Global demand for asparagus may be growing, but the high cost and shortage of labour is hampering many producers and fuelling interest in harvest mechanisation. One of the challenges facing mechanisation is today’s cultivars have been selected for hand harvesting and their traits don’t lend themselves to machine picking. For hand harvesting, desired characteristics include high total yield, excellent spear quality, good spear thickness, and improved earliness. When it comes to machine harvesting, desirable criteria include uniform and consistent spear emergence, a better knowledge of what triggers bud break and spear elongation, a good understanding of bud and bud cluster dominance, more uniform spear position in the planted row, spear growth regulation, and harvest termination details. In his article “Asparagus breeding: Future research needs for sustainable production”* (Front. Plant Sci., 27 March 2023), professor Daniel Drost, from the Utah State University’s Department of Plants, Soils and Climate, says a better understanding of such matters would bene fit producers across the world.
What can be learned from the development of mechanised tomato harvesting?
Drost writes that the history of the mechanical harvesting of tomatoes provides an analogy for how to approach the mechanisation of the asparagus harvest. Prior to the development of a harvesting machine for tomatoes, all tomato fruits were hand harvested and imagining how a mechanical harvester might work and what it might look like was difficult. Those developing the system figured that to allow for machine-picking, completely new varieties of tomato would be needed. Such new plants would need to produce fruit with greater firmness to protect them from machine damage, and have a very short fruit-set period, so that fruit ripening occurred over a concentrated period. A first step in the development of a machine-harvestable tomato variety was the release of the small determinate cultivar “Red Top.” Through selective breeding and screening, the VF 145 lines were later developed. These first mechanically harvested processing tomato cultivars paved the way for the present day preeminence of mechanisation in the harvesting of canning tomatoes, he says.
Apple tree selection has improved labour efficiency
When it comes to apple orchards, Drost says a primary objective is to grow trees that produce high-quality fruit with high productivity. Tree forms have changed over time via selection of di fferent
plant types (dwarfing rootstocks) and the adoption of new training and pruning approaches. It’s been found that planting density has a stronger impact on fruit quality, growth, and light interception than training systems (shape) do at the same spacing. Similarly, light distribution within the canopy is more crucial than total light interception in terms of fruit quality. Efforts to increase fruiting spurs, as opposed to vegetative shoots, have signi ficantly improved plant performance, he writes. “More linear growth habits (spur or central leader type) optimise light capture while reducing the need for extensive training or pruning. Therefore, tree selection (or manipulation) creates new possibilities in orchard management, improves labour e ciency, and reduces inputs (fertiliser, water, pesticides, etc.) while ultimately enhancing productivity”, Drost says.
Fewer buds and bud clusters could simplify production without sacrificing productivity
Like tomato and apple, asparagus breeders need to identify new plant types – particularly identifying different crown architecture – more conducive to mechanical harvesting. Historically, asparagus breeding focused on high productivity and, more speci fically, on the identi cation of all-male lines, disease resistances, high spear yields (large crowns; lots of bud clusters), and spear quality. “Bigger plants do produce higher yields, but they do this randomly. Creating simpler systems (fewer buds and bud clusters) and identifying alternative plant forms could simplify production without sacri ficing productivity”, Drost says. He writes that current asparagus cultivars have highly di erentiated rhizomes (high branching; big crown), similar to indeterminate tomatoes. Existing asparagus varieties also have many bud clusters and buds, so there is the potential for high yields. But as plants develop, growth progresses in too many directions and bud break follows no real pattern or predictability. “These plant types are less adapted to mechanical harvesting due to the randomness of spear position, timing, or growth. Presently, there is no known way to regulate the timing of spear emergence, and it is di fficult to determine when or where spears will emerge. So, existing asparagus plant types and present-day asparagus harvesters are less compatible, and this reduces harvest e fficiency and increases harvest costs.” he says.
Stronger apical dominance a desired trait for mechanised harvesting
Ideally, asparagus plants with a more centralised growth habit (fewer secondary bud clusters or single/ limited axis of rhizome growth) would help concentrate maturity or overcome the uncertainty of where spears may emerge in the field, Drost says. In theory, asparagus crowns with stronger apical dominance may have fewer secondary bud clusters or may suppress those bud clusters from growing spears. Also, asparagus plants with a centralised rhizome (if identi fied) could be organised, much like spur or spindle apples, into narrow rows (high plant populations) and oriented in rows with distinct arrangements (bud clusters positioned directionally), which would better suit mechanical harvest. “This does not overcome the problem of regulating spear emergence but does regulate the field position of the spears.” While fewer bud clusters means fewer buds and lower spear yield per plant, lower perplant yields could be overcome by increasing plant populations. “Asparagus breeders need to not only consider traditional selection metrics but also keep a lookout for the unusual or off-types that may fit into nontraditional production systems,” Drost says.
All asparagus growers stand to bene fit
In summary, “changing asparagus production systems for mechanical harvesting would require additional understanding of apical dominance within the bud clusters, renewed e orts to synchronise spear emergence, and the need for asparagus breeders to look for and identify alternative plant types specifically for the machine harvest industry.” Drost also says: “While many regions of asparagus production with high acreage still rely on hand harvesting methods, architectural changes to the asparagus plant would and could bene t all asparagus production
regions making harvest more e fficient and cost-effective. Furthermore, while much is known about asparagus growth, further work on the regulation of spear growth can help with understanding the dynamics and timing of spear elongation. Through the combined e fforts of breeders and physiologists, the changes described can improve asparagus productivity, adapt the plant to mechanisation, and still maintain the level of productivity and quality needed to make the system sustainable and profitable.”
