In arguing for expanded agricultural guestworker programs, lobbyists for farmers often claim that fruit and vegetable production in the United States would be impossible without large and continuing infusions of foreign labor. For instance, the California Farm Bureau Federation has argued for a guestworker program by claiming that, “Without an adequate supply of workers to fill seasonal labor-intensive tasks such as harvesting, U.S. growers will become uncompetitive, and be forced to reduce production of labor-intensive crops.”
This paper by three prominent agricultural researchers suggests that attempting to maintain the status quo of labor-intensive harvesting is not in the long-term interests of American agriculture. They found that progress in harvest mechanization stalled after 1980, both because of the large supply of farmworkers, many of them illegal, and because of an anti-mechanization policy pursued by the federal government since the Carter Administration. The authors argue that this slowing of progress in harvest mechanization has undermined the competitive position of American farmers and allowed foreign countries to leap ahead of the United States in developing new mechanical harvesting technologies.
The hand harvesting of fruit and vegetable crops in the United States is a labor-intensive operation that accounts for about 50 percent of total production costs. The number of crops and percentage of crop acreage that are mechanically harvested today have increased somewhat since the late 1970s. Most of these crops are used for processing. However, at least 20 to 25 percent of the U.S. vegetable acreage and 40 to 45 percent of the U.S. fruit acreage is totally dependent on hand harvesting. The crops represent about 30 percent of the U.S. fruit, nut, and vegetable acreage and have an annual farm-gate value of over $13 billion. Declining labor availability and increasing labor costs are reducing U.S. growers competitiveness with foreign suppliers. Harvest mechanization and improved production technologies show promise for keeping U.S. growers in business.
The fruit, nut, and vegetable industries in the United States are major agricultural endeavors, with a significant economic and social impact on both the domestic and foreign markets. According to the 1997 census of agriculture, a total of about 12 million acres of fruit, nut, and vegetable crops were harvested in the United States in 1997. They represented a farm-gate value of about $23 billion, which was about 25 percent of the total U.S. value of crop production 20.
The supply of workers available for hand harvesting is decreasing steadily and true shortages are occurring. Without enough workers when needed for a few weeks each year, part of most hand harvested crops will be lost.
The objectives of this paper are to: 1) report on the current status of harvest mechanization of fruit, nut and vegetable crops in the United States; 2) point out several important crops that are still locked in to labor-intensive hand harvesting, and; 3) suggest that the continuation of many of these crops as vital food industries in the United States requires a massive effort to develop labor-efficient production and harvesting technologies.
Harvesting Horticultural Crops
Tables 1 and 2 show summary data for the crops that are hand harvested from more than 2,000 acres in the United States. A review of the Lh/ac (labor-hours/acre) required for hand harvesting these food crops shows that the range is from 25 to 530 and an acreage-weighted average value is about 70. This indicates that two workers must spend a 35-hour week to hand harvest each acre of these horticultural crops.
Commercial hand harvesting of fruit, nut, and vegetable crops is hard, tedious, and time-consuming work. The amount of labor needed for harvesting is often well over 50 percent of the total annual labor requirement (18, 19). Hand harvesting 12 million acres annually would require at least 840 million Lh/year (1.05 million workers for 20 weeks/year), and would easily exceed the labor force of workers willing to do seasonal crop harvesting. Fortunately, several of the crops are now already harvested mechanically, and some are harvested using labor-aid machines or devices that make the work easier, as shown in Table 3. However, Tables 1 and 2 indicate that successful mechanical harvesting technologies are still not available for all horticultural crops destined for either the fresh market or for processing.
History of Mechanization
The introduction of harvesting mechanization began with innovative farmers and local mechanics. They were looking for opportunities to reduce the drudgery of farm labor, increase its productivity, improve the farmers’ ability to perform operations in a timely manner, and to reduce the financial risks associated with the need for large amounts of seasonal hand labor for short periods of time. Subsequently, agricultural engineers within the public sector — federal and state government research agencies and land-grant universities — working in conjunction with growers and equipment manufacturers, put a lot of effort into developing appropriate harvesting technologies for a variety of horticultural crops (2, 3, 4, 7, 11, 12, 13, 14, 22).
Major progress has been made in the past 40 years in developing mechanical harvesters. The 15-year period 1960-75 was a period of relatively massive mechanization, as growers responded to the labor shortage resulting from the 1964 termination of the Bracero Program (PL 78). However, the average harvesting Lh/ac for all horticultural crops decreased only by 20 percent by 1976 (2). Predictions for a continual gradual increase in the acreage and number of crops harvested mechanically, unfortunately did not materialize (2, 5, 22). Growers were not forced to change because worker supply remained good during the period 1980-95, although many workers were illegal aliens using falsified papers.
Commercial use of harvest machinery also slowed because of several technological hurdles. Mechanical harvesting technology may be adopted by one production area, but not by another because of the unique differences in factors such as climate, soil, terrain, labor, crop mix, market, utilization, variety, and tree or plant type. Lack of uniform maturity and differences in criteria for readiness for harvest between different horticultural crops, and even between species and varieties, made it very complicated to substitute machines for human judgment and dexterity. Selecting only mature product for harvest, as practiced with many hand harvested fruits and vegetables, is still very difficult to achieve with machines.
Most important of all is the mechanical damage incurred during mechanical harvest, which has been the major deterrent to continued development of mechanical harvesting systems for fresh fruit and vegetable crops. Most currently available mechanical harvesting systems for processed crops often cause produce damage that cannot be tolerated by the fresh market industry. This is in contrast with the mandatory requirements for high post-harvest produce quality which is becoming extremely important as more fresh fruits and vegetables are being exported to distant markets in Asia, Europe and Latin America, and must reach these markets in optimal condition.
Because of the complexity involved in the introduction of mechanized systems, growers generally use hand labor as long as workers are available when needed, and can be hired at a reasonable cost. Mechanical harvesting is usually adopted only when labor is not available or when appreciable savings will result. Mechanization often also requires a large capital investment and can reduce the growers flexibility to change from one crop to another, or from one market to another. Thus, for many horticultural crops, harvesting has continued to be entirely a hand operation performed by seasonal migrant workers. When local labor sources dwindled, supplementary imported workers have been used under the authority of appropriate Public Laws (8, 9, 17).
Present Use of Mechanical Harvesting
Table 3 lists the nut, deciduous tree fruit, berry, citrus, and vegetable crops grown commercially for food in the United States for which mechanical harvesting systems have been adopted for over 50 percent of their acreage. Note that all nut crops (over 2.5 million acres), potatoes (over 1.3 million acres) and most below ground vegetable crops (over 0.4 million acres) rely totally on mechanical harvesting. Harvesting mechanization of these crops was initiated as early as the late1930s, and is a common practice now (4, 22). The macadamia nut, grown primarily in Hawaii, is the only commercial nut crop which is not yet fully mechanized because the rough Hawaiian soils make it difficult to optimally operate pickup machines. Where soil conditions are maintained the mechanical system is very feasible.
Tables 1 and 2 also list the crops that have limited or no feasible mechanical harvesting systems, and details of their harvested amounts and labor needs. The data listed for Lh/ac, harvest weeks, harvest workers, and percent processed are estimates, but are believed to represent commercial values. Because many conditions may be different for a particular grower or region, the Lh/ac values experienced by a specific grower may be up to 50 percent different from our values.
Types of Mechanical Harvesting
Mechanized harvesting can be divided into three categories: labor-aids that facilitate harvesting work, labor-saving machines that improve productivity and reduce harvest labor needs, and robotic harvesting or automation. Labor-aids are aimed at reducing the effort and endurance required for doing a task, thus helping stabilize the labor force and increase the potential pool of laborers. They also reduce worker drudgery and injuries.
Labor-aids generally do not greatly increase labor productivity or reduce harvesting costs, unless coupled with mechanical harvesting or packing operations. The use of labor-aids for fruit and vegetable crops is more common in other countries (e.g. Italy, Spain, Australia, and Israel). While increased labor productivity has not been recorded in these countries, it does reflect the necessity to increase labor availability and provide higher emphasis on working conditions.
Labor-saving machines do the work of hand harvesting, generally by harvesting the crop as a mass (shaking a tree or a bush, digging a row of below ground vegetables, or cutting a row of above ground vegetables). This will reduce total harvest costs if the equipment can be purchased and operated cheaply enough. Harvest labor productivity can increase by more than 10 fold. More workers may be required at packing or processing to remove cull product. Labor-saving machines offer several advantages to workers. By reducing seasonal labor demands at harvest time, they foster development of permanent jobs, which are more likely to be higher paying and have better employee benefits.
An alternative to the more conventional mechanical harvesting systems that replace hand methods is the method of automated picking with a robotic system, which emulates the human picker for decision making and picking. Conceptually, such a system provides the same or better quality produce, at a much faster rate and works more hours/day than can human pickers. To accomplish these goals the robotic systems are generally equipped with machine vision systems for crop detection and robotic arms with special grippers (end-effectors) for crop detachment, collection, and transport. Great progress has been made as computers, vision systems, and motion/manipulation control technologies have been enhanced. Encouraging results have been obtained by development work done primarily in the EEC countries. However, the cost of robotic harvesting is still higher than conventional harvesting and 20 to 30 percent of the crop cannot be retrieved, so under current conditions a picking robot is not commercially feasible (15).
Because of the large number of horticultural crops on the one hand, and common use of a specific mechanized harvesting system for several crops on the other hand, no attempt will be made in this paper to address each crop separately with the relevant harvesting technology. Numerous publications are available, covering a span of more than 50 years of research and development worldwide, describing mechanization achievements for a variety of horticultural crops. The reader should refer to these publications for more details on a specific crop or harvesting system (4, 13, 22).
A relationship exists between the utilization of a crop and the extent of mechanization that may be possible. Fruits and vegetables are classified according to their use into processed and fresh categories. While the shift to mass harvesting systems offers much higher potential for saving labor, its inherent limitations in regard to the quality of the produce limits its use mostly to crops destined for processing. The processes of canning, drying, or freezing stabilize the final produce quality. By processing quickly after mechanical harvesting, minor damage is not noticeable in the final product.
Examples of Mechanical Harvesting Success
The harvester for processing tomatoes is an example of a successful labor-saving machine, demonstrating the potential for reducing costs through mechanization. Developed in California beginning in 1950, it was commercially available by the end of the Bracero Program in 1964. The harvester eliminated entirely the need for hand-carrying 40 to 50 pound field boxes and made 24 hour/day harvesting possible. Hand harvest required five Lh/ac, while machine harvest today uses only 0.4 Lh/ac. California processing tomato production increased from 2.3 million tons in the early 1960s to more than 10 million tons today, making California the world’s leading producer of processing tomatoes. Total labor use dropped from 13.5 million-hours in the hand harvest years to about 3.8 million today. Mechanization has allowed growers to become more competitive by reducing harvest cost. Hand harvest was quite costly, accounting for almost 50 percent of total production costs to the grower (as is the case with many other crops). With mechanized harvesting, the harvest costs are only about 16 percent of the total costs incurred by an average farmer (16).
At the end of the Bracero Program, two fruit crops that had very high hand labor requirements — tart cherries and prunes — both destined for processing, went from less than 10 percent mechanical harvesting to more than 80 percent mechanical harvesting in six to eight years (3, 4). Without mechanization or an adequate harvest labor supply, the survival of those industries may have been questionable. Harvest costs might have risen to a point where hand harvesting was no longer economically feasible. Sweet cherries for processing were soon mechanized using similar technologies. Mechanical harvesting has offered at least 10 times more labor productivity than did hand harvesting of these crops, and the industries continue to prosper.
Unfortunately, similar breakthroughs have not been repeated with apples, apricots, avocados, peaches, pears, or nectarines, although the labor supply in all fruit growing regions has become worse. The non-selective nature of the tree shaking approach, the lack of uniform maturity of the fruit, and most importantly, the excessive mechanical damage associated with the traditional shake-catch system, hindered its acceptance by the processing and fresh-market industries.
The shake-catch and shake-pickup harvesting systems, utilizing various principles for fruit detachment — limb, trunk, foliage, canopy or air shakers — have been the most commonly used methods for mechanized harvesting of tree fruits and nuts. With some fruit the use of abscission chemicals is required to ease fruit detachment. But in general, the overall characteristics of mechanical harvesting for fruit offer a major increase in labor productivity because of its typical high-capacity mass-harvesting performance.
A small percentage of the processing cling peach crop in California is harvested mechanically with shake-catch systems, although their use was reduced in recent years due to the lack of uniform fruit maturity and because high fruit damage levels caused problems for processors.
The use of mechanical harvesters for juice and wine grapes has approached 100 percent in recent years, although a few high-quality small plot vineyard owners still prefer the use of hand harvesting. All fresh market grapes are still hand harvested.
A significant change in recent years in the application of harvest mechanization has been the renewed interest in mechanizing the harvest of citrus for processing. Florida citrus growers, faced with strong price competition from other citrus growers in other countries and a decreasing supply of hand harvesters, have realized that adoption of mechanical harvesting may be the only way to stay competitive in the years ahead. In 1995 they initiated a massive harvesting improvement program. Inventors and manufacturers are funded by growers to develop an array of mechanical harvesting methods to meet the specifications of virtually any processed citrus grove.
The results show that it will be possible to reduce harvesting costs at roadside by as much as 25 to 75 percent over the next 10 years and to increase labor productivity by three to 15 times. High-density groves may be required to achieve the highest labor productivity and lowest cost. Adoption of commercial shake-catch harvesting has started and mechanization will increase as the labor supply shrinks and more types of harvesting systems are perfected.
Apples could be shake-catch harvested for juice, sauce, and canned or frozen slices. They were harvested mechanically in the 1970s, but fruit damage when harvesting standard and semi-dwarf trees caused storage decay, trimming cost, and product quality problems for the processors that still need to be solved.
Likewise, apricots, peaches, and olives (black) use mechanical harvesters on a very small scale, because of problems of non-uniform maturity, excessively bruised fruit and incomplete fruit removal (olives). Processors will need new very high speed and high volume automated sorting systems to quickly separate fruit with cuts, punctures, and large bruises from those that are basically undamaged. Smaller and appropriately trellised trees most likely will be required for productive and low-cost mechanical harvesting. All fruit crops destined for the fresh market (with the exception of cranberries) are hand harvested, sometimes selectively by size and color, and are not likely to be mechanically harvested commercially in the near future unless some drastic changes occur. New cultural and harvesting approaches are needed for these crops.
Harvest system development work for processing hot peppers, fresh market sweet corn, and fresh market snap beans has been sustained since the late 1970s by equipment manufacturers and growers. Some of the approaches were started in prior years by public agency research. Hand harvest costs were high, and these crops require a reliable timely harvest to provide optimum yield and grower profit. Today, these crops have feasible mechanical harvesting technologies available and will adopt them on more acreage as the labor supply shrinks.
Mechanization of harvesting for fruits and vegetables has progressed significantly since its initiation in the late 1930s. However, the present status shows that while nut crops are almost 100 percent machine harvested, it is estimated that about 20 vegetable crops and 25 fruit crops still lack feasible mechanical harvesting options today (Tables 1 and 2).
These crops total over 1.4 million acres of vegetables (20 to 25 percent of the U.S. total) and over 2.2 million acres of fruit (about 40 to 45 percent of the U.S. total). Labor aids are used to some extent for picking both fruits and vegetables, but they do not offer any saving in labor or cost reduction. Robotic harvesting, while quite impressive in its achievements, is still far from being feasible.
Some of the most significant fruit and vegetable crops that need new productive harvesting technologies appear to be:
- Fruit: apples, apricots, avocados, fresh sweet cherries, fresh grapes, kiwifruit, nectarines, olives, peaches, plums, pears, all citrus, fresh blueberries, and strawberries.
- Vegetable: asparagus, broccoli, cantaloupe, watermelon, cauliflower, eggplant, fresh cucumbers, fresh tomatoes, sweet pepper, fresh snap beans, lettuce, and squash.
In nearly every case with these crops, feasible mechanical harvesting systems have never been developed. Creating a successful mechanical harvest system for these crops will require many years of research and development. The first commercial-scale tomato harvesters, for example, weren’t available until 12 years after research began. Harvesting mechanization of fruits and vegetables is also closely tied with changes and improvements in plant breeding, cultural practices and growing techniques. For example, tree structure and machine component design need to be compatible. Dwarf, or semi-dwarf trees and cultural modifications would make it much simpler to hand harvest, employ mass harvesting (shake-catch systems) and even facilitate the operation of a robotic system. New varieties, new cultural practices, and pre- or post-harvest treatments could improve ripeness uniformity and decrease both the susceptibility to and consequences of produce damage.
Harvest Mechanization Is Needed
Although hand harvesting is still practiced widely in the United States, those fruit and vegetable growers who are dependent on hand harvesting are now facing two significant problems that could very well determine the future of their business — availability of labor and price competitiveness in the world markets:
The supply of workers for seasonal harvesting continues to shrink each year. U.S. Government authorities are enforcing the provisions of the 1986 Immigration Reform and Control Act to curtail the entry and employment of illegal foreign workers. In addition, there are fewer workers available to harvest because of a continuous outflow of workers from agriculture to better paying year-round jobs in other industries (10, 17). The 1992 report of the Commission on Agricultural Workers (21) cited several things that should be accomplished for the U.S. agricultural work force, but programs were not funded to make change occur. It had been already commonly accepted (although not a real effort made to circumvent it) that relegating man to the status of work animal is not a socially acceptable practice. Social issues and working conditions, while having been neglected in the past, are becoming serious considerations with added costs.
Domestic workers no longer seek seasonal agricultural jobs in sufficient numbers to fill the needs. The H-2A offshore labor program is an option for some growers who cannot hire enough domestic workers, but it is cumbersome and costly.
World Market Competition
The high costs of producing food in the United States, compared to the costs in less developed countries that can sell in the U.S. markets, are pushing American growers out of business. Foreign competitors, such as orange growers in Brazil, peach growers in Chile, apple growers in China, fresh tomato growers in Mexico, and olive growers in Italy and Greece, relying on a good labor supply and low wages, can easily sell their produce or processed products in American markets. These expanding supplies of imported produce, especially after the lifting of import tariffs, put downward pressure on market prices, thus further aggravating the competitiveness issue for American growers. If these trends continue, foreign fruit and vegetable growers with lower cost levels will increasingly be able to underbid American growers. As a result, the American growers will lose their profitability and be forced to shift into some other industry (17).
Nevertheless, underbidding by foreign suppliers should not be viewed as an impossible situation, but rather as a catalyst for improving the competitive position of the American growers and their employees.
Harvest labor productivity must be greatly increased so that production costs can decrease and worker income can increase. This is a key factor that the U.S. Government has been neglecting since 1979, when the U.S. Secretary of Agriculture of that time, Bob Bergland, stated, “I will not put federal money into any project that reduces the need for farm labor.” This policy supported an anti-mechanization movement that had brought a lawsuit against the University of California for using public funds to conduct mechanization research. The Court eventually dismissed all charges, except the need for a public interest representative on the project review committee. However, the Bergland policy has gradually ended the availability of public funding for research and development projects focused on reducing the cost and increasing the labor productivity for harvesting horticultural crops. Today, the USDA has only one poorly funded harvest mechanization project (13). Higher wages can be paid when workers are much more productive, but since 1979 the Bergland policy also has reduced that opportunity.
The produce industry is becoming more vigilant at ensuring bacterial safety of their products. Human contact with food can be a major source of pathogen contamination. Harvest machinery can potentially reduce human contact with food and reduce contamination possibilities.
Potential for Future Mechanization
Since the 1970s, when publicly funded research declined, there have been a number of technological innovations that may allow the increased use of harvest machinery. The capability of computers and sensor technology has developed rapidly. Machine sorting equipment is becoming available that may allow machine-harvested products to be automatically sorted for damage. This would allow product that is acceptable for the fresh market to be selected and packed. Product damaged in harvest could be diverted for processed uses.
Plant breeders have continued to develop varieties that may allow greater feasibility of mechanical harvest. New varieties mature more uniformly, have higher yields, and spread the harvest season over more months. All these changes improve the productivity of harvest machinery and allow capital costs to be spread over greater amounts of product. Many tree fruits are produced on smaller trees. This tends to reduce the number of branches that a fruit may contact and be damaged by a shake-catch harvest system.
One of the biggest changes in the produce business has been a great increase in the amount of vegetables sold as fresh-cut salad mixes and other trimmed and pre-cut products. This processing could allow removal of damaged portions caused by machine harvest. Fruits have been more difficult to adapt to this marketing approach, but a great deal of private enterprise and public institution research is being done to develop methods for marketing fresh-cut fruit.
How things will progress in the years ahead is hard to predict and past predictions have always proven to be wrong. There are some doomsday prophets who predict that the United States, while currently producing at its peak, is going out of the food business (1). Would this be also true for the fruit and vegetable industries that do not have feasible mechanical harvesting systems? There is almost a worldwide consensus that competitiveness is crucial to maintaining our agricultural industry. New technologies and mechanization appear to offer the only solution to significantly reduce production costs and maintain competitiveness.
U.S. growers who are facing the prospect of an inadequate hand harvest labor supply and resulting high costs appear to have four choices — maintain the status quo as long as possible, switch to less labor-intensive crops, leave agriculture voluntarily, or try to adopt/develop suitable mechanical harvesting systems.
Discussions with growers, packers, and processors should be held to determine what their needs are for improved harvesting labor productivity and harvesting cost reduction. National or state commodity boards or research committees exist for some of the crops. Unfortunately, all of the horticultural crops are regarded as “minor” parts of U.S. agriculture, whereas grains and animals are “important.” The reasons to use mechanical harvesting are the same today as they were in the past, and the justifications for commercial implementation are even stronger today. Action needs to be initiated soon, at the federal level, to help these commodities become labor efficient and farmers to become low cost producers.
Much of the research expertise in mechanical harvesting that was developed in the past 50 years has been lost, since most projects have been discontinued. Hence, considerable lead time will be required to develop facilities, hire personnel, and develop new programs if and when a consensus is reached that the fruit and vegetable industries in the United States would be able to survive and compete only if their harvesting operations are mechanized. It is interesting to note that, while the United States was in the past the leading country in the world in mechanical harvesting, the majority of the research work in recent years is conducted outside the United States. This is another factor that needs to be considered if the United States wants to remain competitive in the world.
History has shown that technology always kept American agriculture competitive in the world’s markets by keeping prices of commodities low in real terms. So what is appropriate today?
If mechanization will be recognized as the only viable approach for staying in business, a concerted effort, renewed funding, and focused R&D efforts on harvest mechanization research should be initiated. Individual development of such systems is financially impossible for most growers. Hence, public agencies and private companies should begin a sizeable investment program in mechanization, development of new plant varieties and growing practices adapted to mechanical methods, and packinghouse or processing plant technologies for horticultural crops. A model like the one used for the harvesting program now operated by the Florida citrus growers may work well for growers of other crops, as well as for the USDA.
A transition period from labor intensive conventional technologies to labor efficient new production and harvesting technologies will be required. An adequate legal labor supply for seasonal harvesting work will be essential for at least the next 10 years, or until feasible mechanical harvesting systems are realities for these crops. Labor costs will escalate during this period, due to labor shortages and government regulations.
Technology has the potential to provide the answers to most of the challenges associated with mechanical harvesting, while offering in the long run lower or more stabilized prices, reduced human physical effort and drudgery, and better jobs with higher income. Maintaining the status quo for several more years will not be in the best interests of U.S. growers, workers, or consumers.
1 Blank, S.C. 1998. The end of agriculture in the American portfolio. Quorum Books, Westport, Conn.
2 Brown, G.K. 1980. “Harvest mechanization status for horticultural crops.” ASAE Paper No. 80-1532, presented at the 1980 ASAE Winter Meeting, Chicago, Ill.
3 Brown, G.K., D.L. Peterson, and J.H. Levin. 1983a. “Tree fruit harvesting systems: Individual trees,” P. 575-587. In: O’Brien, Cargill and Fridley (Eds.), Principles and Practices for Harvesting and Handling Fruits and Nuts. AVI Publ. Westport, Conn.
4 Brown, G.K., D.E. Marshall, B.R. Tennes, D.E. Booster, P. Chen, R.E. Garrett, M. O’Brien, H.E. Studer, R.A. Kepner, S.L. Hedden, C.E. Hood, D.H. Lenker, W.F. Millier, G.E. Rehkugler, D.L. Peterson, and L.N. Shaw. 1983b. “Status of harvest mechanization of horticultural crops.” ASAE Spec. Publ. 3-83.
5 Brown, G.K. 1985. “Fruit and vegetable mechanization.” In: Martin (Ed.). Migrant labor in agriculture, an international comparison. Giannini Foundation of Agricultural Economics and the German Marshall Fund of the United States, Washington, D.C.
6 Dale, D. 1999. Farmers offering incentives to keep needed workers in fields. California-Arizona Farm Press 21(2):26-28.
7 Fridley, R.B., L.L. Claypool, and P.A. Adrian. 1969. “Statistical summary of harvest mechanization outlook - apricots, peaches, pears, plums, and prunes.” P. 751-754. In B.F. Cargill and G.E. Rossmiller (Eds.). Fruit and vegetable harvest mechanization: technological implications. Rural Manpower Center, Mich. State Univ., RMC Rpt. 16.
8 Martin, P.L. and R. Mines. 1983. “Immigration reform and California agriculture.” Calif. Agr. 37(1&2):14-15.
9 Martin, P. and A. Olmstead. 1985. “The agricultural mechanization controversy.” Science 227:601.
10 Mines, R. 1991. “The future of farm labor in the 1990’s, the horticultural, fruit and vegetable industries.” Labor Outlook and Mechanization Research Void. ASHS Workshop, Pennsylvania State University, State College, Pa.
11 Perry, R.L. 1965. “Harvesting aids and the outlook for mechanical harvesting.” Calif. Citrogr. 51:61-70.
12 Peterson, D.L. and S.S. Miller. 1988. “Advances in mechanical harvesting of fresh market quality apples.” J. Agric. Engng. Res. 42:43-50.
13 Peterson, D.L. 1992. “Harvest mechanization for deciduous tree fruits and brambles.” Hort. Tech. 2(1):85-88.
14 Sarig, Y. and G.E. Coppock. 1988. “Harvesting techniques.” P. 205-226 In: Wardowsky, Nagy and Grierson (Eds.) Fresh Citrus Fruit. AVI Publ. Westport, CT.
15 Sarig, Y. 1993. “Robotics of fruit harvesting: a state-of-the-art review.” J. Agric. Engng. Res. 54:265-280.
16 Thompson, J.F. and Blank, S.C. 1999. “Harvest mechanization.” Calif. Agric.
17 Torres, A. 1988. “Impact of the immigration reform and control act of 1986 on California agriculture.” Rept. Joint Com. Refugee Resettlement, Int’l. Migration and Coop. Dev.
18 USDA. 1964. “Labor used to produce vegetables, estimates by states, 1959.” Stat. Bull. No. 341, Econ. Res. Serv., Wash., DC 20250.
19 USDA. 1969. “Farm labor used for fruits and tree nuts, 1964.” Stat. Bull. No. 436, Econ. Res. Serv., Wash., DC 20250.
20 USDA-NASS. 1997. “United States Crop Rankings - 1997 production year, Citrus Fruits 1997-98, Non-Citrus Fruits and Nuts Final Estimates 1992-97, Vegetables Final Estimates 1992-97,” http://www.usda.gov/nass.
21 US Gov’t. 1992. Report of the Commission on Agricultural Workers. 188 pp. Library of Congress, serial and gov’t pubs. 101 Independence Ave., Wash., DC 20504.
22 Zahara, M. And Johnson, S.S. 1979. “Status of harvest mechanization of fruits, nuts, and vegetables.” Hort. Sci. 14(5):578-582.
23 Zepp, G.A., R.K. Conway and F.L. Hoff. 1985. “Trade patterns in fruits and vegetables.” In: Martin (Ed.), Migrant labor in agriculture, an international comparison. Giannini Foundation of Agricultural Economics and the German Marshall Fund of the U.S., Wash., DC.
Yoav Sarig, Agricultural Research Organization, Israel; James F. Thompson, Department of Biological and Agricultural Engineering, University of California, Davis; Galen K. Brown, Florida Department of Citrus