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Immokalee Report Fritz Roka and Robert Rouse, Southwest Florida Research and Education Center For more than 40 years, Florida citrus growers have looked to the Savage yield tables as benchmarks of production performance. Zach Savage, an agricultural economist at the University of Florida, compiled yield statistics from grower records to develop yield tables for seven categories of citrus – early, mid, and late season oranges, seedy and seedless grapefruit, tangerines and Temple oranges. Savage’s tables plotted average production (boxes per tree) against tree age. The yield tables reflected the deep sand conditions of central Florida and tree densities of between 48 to 70 trees per acre. So long as growers faced similar conditions, the Savage yield tables provided a valuable reference from which to make reasonable production projections. Severe freezes during the 1980s devastated much of the central Florida citrus region. Consequently, 20% of the industry shifted into southwest Florida where acreage increased 175% from 65,000 acres in 1984 to more than 179,000 acres in 1996. Many of the growers who initiated these citrus plantings had little or no experience with the flatwood soils and general growing conditions of southwest Florida. Further, tree densities in the new plantings were increased to an average of 150 trees per acre. Growers realized that the Savage yield tables were no longer appropriate for their circumstances. In particular, growers were concerned with the long term yield expectations from the higher tree density plantings. Trees formed hedgerows at an earlier age, suggesting that the Savage tables could not accurately represent the increase in per tree yields. Through the Citrus Advisory Committee at the UF/IFAS Southwest Florida Research and Education Center, growers requested that new yield benchmark tables be developed to represent southwest Florida conditions. This article summarizes the results of a study begun four years ago. Several growers in southwest Florida were solicited to provide yield information on selected blocks. The size of a block corresponded to a harvesting unit on which yield information was conveniently recorded. Only blocks that had been planted since 1985 and have been uniformly managed to a single rootstock / scion combination were selected. Eight scion-rootstock combinations were selected for data collection – Hamlin, Valencia, Rohde Red Valencia, and grapefruit varieties on Swingle and Carrizo rootstocks. In addition to the annual yield information, cooperators filled out a data sheet for each block, describing the predominate soil type, tree spacing, bed configuration, type of irrigation, and general fertilization practices. To date, nine cooperators have provided annual yield information on 65 blocks, representing more than 4,000 acres. Tree ages range from four to ten years. Table 1 presents the average boxes per acre and pounds solids per acre by tree age. To a large extent, seasonality has been removed by averaging together the production by tree age of any block that had been planted since 1985. As yet, the data has not indicated any significant yield differences attributable solely to Swingle or Carrizo rootstocks. Therefore, the yield summaries presented in Table 1 have not been separated by rootstock. Until age four, the data is showing little yield difference between Hamlins and Valencias, both in terms of boxes and pounds solids per acre. After age five, Hamlins outproduce Valencias. By the seventh year, Hamlins produce more than 120 boxes more than Valencias. The differences are not as pronounced when yields are measured in terms of pounds solids per acre. Hamlins, between seven and nine years old, produce on average 2,900 pound solids per acre. The same aged Valencia trees produce nearly 2,600 pound solids per acre. Setting a reference date from which to mark tree age was a topic of considerable discussion among citrus horticulturalists at the Immokalee and Lake Alfred IFAS centers. For this study, May 1st was selected. Any trees planted between 1 May and 30 April of the following year were considered to be the same tree age. Changing the reference date earlier or later will alter the ages of several blocks, thereby influencing the expected yields of young trees. However, it is expected that by the fifth year of production any yield differences dependent on the reference date should be minimal. So long as the cooperators are willing, the study will march onward. With each new harvest season, Table 1 expands by one row, reflecting another year in the tree age- yield profile. All the yield data summarized in Table 1 comes from Hendry and Collier Counties. Work is ongoing to recruit additional cooperators and expand the study area, not only within southwest Florida, but also to include Indian River and central Florida. In addition, work is underway to correlate soil differences and cultural practices with the observed yields. Two research efforts, both funded by the Florida Citrus Production Research Advisory Council, are utilizing these data. One effort, headed by Tom Spreen and Ron Muraro, is investigating tree density and hedging practices on overall yield. The second project, headed by Bill Castle and Gene Albrigo, is comparing yields and tree size with soil types. Table 1: Boxes per acre and pounds solids per acre by tree age from Hamlin and Valencia trees.
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