Gibberellin Applied to the Rice Main Crop Increases Ratoon-Crop Yield

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Elliott W. Rounds, Graduate Student, Abdul R. Mohammed, Graduate Student, and Lee Tarpley, Assistant Professor, 1509 Aggie Drive, Agricultural Research and Extension Center, Texas A&M University, Beaumont, TX 77713

Abstract

Ratoon rice (Oryza sativa L.) refers to the second crop made from the regrowth following main-crop harvest. The ratoon crop is receiving increasing attention in US rice production systems due to the increasing prevalence of early-maturing genotypes with good ratoon-yield potential, and the economic pressure on producers to maximize seasonal yields. Field studies were conducted in 2002 through 2005 to evaluate the influence of gibberellin (GA) applied to the main crop on ratoon-rice yields. Gibberellin at 10 g ai/ha was applied when the main-crop was between 3 days post-flowering and soft-dough. Various cultivars were tested in field research plots at two locations in Texas. The data were analyzed using a paired t-test of medians to evaluate GA-treated plots compared to nontreated plots. Main-crop yields were not influenced by application of GA, while ratoon-crop yields were increased 264 kg/ha by the GA application. Ratoon-crop yields were higher in GA-treated plots vs. nontreated plots in 14 of 17 studies. The very early-maturing vigorous hybrids responded strongly to GA treatment, with average increases in ratoon yield due to the GA treatment of 697 kg/ha. The GA treatment slightly delayed main-crop development.

Introduction

Rice (Oryza sativa L.) production practices along the US Gulf Coast, including southeast Texas, southwest Louisiana, and parts of Florida, have long included ratoon cropping because the relatively long growing season is favorable for growth of the ratoon, or regrowth, rice. With the increased use of early maturing cultivars, and with increasing economic pressure on producers to maximize seasonal yields, ratoon-crop production is receiving attention in other US rice-growing regions. Usually ratoon rice is cut 60 to 90 days after main-crop harvest, and yields reach nearly 50% of the main crop (8). Previously, rice producers noted that ratoon production is limited by inconsistent stands and uneven maturation of the ratoon crop. A shorter cutting height of the main-crop stubble along with higher-input ratoon-crop management has helped improve ratoon-crop yield, probably by alleviating these inconsistencies through enhancing uniformity among the ratoon tillers. The lower cutting height, however, increases the developmental period of the ratoon crop (6).

Gibberellin is a plant hormone that promotes cell elongation related to stem growth (9). The rice cultivar ‘Jodon’ showed increased plant height, internode length, and stem elongation in response to GA application (4). At the seedling stage of the rice cultivar ‘Tebonnet,’ height was increased by enhanced leaf elongation, as attributed to a reduction in the leaf blade to sheath length ratio (2). Gibberellin can also stimulate seed germination and pre-emergence seedling vigor and is often used as a seed treatment. Seedlings of GA-treated ‘Lemont’ rice emerged two days earlier than the nontreated control seedlings due to increased mesocotyl elongation (1). Furthermore, ‘Cypress’ and ‘Lemont’ rice plants demonstrated increased seedling height and enhancement of seedling vigor in response to GA applications (3,5).

Gibberellin, furthermore, is considered by EPA to be exempt from tolerances at rates considerably higher than the application rates used in the above cited studies, as well as those proposed in this manuscript, and therefore has the potential for rapid adoption in novel management situations.

Gibberellin application in late development of the main crop was hypothesized to increase ratoon-crop yield, by promoting ratoon-tiller vigor and early establishment of the ratoon-crop stand. The objective of this study was to determine if GA application at post-flowering to mid-grain fill of the main crop can increase ratoon-crop yield in field-grown rice without effects on main-crop yield.

Plant Material

This was a 4-year field study conducted in 2002 through 2005 at the Texas A&M Agricultural Research and Extension Center located at Beaumont, Texas (29°57’N, 94°21’W) and at Eagle Lake, Texas (29°37’N, 96°21’W). The soil type at Beaumont was a Beaumont clay soil (fine, montmorillonitic, thermic, Entic Pelludert). At Eagle Lake in 2002, 2003, and 2005, the soil type was a Nada fine sandy loam (fine-loamy, siliceous, active, hyperthermic Albaquic Hapludalfs). In 2004 at Eagle Lake, the soil type was an Edna fine sandy loam (fine, smectitic, hyperthermic Aquertic Chromic Hapludalfs).

At each location, one to several cultivars were grown in each small field. Each small field was contained within levees; the levees contain the water level at the desired level without affecting neighboring fields. Within each cultivar in a field, research plots were randomly assigned to the chemical treatment. Each treatment was replicated 4 to 6 times.

The cultivars evaluated during the various growing seasons were ‘Cheniere,’ ‘CL131,’ ‘CL161,’ ‘Clearfield XL8’ (CL-XL8), ‘Cocodrie,’ ‘Cypress,’ ‘Jefferson,’ ‘Presidio,’ ‘Saber,’ ‘XL7,’ and ‘XL723.’ Table 1 provides the locations and years in which the particular cultivars were evaluated, as well as the parameters measured.

Table 1. Cultivars used for studies by year, location and parameters measured.

Cultivar	Year	Location	Measured parameter
Cultivar	Year	Location	Main and ratoon crop yields	Milling
Cheniere	2003	Eagle Lake	Yes	No
Cheniere	2004	Eagle Lake	Yes	Yes
Cheniere	2005	Eagle Lake	No	Yes
CL-XL8	2004	Eagle Lake	Yes	Yes
CL-XL8	2005	Eagle Lake	No	Yes
CL131	2005	Beaumont	Yes	No
CL161	2003	Eagle Lake	Yes	No
CL161	2004	Eagle Lake	Yes	Yes
CL161	2005	Eagle Lake	No	Yes
Cocodrie	2002	Eagle Lake	No	Yes
Cocodrie	2002	Beaumont	No	Yes
Cocodrie	2003	Eagle Lake	Yes	No
Cocodrie	2004	Eagle Lake	Yes	Yes
Cocodrie	2005	Eagle Lake	No	Yes
Cocodrie	2005	Beaumont	Yes	No
Cypress	2005	Beaumont	Yes	No
Jefferson	2004	Eagle Lake	Yes	No
Jefferson	2005	Beaumont	Yes	No
Presidio	2005	Beaumont	Yes	No
Presidio	2005	Beaumont	Yes	No
Saber	2002	Beaumont	No	Yes
XL7	2002	Beaumont	No	Yes
XL7	2003	Beaumont	Yes	No
XL7	2004	Eagle Lake	Yes	No
XL723	2005	Beaumont	Yes	No

The fields at the Beaumont location were planted with a Tye Pasture Pleaser 8-row drill (Tye Co., Lockney, TX) on 0.22-m (8.5") row spacing. In 2003 and 2004, the plot size was 9.9 m², while in 2005 the plot size was 8.4 m². The main crop was harvested on the center 6 rows of each research plot and the ratoon crop was harvested on the center 4 rows using a Massey Ferguson 8XP grain combine (AGCO Parts Division, Batavia, IL) with a 1.5-m grain header.

At the Eagle Lake location, cultivars were planted with a Kincaid cone, 10-row, grain drill (Kincaid Equipment Manufacturing, Haven, KS) on 0.19-m (7.5") row-spacing. The plot size was 9.3 m². The main-crop harvest was on the center 8 rows using a Massey Ferguson 8XP (AGCO Parts Division, Batavia, IL) with a 1.5-m grain header. Ratoon-crop harvest was on the center 4 rows of each research plot using a Kubota RX 1300 spiketooth combine (Kubota Tractor Corp., Torrance, CA) with a 0.76-m grain header.

Planting dates, fertilizer inputs and other cultural inputs were those recommended in the Texas Rice Production Guidelines (10) and, for hybrids, by RiceTec, Inc. (Alvin, TX).

Chemical Applications

Treatments were applied as early as several days after peak flowering of the main crop and as late as soft dough. The GA used in this study is commercially available as Release and RyzUp from Valent BioSciences Corporation (Libertyville, IL). The rate used was 10 g ai/ha with 0.5% (v/v) Latron AG-98 Spreader Activator (Rohm and Haas Co., Philadelphia, PA), applied with a backpack sprayer. The propellant used was CO₂ at 103 to 117 kPa (15 to 17 psi). There were four nozzles with 0.41-m (16") spacing. The nozzle tips used in 2003 and 2004 were TeeJet 8002VS, and in 2005 the nozzle tips used were TeeJet LP11001VS, with TeeJet Strainer and Check Valve with 100-mesh brass body (R&D Sprayers, Opelousas, LA). The sprayer was calibrated to deliver 93.5 liter/ha (10 gpa).

Harvest and Post-Harvest Measurements

Yields were calculated based on 120 g/kg grain moisture. Ratoon-crop yields as a proportion of main-crop yields were determined at the individual plot level by dividing the ratoon-crop yield into the main-crop yield.

Grain moisture at harvest was determined through the use of a Dickey John GAC 2000 grain moisture computer (DICKEY-john Corporation, Auburn, IL) (Eagle Lake) or a Dickey John GAC II (Beaumont). The instrument was previously calibrated from 100 to 400 g/kg grain moisture against the oven-drying method.

At Eagle Lake, rough rice samples in excess of 125 grams (typically 300 g) were collected from the combine sample at harvest. These rough rice samples were placed in labeled open paper bags in an air conditioned room with good air circulation to dry to 12% grain moisture. At Beaumont, the combine samples were forced-air dried to 120 g/kg grain moisture while in cloth bags. Subsamples were then collected, and stored in airtight containers at 22°C until milled.

Milling quality was determined by analyses of selected cultivars and years (Table 1). At Eagle Lake, the rough rice sample was aspirated and 125 g of rough rice weighed out for the milling sample. The rough rice milling sample was placed in a McGill #2 rice mill (H. T. McGill, Houston, TX) with the weight arm and weight totaling 2136 g. The weight was placed at 153 mm from the pivoting pressure plate attachment point, and the mill run for 30 s. The milled samples were then placed in baggies for 24 h to allow equilibration. Milled samples were then weighed and weights recorded for "total" milling weight calculations. Samples were then "sized" to remove broken kernels using a #10 (3.5 mm) plate size in a shaker. Whole kernel weights were then recorded for milling calculations. At Beaumont, milling was also performed on rough rice using a McGill #2 mill. The remaining procedures were similar to those at Eagle Lake.

Statistical Analysis

The effects of GA treatment on main-crop yield, ratoon-crop yield, and ratoon-crop yield as a proportion of main-crop yield were evaluated as paired t-tests. The median yields from GA-treated plots were compared to those from nontreated plots across year, location, and cultivar using the methods described by Ott and Longnecker (7). The effect of GA treatment on milling quality (percent total milled and percent whole kernels) was analyzed by ANOVA, in which GA treatment was randomized within cultivars for each year and location combination, using the general linear model procedure from SAS version 9.1.3 (SAS Institute Inc., Cary, NC). A Wilcoxon signed ranks test was used to evaluate the significance of the number of studies in which ratoon yield was increased by GA treatment. A separate supporting data analysis was used to compare the 2005 Eagle Lake data of percent whole kernels and grain moisture content at harvest. In this analysis, the medians from the GA-treated plots were compared to those of the nontreated plots to see which was greater for each cultivar, then the likelihood of these results being obtained across the six studies was determined using simple probability calculations. In all analyses, the differences were evaluated for significance at α < 0.05.

The 2005 Eagle Lake ratoon-crop data were omitted from the analyses because there was a period of unusually high temperatures in the region that occurred after Hurricane Rita during ratoon-crop development. The ratoon-crop yields at the Eagle Lake site were consistently lower than usual for all studies at the station (Garry McCauley, Texas Agricultural Experiment Station, Eagle Lake, personal communication, 2006). However, the 2005 Eagle Lake main-crop milling data were used in the analysis because the high temperatures did not influence the main-crop milling quality data. The data from the 2005 main crop at Eagle Lake were also used for a supporting analysis examining variation in grain moisture content at harvest between GA-treated and nontreated plots.

Effect of Gibberellin on Grain Yield

Main-crop yield was not influenced by the GA application, as indicated by the results from the paired t-test analysis (13 kg/ha difference; P > 0.4) when GA was applied to field-grown rice during the late-flowering to soft-dough period (Fig. 1, Table 2).

Fig. 1. Cultivar response to GA (gibberellic acid) application late in main-crop development. The studies were conducted between 2002 and 2005 and at either Beaumont or Eagle Lake, TX.

Table 2. Yield differences between gibberellin-treated and nontreated plots of the main and ratoon crops.

Cultivar	Year	Location	Yield difference (kg/ha)		Proportion difference^x
Cultivar	Year	Location	Main crop	Ratoon crop	Percent
Cheniere	2003	Eagle Lake	240	40	-0.7
Cheniere	2004	Eagle Lake	-250	74	1.8
CL-XL8	2004	Eagle Lake	-331	-38	1.1
CL131	2005	Beaumont	79	590	6.2
CL161	2003	Eagle Lake	-103	218	3.1
CL161	2004	Eagle Lake	-82	78	1.2
Cocodrie	2003	Eagle Lake	148	-121	-2.0
Cocodrie	2004	Eagle Lake	175	335	2.6
Cocodrie	2005	Beaumont	249	501	4.4
Cypress	2005	Beaumont	-228	-166	-1.0
Jefferson	2004	Eagle Lake	-318	40	2.5
Jefferson	2005	Beaumont	6	427	4.8
Presidio	2005	Beaumont	30	322	3.3
Presidio	2005	Beaumont	-130	89	1.5
XL7	2003	Beaumont	79	1170	13.5
XL7	2004	Eagle Lake	-336	331	5.0
XL723	2005	Beaumont	544	591	4.2
Mean			-13	264	3.1
P-value			> 0.4	< 0.025	< 0.025
95% CI Upper			113	485	4.9
95% CI Lower			-139	41	1.3

^x The proportion of ratoon-crop yield of main-crop yield was obtained by dividing ratoon-crop yield by main-crop yield.

Using a number of widely-used cultivars evaluated at two locations and in different years, the results from the paired t-test analysis indicated that GA increased ratoon yield. The ratoon-crop yield was increased an average of 264 ± 221 kg/ha (95% c.i.; P < 0.025; Fig. 1; Table 2). Results from a Wilcoxon signed ranks test, which evaluates the relative number of times that the treatment increased yield as well as the relative rank in absolute differences in yield, also indicated that the GA treatment was highly effective (P < 0.001) in increasing ratoon yield. The very early maturing hybrids, XL7 and XL723, responded best to the GA applications, with an average 700 kg/ha increase in ratoon-crop yield (Fig. 1).

It has been reported that ratoon-crop yields can often reach 50% of main-crop yields (8). The paired comparison of medians in this study showed that the average increase in ratoon-crop yield as a proportion of main-crop yield due to GA application was 3.1% (P < 0.025). The average percentage of the GA-treatment ratoon-crop yield was 42.3% of main-crop yield while that of the nontreated plots was 39.2% of main-crop yield (data not shown). The highest percentage was observed with XL7, in which the ratoon-crop yield contributed over 52% that of main-crop yield across treatment, year and location (data not shown). Furthermore, XL7 demonstrated two of the highest increases due to GA in terms of ratoon-crop yield expressed as a proportion of main-crop yield. These results suggest that the very early-maturing, vigorous hybrids may prove more responsive to the GA treatment in terms of increased ratoon yield as a proportion of main-crop yield, as well as in terms of absolute ratoon-yield increase.

Effect of Gibberellin on Main-Crop Milling Quality and Grain Moisture Content at Harvest

Milling data were collected on selected cultivars. The results indicated that main-crop milling quality was negatively affected by GA application. Using ANOVA, the percent total milled and percent whole kernel differences indicated that the GA application reduced the rice milling quality compared to the nontreated (both with P < 0.0001). The percent milled was reduced by GA application from 71.2% of the control to 70.5% of the GA-treated, and the percent whole kernels was reduced from 57.2% of the control to 54.6% of the GA treated (data not shown).

It is our hypothesis that the GA treatment’s effect on main-crop milling quality was due to an effect of GA on main-crop development. The plant growth regulator, gibberellic acid, used in this study is best known as the plant growth hormone, and it is well established that the application of GA can alter the developmental rate of plants. Thus, the GA application may have altered grain age at harvest relative to that of grain from neighboring nontreated plots. Different developmental ages of the grain would be reflected as different moisture contents of the grain. The different moisture contents of the grain at harvest would be reflected as different milling qualities among plots. Grain moisture contents are routinely used as an indicator of grain readiness for harvest, so the main-crop milling quality (percent whole kernels) and grain moisture contents at harvest from the 2005 Eagle Lake studies were compared for the GA-treated and nontreated plots. Consistent with the comprehensive analysis described above, the percent whole kernels was less for the GA-treated plots than the nontreated plots in all of the six studies analyzed (P < 0.025). Consistent with the notion that GA treatment was altering development of the main crop, the harvest moisture content of the grain from the GA-treated plots was higher than that from the nontreated plots in each of the six studies (P < 0.025). The complete correspondence of the milling-quality and moisture-content observations in this analysis strongly suggests that the GA treatment delayed development of the main crop, which in turn affected the milling quality as described above. The effect of GA on milling quality was therefore an artifact of our research methodology that emphasized accurate determination of the GA effects on ratoon-crop yield. Under typical production conditions, an entire field will be treated with GA, so that the effect of GA on altered maturation of neighboring plots, and thus their milling quality, can be dismissed. The effect of GA treatment to delay main-crop development while increasing ratoon yield needs to be considered, however, when planning the harvest of main-crop fields, even though main-crop yields are not affected.

Future research should focus on GA application later in grain development, the feasibility of which was shown by the applications at soft dough used in some of the 2005 studies presented in this manuscript. The primary advantage of such a timing would be the ability to add the GA as a tank-mix partner with insecticides that are often applied at this developmental stage for control of stink bugs. This would save on application costs. Preliminary results from a study conducted in cooperation with M. O. (Mo) Way and Luis Espino from the Texas A&M Agricultural Research and Extension Center at Beaumont indicated that GA can be tank-mixed with insecticides applied during main-crop grain filling without loss in efficacy of either compound. Additional unpublished results from a 2005 study examining timing of the GA application, which was conducted by the authors in cooperation with Richard Dunand (LSU AgCenter, Crowley, LA) at Beaumont and Crowley, suggested that application during grain filling was effective in increasing ratoon-crop yield.

Conclusions

Gibberellin can be applied between several days post-flowering and the soft-dough stage of the main crop of rice to increase ratoon-crop yield, without influencing main-crop yield. The average increase in ratoon-crop yield of 17% for the very early maturing hybrids suggest that the GA treatment has especially good potential for increasing their yield. Preliminary work suggests that the GA can be tank mixed with insecticides, which could provide a very economical way to increase rice-crop yields. The effect of the GA treatment to slightly delay main-crop development should be considered when planning harvests.

Acknowledgments

The authors thank Ronnie Porter, Charles ‘Bo’ Dixon, Wilbert Moore, Marcus McCabe, Casey Hall in Beaumont, and Jack Vawter and the Eagle Lake staff, for assistance in maintaining the research plots from planting to harvest and collecting post-harvest measurements. Also, Cynthia Tribble provided assistance in the manuscript preparation and review.

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