest. This was determined by GC analysis of samples of oil from peppermint herbage.
Double harvest research plots were established on sites on a research field and on a large-scale commercial property. Times of harvest coupled with double harvest treatments were set up and overlaid with nitrogen treatments. The nitrogen treatments included four rates of application which covered 100, 200, 300, and 500 kg/ha actual N. The highest rate was deliberately chosen to be “over the top”.
A further trial was established to examine the role of sources of nitrogen fertiliser. Ammonium nitrate and urea were employed at equivalent rates of 100, 200, 300 and 500 kg/ha actual N.
Treatments were applied in six split equal applications with dates from the middle of November until the middle of January Generally, no long-term benefits were seen in the exercising of double harvesting peppermint oil crops in this study. In most seasons during the study the yield under a double harvest regime aggregated to less than that for a normal single harvest. The highest increase over a single harvest was 27% in only one year. In later years there was a substantial loss of yield due to double harvest.
The plots were positioned on the same sites for the duration of the project to test the longer-term effects of the continual removal of photosynthetic material. In the fourth year, 65% of the plots marked for double harvest were void of any herbage at all.
In general the optimal nitrogen application was between the 200 and 300 kg/ha per year actual N.
This provides a practical ceiling for effective nitrogen applications each season for peppermint crops grown under Victorian conditions. Higher rates of nitrogen application can have negative effects both economically and environmentally. In both the double harvest and the source of nitrogen trials there was a very clear inverse relationship between nitrogen and menthofuran. Menthofuran is the main negative component in peppermint oil and it must be kept to a minimum to achieve premium prices and status. Results also showed that higher rates of nitrogen can cause oil yield depression, but the extent tended to be affected by seasonal factors.
Problems were encountered when distilling in the cooler months. The cooler conditions caused later oil “break-through” and made “end-point” determination more difficult. This has the potential to increase costs and reduce yield during late harvests.
The Cardy Sap/Stem Nitrate meter (or similar) has been used successfully for many years to provide an “in-the-field” indicator of nitrogen status of a wide range of growing crops from cereals to tree crops through out the world. However, due to the inability to obtain consistently reproducible results it was concluded that the Cardy meter was not suitable for prolonged use in the peppermint industry as an indicator of nitrogen status.
Quality of locally produced peppermint oil is situated in the upper levels in the world market place.
Double harvesting of peppermint by necessity brings into question the economic value of various oils as the profile of an early cut oil differs greatly from that of a late cut oil, as can the profile of a single normal cut vary from a double cut oil.
The results clearly indicate that double harvesting of Victorian peppermint crops is not a sustainable economic practice. Continuing double harvest across successive seasons will result in a rapid depletion of yield. Quality of oils from double harvest regimes was generally inferior to normal and on an open market would be of lower value.
If maximisation of peppermint oil yield is a key goal of producers then significant savings on losses could be made at the harvest distillation stage through modification of harvest techniques and adoption of better procedures for preventing over drying and therefore shattering of herbage. These and other similar topics are canvassed in the discussion section of this report.
No Victorian peppermint crops should be double harvested unless a premium price can be assured for the resultant oil and it can be ascertained that the crop can survive the negative effect on plant vigour over the long term.
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