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Rice Hybrids: Yield and Stability Comparisons

Rice plot
figure 1.
figure 2.
jupiter flower

Sterling Brooks Blanche and Steven D. Linscombe

Since their introduction into the U.S. rice market, hybrid rice varieties have been grown on an increasing amount of Louisiana acreage. As a group, they have demonstrated excellent yield potential and disease resistance but lower milling yields and an increased incidence of lodging – or falling over before harvest. Improvements in yield potential, grain quality, milling quality and plant characteristics have resulted in increased adoption by Louisiana and other Midsouth rice producers. The expansion of hybrid rice acreage has been limited, in part, because hybrid seed is much more expensive than pureline varietal seed, and significantly higher yields are necessary for the economics to favor hybrid rice production.

On the genetic level, hybrid and varietal rice varieties are entirely different. Hybrid rice is created by fertilizing the female organs of one plant with the male pollen of a separate, genetically different plant. The resulting seed is harvested as uniform first-generation F1 hybrid seed containing half of its genetic material from each parent. Hybrid plants with different alleles (portions of chromosomes) from two distinct parents are referred to as heterozygous.

Pureline rice varieties are developed by crossing two distinct parents. Subsequent inbreeding is performed until the lines breed true, at which point they are considered to be homozygous inbreds. Homozygotes typically have identical alleles on both chromosomes. For these reasons, hybrid rice plants segregate in subsequent generations and yield a tremendous amount of variation in the progeny while varietal rice varieties remain similar as subsequent generations are grown.
Research in other crops has indicated that heterozygosity compared with homozygosity provides a stability advantage in terms of broad adaptation to multiple environments. Field studies were conducted at the LSU AgCenter Rice Research Station in 2005 and 2006 to compare rice hybrids and varieties for grain yield and milling quality to test this phenomenon in rice. For the experiment, stability was an estimate of how consistently varieties yielded across a diverse range of environments. Multi-location trials were conducted in 15 environments including the Rice Research Station and on-farm test sites in Evangeline, Vermilion, Acadia and Jefferson Davis parishes. A total of 13 varietal varieties (long-, medium- and short-grain rice) and two hybrid rice varieties were included in the analysis. Researchers were interested in both the mean performance and stability of rough rice yield and the percentage of whole, milled rice.
Grain yield of all varieties ranged from 6,759 to 9,902 pounds per acre. The two hybrid rice varieties, XP723 and CLXL730, had significantly higher rice yields than all inbred varieties (Figure 1). Jupiter, a medium-grain variety, was the highest-yielding of the varieties. For stability evaluations, the coefficient of variation (CV) was used to estimate the yield variation associated with each variety. A low CV suggests consistency across the 15 environments.

In evaluations of both grain yield and stability, hybrids were more desirable because of high grain yields and low CVs. Jupiter, with high yield potential and a relatively low CV, was the best option among varieties.

Whole, milled rice percentage is the proportion of whole, unbroken rice kernels remaining after the dehulling and milling processes. It affects the value of a rice crop. The highest milling yields were observed with the inbred varieties, and CL161 was among the best for high milled-rice percentage. The CV of CL161 was among the lowest in the test, suggesting it is a good candidate for high and stable milling yields. Milling yields of the hybrids were among the lowest and most variable in the study, particularly for CLXL730.

The hybrid varieties XP723 and CLXL730 exhibited higher rough rice yield than traditional inbred varieties and were consistent across the 15 environments included in the study. Their whole milled rice percentages, however, were among the lowest of all varieties. This suggests that to obtain maximum milling yields with hybrid rice varieties, harvest at the optimum time and moisture level is critical. A stable-milling variety, such as CL161, may be more resilient if inclement conditions prevent a timely harvest.

Ultimately, the profit potential of a hybrid or an inbred variety is the most important characteristic for rice producers. It should be noted that hybrid rice seed is more expensive than pureline variety seed; thus, a large yield advantage is necessary for hybrids to be economically competitive with rice varieties. Each system has advantages and disadvantages, and the decision should ultimately be based on specific variables such as planting date, the likelihood for a successful second crop and anticipated disease pressure. For example, hybrids are more likely to shatter (shed their grain prior to harvest) and lodge – which reduces yield, grain and milling quality, and harvest efficiency – during strong winds, whereas losses would be less in varietal rice after such an event. In addition to grain yield, economic factors such as price premiums or discounts because of grain and milling quality must be considered as well.

This study characterized rice varieties, including homozygous varieties and heterozygous hybrids, for rough rice yield and whole, milled-rice percentage and stability. Use of the CV provided a good understanding of the performance and level of stability of hybrid and varietal rice varieties across a diverse range of environments for two economically important traits.

Sterling Brooks Blanche, Assistant Professor, Dean Lee Research Station, Alexandria, La.; and Steven D. Linscombe, American Cyanamid Professor, Rice Research Station, Rayne, La.

(This article was published in the summer 2010 issue of Louisiana Agriculture.)

Last Updated: 9/29/2010 9:34:36 AM

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