Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2004
Publication Date: 4/27/2005
Citation: Gesch, R.W., Cermak, S.C., Isbell, T., Forcella, F. 2005. Seed yield and oil content of cuphea as affected by harvest date. Agronomy Journal. 97:817-822.
Interpretive Summary: Cuphea is a plant whose seed is rich in oil similar to that of tropical plants such as palm. The U.S. imports billions of pounds of palm oil to manufacture soaps, detergents, personal care products and industrial lubricants. On the other hand, cuphea can be grown in the U.S. and serve as a replacement for imported oil. However, because cuphea has only been domesticated a short while, it still has some bad traits that make it difficult to grow on a farm. For instance, a single plant can produce as many as 30 to 40 flowers, but they do not all come out at the same time, a characteristic called "indeterminacy." This means that, likewise, the seed from all of those flowers will not ripen at the same time. To make matters worse, the oldest flowers lose their seed if they stay on the plant too long, which is called "shattering." Because of these problems, it is difficult to know exactly when to harvest cuphea in order to get the highest seed yield and amount of oil from the seed. That is why we conducted a field experiment at Morris, Minnesota to determine the best time to harvest the varieties of cuphea that we currently have available. We found that the best time to harvest cuphea to get the greatest amount of seed and oil was within about the last week in September to the end of the first week in October when it was planted in mid May. More work, however, is needed to determine the best method for harvesting cuphea that will reduce the amount of seed lost to shattering. For the future, cuphea breeders need to concentrate their efforts on selecting new varieties that flower at the same time (determinate) and do not shatter their seed.
Technical Abstract: Cuphea (spp.) can serve as an oilseed crop substitute for small and medium-chain triglycerides, which are in high demand for chemical manufacturing. Domesticated genotypes of cuphea show good potential for agricultural production in the upper Midwest. Cuphea has an indeterminate growth habit and shatters seed if left in the field too long. Little information exists on when to harvest cuphea to obtain optimum seed yield and oil content. Therefore, a study was conducted on a Barnes soil in west central Minnesota to determine the best time to harvest cuphea when sown at an optimum time in the spring. Harvests were taken at one to two week intervals from mid Aug. through about mid Oct. during 2001 and 2002. Seed yields were greatest within a relatively narrow window of about 15 d in late Sept. to early Oct. Soon after a hard frost (less than or equal to -2 deg C), which occurred DOY 278 and 282 in 2001 and 2002, respectively, yield declined sharply at a rate of 11.6 kg ha**-1 d**-1, likely due to increased shattering. However, shattering due to mechanical harvesting was greater than that from natural causes. Total seed oil content was also influenced by harvest date. Across years, oil content averaged 247 g kg**-1 in August, increasing to 304 g kg**-1 by late Sept. before remaining stable. For greatest seed yield and oil content the optimum time to harvest cuphea is in late Sept. to early Oct. in west central Minnesota. However, until more shatter resistant, determinate genotypes are developed, improved harvest management is needed to reduce shatter-induced yield loss.