- Author: Brad Hanson and O. Adewale Osipitan
Article also published in California Weed Science Society Journal (March 2021 issue)
-Brad
Glyphosate is one of the most commonly used herbicides in orchard crops in California both in terms of treated acres and amount of active ingredient applied. Weed managers are generally familiar with the attributes of glyphosate as a postemergence herbicide. Duke and Powles (2008) published an article in Pest Management Science entitled “Glyphosate: a once-in-a-century-herbicide”. Suffice to say, it's a pretty useful herbicide, if that's your thing. It has also been the subject of several controversies in the past couple of decades.
One of those controversies was a source of frequent extension questions from California farmers and Pest Control Advisors about 10 years ago. In the late 2000's there were several researcher articles from work done in glyphosate-tolerant soybeans that suggested there might be some nontarget impacts of glyphosate on crop nutrient status, plant disease interaction, and microbial community effects. In California tree crops concern generally focused around whether glyphosate in the soil could chelate micronutrients and lead to nutrient deficiencies and/or if repeated use could cause enough glyphosate to accumulate in soil and have direct impacts on the trees.
In a 2012 review article on the impacts of glyphosate in the soil environment, Duke et al. suggested that “significant effects of glyphosate on soil mineral content is unlikely” and there is no clear negative trend in orchard productivity data in California even after decades of glyphosate use. However, because this was a concern for the California orchard crop industries, we conducted a long-term research project from 2013 to 2020 to help address the issue in representative orchard crops.
If you're REALLY interested, the full report was published in summer 2020 as an open-access article in the journal Weed Technology which can be accessed HERE.
If you're only a LITTLE interested, here is the essence of the project conduct at the UC Davis Plant Sciences Field Facility in Davis, CA:
- Crops (three crops were considered separate experiments):
- Almond (Nonpareil on Lovell)
- Prune (Improved French on Lovell)
- Cherry (Coral on Emla-Colt)
- Planting site:
- At planting, all tree sites were excavated with a 36-inch diameter augur to a 2-ft depth
- Half were refilled with the native silty clay loam soil
- Half were refilled with Delhi sandy loam
- Glyphosate treatments:
- From 2014 to 2019, treatments were applied 3 times per season (~between Apr-Nov)
- Rates were 0, 1, 2, or 4 lb ae/A equivalents (Roundup PowerMAX plus ammonium sulfate). Applied to an area of about 6x6 ft around each tree
- In 2014, trunks were protected with cartons, but no trunk protection in 2015-19
- Drench
- In the first two years of treatment (2014 and 2015) there was a split-plot factor in which half of the trees had a small berm built up around the base of the tree and were “drenched” with water to simulate a 1-inch irrigation immediately after each application (intended to increase leaching into the relatively small tree root zone and crown area).
- Evaluations
- In the first year of treatment (2014) leaf samples were collected 14 days after each glyphosate application and assayed for shikimate accumulation (which would indicate direct herbicidal effects of glyphosate).
- In the first two years of treatment, relative chlorophyll content was measured in leaves from each tree 30 days after each glyphosate application (which could indicate either direct glyphosate effects or indirect effects of micronutrient limitations).
- Trunk diameter measurements were made before the first application in 2014 and during the winter after each subsequent season to evaluate relative tree growth.
- After the 6th year of treatment, leaf samples were collected in the fall from each tree but combined over soil type and drench subplots. The nutrient status of these leaf samples was determined by the UC Davis Analytical Lab using appropriate techniques.
- So, to recap, the worst case scenario had:
- Very coarse soil in the planting site of bare root almond, cherry, and prune nursery trees.
- In the first two years, some plots had a simulated acre-inch of irrigation immediately following each glyphosate application.
- The highest rate treatment of 4 lb ae/A glyphosate applied 18 times over a six year period. That's 114 fl oz/A of Roundup PowerMAX at each application. Over the course of the experiment that's 72 lb ae/A glyphosate or 16 gallons of Roundup PowerMAX. It's a lot!
- Results:
- Shikimate levels were similar among treated and non-treated trees with no clear dose-response or soil-related parameters. This suggests little or no direct effect of glyphosate on these tree crops via root uptake even at fairly extreme rates over multiple years.
- Chlorophyll content mostly indicated no differences among treatments. Where there were statistically significant main effects or interactions, there was no consistent pattern with regard to glyphosate rate, coarse soils, and post-treatment drench, which suggests they may be due to random variation or experimental artifacts.
- Leaf nutrient analysis after 6 years of treatment did not provide evidence of negative impacts on crop nutrient status.
- Trunk diameter increase over six growing seasons was not negatively impacted by glyphosate treatments (Figure).
- Observationally, over the six year period, there was no evidence of treatment-related trunk cankers, trunk or limb malformations or unusual die-back.
Scientists will always remind you that you “can't prove a negative”, and that remains true. However, we think these data suggest that it is probably not easy or common for almond, cherry, or prunes to be negatively impacted by glyphosate residues in the soil either due to direct herbicide effects or to micronutrient deficiencies.
Acknowledgments:
This work was initially supported by the Almond Board of California, the California Dried Plum Board and with nursery stock provided by Sierra Gold Nurseries. Over time, the experiments were maintained with general program support from the crop protection industry, orchard commodity groups, and agricultural input suppliers. We gratefully thank our colleagues who contributed to this long-term project along the way.
References:
Duke SO, Powles SB (2008) Mini-review. Glyphosate: a once-in-a-century herbicide. Pest Manag Sci 64:319–325
Duke SO, Lydon J, Koskinen WC, Moorman TB, Chaney RL, Hammerschmidt R (2012) Glyphosate effects on plant mineral nutrition, crop rhizosphere microbiota, and plant disease in glyphosate-resistant crops. J Agric Food Chem 60:10375–10397
Osipitan OA, Yildiz-Kutman B, Watkins S, Brown PH, Hanson BD (2020). Impacts of repeated glyphosate use on growth of orchard crops. Weed Technol. doi: 10.1017/wet.2020.85 (available at: https://www.cambridge.org/core/journals/weed-technology/article/impacts-of-repeated-glyphosate-use-on-growth-of-orchard-crops/26697F334626A587765C75C2B0F76A3D)
This blog originally appeard in the UC Weed Science Blog
https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=46316
- Author: Ben Faber
There is a wild world in those avocado orchards. A study was done locally of some of the carnivores besides humans that show up there.
Carnivore Use of Avocado Orchards Across and Agricultural-Wildland Gradient
Wide-ranging species cannot persist in reserves alone. Consequently, there is growing interest in the conservation value of agricultural lands that separate or buffer natural areas. The value of agricultural lands for wildlife habitat and connectivity varies as a function of the crop type and landscape context, and quantifying these differences will improve our ability to manage these lands more effectively for animals. In southern California, many species are present in avocado orchards, including mammalian carnivores. We examined occupancy of avocado orchards by mammalian carnivores across agricultural-wildland gradients in southern California with motion-activated cameras. More carnivore species were detected with cameras in orchards than in wildland sites, and for bobcats and gray foxes, orchards were associated with higher occupancy rates. Our results demonstrate that agricultural lands have potential to contribute to conservation by providing habitat or facilitating landscape connectivity.
Read on:
http://www.avocadosource.com/journals/plos_one/nogeiretheresa2013.pdf
- Author: Gary S. Bender
Since our last article on high-density avocados appeared in Topics in Subtropics we have had a lot of questions from growers concerning the cost of installation of such a grove. We have also had a lot of interest from potential winegrape growers who think this might be the way to go given that wine-grapes use about 25% of the amount of water per acre compared to avocados. For higher quality wines growers use less water, but harvest lower pounds/acre.
I am currently advising a student at Cal State San Marcos who is doing a cost study for removing an avocado grove and installing a winegrape planting in San Diego County. Hopefully we will have an article on that in the near future.
We don’t have a lot of experience in high density avocados but we will try to show some costs to help you decide.
Clearing an older grove can get expensive. In a study we did in 2010 we found that it cost $40/tree to cut a tree down, haul the cut wood down to a firewood stack and haul to small branches to a chipper. The old stump is usually killed with a herbicide and left in place to rot. So, just to remove a tree at 109 trees/acre may cost $4,360 per acre.
For our high density planting we will be planting Hass avocados grafted on Dusa rootstock. We are using this rootstock because it appears from several trials to be a good rootstock for resistance to avocado root rot in the well-draining hillside soils in San Diego County.
The current price is about $30/tree (plus or minus). For a 10’ x 10’ spacing we would plant about 435 trees per acre, this would be an initial cost of $13,500/acre. We are not calculating the cost of a grove road installation, and we are assuming that the entire acre would be planted.
Planting cost: digging, planting, wrapping and staking trees takes about 15 minutes per tree. This includes carrying the tree to the hole. At 435 trees per acre, this would take about 109 hrs. At $14/hr, this would cost $1526/ac. Stakes (435) at $2.25/stake would cost $979/ac.
The irrigation system installation would cost $2,660/ac. This is based on an entire system being installed in a 20 acre block, divided by 20 to get the cost per acre.
There are quite a few other costs that you can see from our complete avocado cost study, but these basic costs will get the trees in the ground (Table 1).
Table 1. Basic costs for the installation of a high density avocado grove. |
|
Trees (Hass on Dusa rootstock) |
$13,500 |
|
|
Planting (digging, planting and wrapping) |
$1,526 |
|
|
Stakes |
$979 |
|
|
Irrigation system |
$2,660 |
|
|
Total |
$18,665/acre |
If you are thinking about planting 10 acres, then you would need $186,650 in the bank to get the trees in the ground. There will be some shifting of costs because this study does not include clearing or building of grove roads, and with grove roads you will be planting less numbers of trees/acre.
Will you make more money in the long run? That is the question we are trying to answer with our high density trial in Valley Center.
Reference
Bender, G.S. and E. Takele. 2010. A guide to estimate compensation for loss of a single avocado tree. University of California Cooperative Extension county publication AV 606.
Takele, E., Bender, G.S., and M.Vue. 2011. Avocado sample establishment and production costs and profitability analysis for San Diego and Riverside counties, conventional production practices. http://coststudies.ucdavis.edu/