Evaluation of main-crop stubble height on ratoon rice growth and development☆
Introduction
The production of a second rice crop in one cropping season is known as ratooning. The ratoon crop develops by regenerating rice tillers from nodal buds of the stubble that was left behind after the first seasonal rice harvest. Grain yield of the ratoon crop is generally one-third of the yield realized in the first, or main, crop (Bollich and Turner, 1988, Turner and Jund, 1993). Ratooning in the USA has been practiced since the early 1960s (Jones, 1993) and has become more attractive to producers during the past decade due to newly released rice varieties and hybrids, which mature earlier and produce higher yields than older cultivars. Ratooning is practiced in the USA predominantly in the semi-tropical rice-producing regions along the Gulf Coast of Louisiana and Texas. The growing season in these areas is longer than the higher latitude, mid-southern USA rice-producing areas, which allows ample time for a ratoon crop to develop and mature prior to the onset of cooler weather in most years. Along the Gulf Coast, rice ratooning is very important because additional rice yields can be achieved with minimal agricultural inputs (Bollich and Turner, 1988, Bond and Bollich, 2006b). Management of the ratoon rice crop differs from that of the first rice crop because chemical inputs such as herbicides, insecticides and fungicides are seldom used. Generally, the only costs associated with ratooning include 84–100 kg ha−1 nitrogen fertilizer (Bond and Bollich, 2006a, Bond and Bollich, 2006b, Saichuk et al., 2007), irrigation, harvest, and grain drying (Linscombe et al., 1999, Tarpley et al., 2008).
Cultural practices and condition of the main crop at harvest impact the growth and development of the ratoon crop (Jones, 1993, Jones and Snyder, 1987). During the growth and development of the main rice crop, various starches and sugars are accumulated in the leaves and culm of the rice plant which are subsequently translocated to the developing grain after flowering (Yoshida, 1972, Murata and Matsushima, 1975, Turner and Jund, 1993). It has also been postulated by others that a second accumulation may occur near main crop maturity in the culms (Bollich et al., 1982) which may attribute to initial ratoon growth and development. Therefore, ratoon-specific agronomic practices applied at or after main-crop harvest may have the potential to alter ratoon growth parameters and increase grain yields and have become increasingly important to rice producers along the Gulf Coast. Ratoon-specific practices include harvesting the main crop at a lower-than-normal height in order to produce shorter ratoon stubble, harvesting the main crop at a normal height followed by flail-mowing the stubble to lower heights, or rolling the main-crop stubble following harvest. Of the aforementioned practices, harvesting the main crop with the combine platform at a lower height is an operation that all producers could implement without purchasing additional equipment or making an additional trip over the field. Main-crop harvest heights are cultivar dependent and generally range between 40 and 45 cm above ground level. Lower harvest heights leaving stubble of 20–25 cm are currently recommended in Texas as a potential ratoon yield enhancement strategy when the main crop is harvested before August 15 (Tarpley et al., 2008).
Previous research on the effects of main-crop stubble height on ratoon production has produced mixed results. A study by Jones (1993) found that harvest heights resulting in 20- and 30-cm main-crop stubble produced higher ratoon yields than harvest heights resulting in 40- and 50-cm stubble from both semi-dwarf and conventional-height rice cultivars. The increase in yield was attributed to more filled grains panicle−1 on the shorter stubble since the number of panicles m−2 was not affected by main-crop harvest height. Others have also found that lower-than-normal main-crop harvest heights can enhance yields (Balasubramanian et al., 1970, Bahar and De Datta, 1977, Jund et al., 2002). Tarpley and Jund (2007a) recently reported that reducing main-crop stubble height to 20–25 cm improved ratoon yields of ‘Cocodrie’ in 3 years at two locations. Ratoon yields during the 3-year period were 440 and 1425 kg ha−1 higher at the Eagle Lake and Beaumont locations, respectively. Conversely, harvest heights resulting in main-crop stubble of 30 cm (Bond et al., 2006, Harrell et al., 2007, Harrell et al., 2008) and 15 cm (Bond et al., 2005) have not shown a yield advantage as compared with traditional harvest heights for currently grown rice inbred and hybrid cultivars. Bollich et al. (2004) indicated that yield was increased at one of two locations when the main-crop stubble was reduced to 16–20 cm. Similarly, in a study with rice hybrids, Tarpley and Jund (2007b) reported that main-crop stubble heights of 20–30 cm increased ratoon yields of ‘XL723’ at one location but not at another when a fungicide was not used. In addition, yield of the imidazolinone-tolerant hybrid ‘CLXL729’was not improved at either location with a reduced main-crop stubble height. Previous ratoon stubble height research has focused primarily on grain yield while additional effects on panicle point of origin over the ratoon developmental period have not received ample attention.
While yield results of reduced main-crop stubble heights seem to vary by location, some studies (Prashar, 1969, Jones, 1993, Bond et al., 2005) and published literature (Tarpley et al., 2008) do indicate that reducing main-crop stubble heights increases time to ratoon grain maturity from 1 to 10 days. In years when the main-crop harvest is later than August 15, thus reducing available ratoon growing time until cooler weather, the delay in maturity could potentially be detrimental and would not be recommended (Tarpley et al., 2008). Other factors affected by reducing combine platform height during main-crop harvest include increased harvest time, increased wear on harvest equipment, and reduced harvest efficiency; however, the latter has been greatly reduced with newer combines (Steve Linscombe and Johnny Saichuk, personal communication 2009).
Ratoon tiller point of origin can differ among rice cultivars. Hoff and Bollich (1990), using a standard 40-cm cutting height, reported that ‘Jasmine 85’ ratoon tillers emerged basipetally (beginning from nodes at the top of the stubble and progressed downward toward the base) and that 60% of the tillers were from axillary nodes. ‘Tebonnet’ ratoon tillers originated from basal nodes 84% of the time, while ‘Maybelle’, ‘Skybonnet’, ‘Gulfmont’, ‘Mars’, ‘Rico’, ‘Katy’, and ‘Lemont’ ratoon tillers originated from near-basal nodes 64, 57, 56, 55, 96, 74, and 62% of the time, respectively. Ratoon panicles were also shown to decrease in size as the point of origin moved acropetally (beginning from the base and progressing upward) on the main-crop stubble. While ratoon tiller origin has been shown to vary by cultivar, little is known about the relative effects of low main-crop harvest height compared with standard harvest height on ratoon-crop growth and development of currently grown semi-dwarf rice cultivars. The objectives of this study were to evaluate the effects of main-crop stubble height on ratoon-crop grain yield, agronomics, and cumulative/weekly panicle growth parameters (density, point of origin, and weight).
Section snippets
Materials and methods
Field trials were conducted in 2006 and 2007 to evaluate the effect of main-crop rice harvest height on ratoon rice growth and development. The trials were established at the Louisiana State University AgCenter's Rice Research Station south farm location, just south of Crowley, Louisiana. Soil type both years was a Crowley silt loam (fine, smectitic, thermic Typic Albaqualfs). Both fields had been in a rice-fallow rotation system for at least 25 years. Representative composite soil samples at a
Crop growth results
Marked differences in main- and ratoon-crop development occurred during 2006 as compared with 2007 (Table 1). In 2006, low soil temperatures delayed emergence and early crop development which lead to a 3-week difference in main-crop harvest dates between the 2006 and 2007 seasons. This delay, coupled with the onset of cooler fall temperatures earlier in 2006, further prolonged growth and development in the 2006 ratoon crop as compared with 2007.
Crop growth as reflected in plant height varied
Conclusions
Rice producing areas in the USA located along the Gulf Coast have longer growing seasons than other mid-southern rice-producing areas. This allows a ratoon rice crop to be produced. In these areas, rice ratooning is important because the total rice yield per area can be increased with minimal additional agricultural inputs. An increase in ratoon grain yield due to harvesting the main crop at a lower-than-normal height increased ratoon yields of Cocodrie and Trenasse in only 1 year of a 2-year
Acknowledgments
This research was funded in part by the Louisiana Rice Research Board. Special thanks to Richard Dunand, now retired, and research associates James Leonards, Ron Regan and Russell Dilly for their contributions to this research.
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Published with the approval of the Director of the Louisiana Agricultural Experiment Station as publication number 2009-266-3684.