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a Dep. of Crop and Soil Science, Oregon State Univ., 107 Crop Science Bldg., Corvallis, OR 97331
b Facultad de Ciencias Agrarias, Pontificia Univ. Católica Argentina, Freire 183, C1426AVC Buenos Aires, Argentina
c Marion County Extension Office, Oregon State Univ., 3180 Center NE Rm. 1361, Salem, OR 97301
* Corresponding author (maria.zapiola{at}oregonstate.edu)
Received for publication July 28, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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-hydroxy methylene)-3,5-dioxocyclohexane carboxylic acid ethyl ester] (TE) was evaluated as a potential alternative to open-field burning for maximizing yield in creeping red fescue over 4 yr. Fall and spring applications of TE in combination with residue management practices, open-field burning and mechanical removal (flailing) of post-harvest residue, were evaluated to determine potential effects on seed yield and dry matter partitioning. Spring TE applications in burn plots increased cumulative seed yield by 38% over the check. In flail plots, spring TE applications increased yield by 30 and 16% over the burn check for the first 2 yr, but no response was observed later. Open-field burning was critical for maintaining high yields in the last 2 yr. Although late-spring TE applications in flail plots resulted in cumulative yields comparable to those of burn check plots, yields were 34% lower than those of burn, TE spring-treated plots. Fall TE applications had no consistent effect on seed yield. Therefore, neither spring nor fall TE applications are an effective alternative to replace open-field burning in creeping red fescue seed production over the life of the stand if seed yield is to be maximized. Spring TE applications plus open-field burning maximized seed yield and had the greatest harvest index, resulting in a more efficient crop.
Abbreviations: DM, dry matter • GDD, growing degrees days • HI, harvest index • RM, residue management • TE, trinexapac-ethyl
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The post-harvest regrowth period is a critical phase of seed crop development because it can strongly influence flowering and seed yield in cool-season perennial grasses (Canode and Law, 1978). According to Chastain et al. (1997), the number and condition of tillers that are exposed to floral inductive stimuli rely on a relatively short regrowth period in late summer and early fall. Meints et al. (2001) reported that burning resulted in larger (>2.0 mm basal diameter), but not taller, fall tillers than the mechanical stubble-removal treatments in the second year crop in creeping red fescue. In addition, fall tiller height was negatively related with fertile tiller production the following spring, which was positively and closely associated with seed yield (Meints et al., 2001). These results are in accordance with Chastain et al. (1997), who showed that height of tillers in fall was inversely related with seed yield in Kentucky bluegrass (Poa pratensis L.).
Trinexapac-ethyl (TE), a foliar-applied plant growth regulator from the acylcyclohexanedione family (Rademacher, 2000) commercialized in the USA as Palisade, results in growth reduction when applied at actively growing stages in grass seed crops. Growth reduction is caused by reduced levels of biologically active gibberellin (GA1) as a result of 3ß-hydroxylation inhibition in the late steps of gibberellin (GA) biosynthesis (King et al., 1997; Rademacher, 2000). Rolston et al. (2003) reported that spring TE applications in forage tall fescue (Festuca arundinacea Schreb.) resulted in shorter, more erect crops that were less prone to lodging. We hypothesized that by applying TE in the fall when the crop is actively growing, the height of fall regrowth could be controlled, and, consequently, seed yield could be increased.
No work has evaluated the potential effects of fall TE applications on seed yield of cool-season perennial grasses. Spring applications of plant growth regulators have been shown to improve seed yields of creeping red fescue and other grasses (Rolston et al., 1997; Chastain et al., 2001, 2003; Gingrich and Mellbye, 2001; Silberstein et al., 2001, 2002; Rolston et al., 2003). In perennial ryegrass (Lolium perenne L.), TE-induced increases in seed yield were related to improvements in floret production and seed set (Chastain et al., 2003). We evaluated open-field burning and TE applications together to find out if spring or fall applications of TE could overcome the loss in creeping red fescue seed yield caused by the absence of open-field burning. In addition, we wanted to determine if there is an interaction between residue management treatment and the use of TE that could result in maximized seed yields.
The objectives of this experiment were to (i) evaluate different timings and rates of TE application to maximize seed yield, (ii) to evaluate the interaction between open-field burning and the use of TE on seed yield and dry matter (DM) partitioning, and (iii) to attempt to develop an alternative management tool to replace open-field burning and maintain high seed yields in creeping red fescue in the Willamette Valley.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The experimental design was a split plot with four replications. Two residue management treatments were applied in main plots, and seven treatments of time and rate of TE application were imposed in subplots. Residue management treatments consisted of (i) burn, in which the full straw load was open-burned after harvest, and (ii) flail, where post-harvest straw was baled and stubble was flailed low (2–2.5 cm) soon after harvest. Trinexapac-ethyl treatments consisted of (i) check, with no TE application, (ii) early fall application of 200 g a.i. ha–1, (iii) early fall application of 400 g a.i. ha–1, (iv) late fall application of 200 g a.i. ha–1, (v) late fall application of 400 g a.i. ha–1, (vi) early spring application of 400 g a.i. ha–1, and (vii) late spring application of 400 g a.i. ha–1.
Residue management treatments were randomly assigned to main plots within replicates, and the seven TE treatments were randomly assigned within each main plot. Each subplot was 3 m wide by 15 m long. Open-field burning was facilitated by the allocation of residue management treatments to main plots (21 by 15 m). In 2000, the first seed production year, the crop was harvested without making any distinction of plots. Residue management treatments were applied subsequently. The trial was then conducted for four additional harvest seasons (2001–2004). Each subplot received the same treatment combination each year.
Trinexapac-ethyl treatments were applied at walking speed using a bicycle-type boom sprayer operated at 138 kPa with XR Teejet 8003VS nozzles as described by Silberstein et al. (2002). Fall TE application dates (Table 1) were selected based on active regrowth and growing degrees days (GDD) accumulated from 1 January using 0°C as base temperature. Early fall applications took place between 3474 and 3974 GDD when weather conditions allowed the application. Late fall applications were made 4 to 5 wk after the early fall applications. Spring TE application dates were selected based on plant growth stage. Early spring applications were made when tillers were rapidly elongating and had flag leaves and some panicles visible. Starting at the end of March, 150-cm2 tiller samples were taken weekly to determine when tillers started active elongation. Late spring applications took place 17 to 21 d (184–233 GDD) later than the early spring applications.
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After the beginning of anthesis (Zadoks 60) but before anthesis was complete (Zadocks 69), two adjacent 0.09-m2 samples were taken from each plot by cutting all the aboveground matter at soil level. Adjacent samples were taken to ensure uniformity of selected rows. Samples were dried for 48 h at 65°C in drying chambers, and total dry weight was assessed. Fertile tiller number was determined by counting all the tillers bearing a panicle in each sample. In the third and fourth year, fertile tillers were separated for estimation of individual fertile tiller weight and fertile tiller proportion of total aboveground DM. In 2003 and 2004, fertile tillers were measured to determine the average tiller height.
Plots were harvested at approximately 250 g kg–1 seed moisture content. The central 1.8 m of each plot was cut with a plot swather (custom modified John Deere 2280 swather; John Deere, Moline, IL) and dried in windrows to about 120 g kg–1 seed water content. Dried windrows were threshed with a plot combine (Model 180; Hege Equipment, Colwich, KS). An on-site scale was used to measure the bulk seed weight harvested from each plot. A seed subsample was taken at harvest and cleaned with an M-2B Clipper (A.T. Ferrell Company, Bluffton, IN) air-screen seed cleaner to determine clean-out, which is the difference in the sample weight before and after the cleaning process. Percent clean-out of the subsamples was used to calculate clean seed yield, which is expressed on a 120 g kg–1 seed water content basis. Cumulative seed yield was calculated by adding the 4 yr seed yield for each plot.
Harvest index (HI) was determined as the proportion of the total aboveground DM represented by seed yield. Stand density was estimated by visual assessment of ground coverage.
Statistical analysis was conducted on a plot mean basis using Proc GLM from SAS statistical package (SAS Institute, 2001). Analyses of variance were performed to test residue management and TE application effects. Fisher's protected LSD values at the 0.05 probability level were used for mean comparisons. Because no differences were found between different fall rates, results are reported as an average of the 200 and 400 g a.i. ha–1 rates for early fall and late fall applications. Barlett's 
2 test showed that seed yield, total aboveground DM, and HI variances were not homogeneous across years (p < 0.005), violating the assumption for repeated measurements analysis. Therefore, each year's results were evaluated separately. Only the types of responses were compared among years, and no attempt to quantitatively compare results between years was made.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Third Year (2003)
The dry weather conditions experienced around flowering, where precipitation was 77% lower than the long-term average for the last 2 wk of May and first 2 wk of June, might have influenced the low yields obtained in 2003. However, we cannot rule out the decrease in seed yield due to the aging of the stand, a phenomenon previously observed in Kentucky bluegrass (Canode and Law, 1975).
In 2003, there was an interaction between residue management and TE treatment for seed yield (Table 2). Spring applications of TE in burn plots showed the highest seed yields again in the third year. Seed yield was increased by 43.5% with spring TE applications on burn plots, whereas there was generally no difference among TE treatments within flail plots (Fig. 2), contradicting the previous years' results. Besides, burn plots resulted in greater seed yields than flail plots regardless of TE application. On average, flail plots resulted in 53% lower seed yield than the burn check and in 67% lower seed yield than the spring-treated burn plots.
Fourth Year (2004)
As in 2003, there was an interaction between residue management and TE treatment for seed yield in 2004 (Table 2). Burn plots produced higher seed yields than flail plots regardless of TE application. Within burn plots, spring applications of TE resulted, on average, in 36.5% greater seed yields than the check and fall TE applications (Fig. 2). Among flail plots, late spring TE application showed a 23% seed yield increase when compared with the flail check but produced 37% less seed yield than the burn check.
Dry Matter Partitioning
Aboveground Total Dry Matter
In the first year, there was no effect of the treatments and their combinations on aboveground total DM production (Table 2). In the second and third year, there was a residue management effect on aboveground total DM (Fig. 3
). Burn plots produced 34 and 56% greater DM than flail plots in 2002 and 2003, respectively. In 2003, there also was a TE treatment effect on total DM (Table 2), whereby early spring treated plots produced 27% less total DM than the check regardless of the residue management treatment. Late spring TE application followed the same trend but was not different from the check. Contrary to the results in previous years, there was an interaction between residue management and TE treatment for total aboveground DM in 2004 (Table 2). Fall TE applications and the check in general resulted in higher DM in burn plots than in flail plots. Spring applications of TE in burn plots resulted in the lowest total aboveground DM production, which was 31% lower on average than in the burn check (Fig. 3). There were no differences in total aboveground DM between TE treatments within flail plots.
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Harvest Index
In 2001, there was no effect of residue management or TE treatment on HI (Table 2); nevertheless, spring-treated plots and burn plots exhibited a trend toward greater HI values (Fig. 5
). In 2002, there was an effect of TE treatment on HI, and spring-treated plots had a 48% increase in HI over the check regardless of residue management. In 2003 and 2004, there was an interaction between residue management and TE application for HI (Table 2). The response of HI to the different treatment combinations (Fig. 5) was similar to that found for seed yield. Harvest index tended to be greater for burn plots regardless of TE application. Burn plots with TE application in the spring exhibited the greatest HI, which was 88 and 100% higher than the burn check HI for 2003 and 2004, respectively. There was no difference in HI within flail plots.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Even though spring TE applications in flail plots increased cumulative yield by an amount comparable, on average, to one extra production year of flail check plots, the increase was not enough to outyield the burn check. The late spring TE application in flail plots was found not to be different from the burn check, indicating at least the possibility of attaining seed yields comparable to those of burn plots where no TE was applied in spring. However, seed yield of late spring–TE treated flail plots was lower than that of burn plots with the same late spring TE application.
The hypothesis that regulating tiller growth in fall with TE would increase seed yield could not be validated. None of the fall TE applications in flail plots showed cumulative seed yield comparable to those of the burn check, confirming that fall TE applications are not a viable and sustainable option to replace open-field burning in creeping red fescue seed production. Trinexapac-ethyl is not effective in fall applications and should be applied in spring if a seed yield increase is to be expected. The lack of comparable response to fall TE applications with that of spring TE applications is in accordance with Chastain et al. (2003), who reported that TE does not have a residual effect.
Important differences were found in the type of response of seed yield to spring TE applications in flail and burn plots for the different years of the stand. In the first 2 yr, while the stand was still young, seed yield increases were consistently achieved with spring TE applications regardless of residue management. However, in 2003 and 2004, flail plots no longer responded to spring TE applications, suggesting a lower potential seed yield for flail plots. No differences were found between early and late spring applications within each residue management treatment in any year, confirming that creeping red fescue has a wide application window for TE (Silberstein et al., 2002).
Although there was almost no difference in seed yield between burn and flail plots in the first year, in the second year there was an increase in seed yield as a result of field burning. Later in the life of the stand, in 2003 and 2004, a subjective greater stand density was observed in flail plots, where rows were almost not detectable, versus burn plots, where rows were clearly defined, and the differences in seed yield between burn and flail plots were considerable. These findings are in agreement with Fairey and Lefkovitch (1996), who showed that seed yield was closely related to and dependent on stand density in creeping red fescue and reported lower seed yields for denser stands as the stand ages. In addition, Deleuran and Boelt (1997) demonstrated a decrease in seed yield in F. rubra var. rubra for greater stand density in the second seed production year. Similarly, in Kentucky bluegrass, mechanical residue removal resulted in greater or no difference in seed production for the first 2 yr, whereas burning produced higher seed yields in the following 2 yr (Canode, 1972; Lamb and Murray, 1999). Kentucky bluegrass and creeping red fescue have comparable creeping growth habits and are less tolerant to residue management without thorough straw and stubble removal than bunch-type species, such as perennial ryegrass and tall fescue (Chastain et al., 2000). Meints et al. (2001) stated that stubble removal to the plant crown is crucial for maximizing seed yield in creeping red fescue. Even though in our experiment post-harvest residue was largely removed with flailing, open-field burning seemed to provide a more complete removal of the residue. Accordingly, supporting the results of Chastain et al. (1996b) and Young et al. (1998), seed yield in creeping red fescue could not be maintained over time without open burning post-harvest residue.
Although we tried to develop an alternative practice to replace open-field burning in creeping red fescue seed production, neither fall nor spring TE applications resulted in a viable option when compared with open-field burning plus spring TE application, which is the treatment combination that maximized seed yield. Further research could be based on our findings to explore the combination of mechanical removal of post-harvest residue during the first years of the stand and open-field burning later in the life of the stand to maximize seed yield.
Total aboveground DM production responded to open-field burning in the second and third year, which is similar to the findings of Lamb and Murray (1999), who reported increases in aboveground DM ranging from 24 to 36% for field burning over nonthermal residue management in Kentucky bluegrass. Also in the third year, early spring–treated plots produced less biomass than the check regardless of residue management. The lack of reduction in DM for late spring–treated plots could be due to the fact that fertile tillers were more developed at the time of TE application. In the fourth year, the observed residue management by TE treatment interaction was unexpected. The smallest aboveground DM of burn spring-treated plots contradicted the previous years' results. Despite burn spring-treated plots having the smallest DM production, they had the greatest seed yields.
Consequently, not only was open-field burning associated with an increase in seed yield, but it was also associated with an increase in HI, as noted by Chilcote et al. (1980) and Young et al. (1998). Spring applications of TE were related to higher HI as well, implying that spring-treated, burned plots were more efficient in partitioning DM to seed yield than fall-treated and check plots, especially later in the life of the stand.
The important difference in fertile tiller proportion between burn and flail plots found in 2003 and 2004 may imply a differential partitioning of photoassimilates favoring fertile tillers in burn plots. The increase in fertile tiller DM in burn plots was due not only to a greater number of fertile tillers per area but also to a greater individual fertile tiller dry weight. The greater seed yields obtained in burn plots, coincident with heavier fertile tillers at the time of anthesis, is in agreement with Canode and Law (1979) and Sylvester and Reynolds (1999), who reported that tiller size was positively correlated with high potential productivity in Kentucky bluegrass.
Considering that Lamb and Murray (1999) and Johnson et al. (2003) reported a strong interaction between post-harvest residue management and different cultivars and accessions of Kentucky bluegrass, it must be noted that only one cultivar of creeping red fescue was used in this experiment. Extrapolation of these results to other production environments and cultivars should be done with caution. Aggressive strong creeping red fescue cultivars, which rapidly fill space between rows, like Shademaster (Rose-Fricker et al., 2000), would heavily rely on open-field burning for obtaining high yields, whereas for nonaggressive cultivars, mechanical means of residue removal may prove reliable.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Therefore, open-field burning is critical for maximizing seed yield in creeping red fescue throughout the life of the stand, and neither spring nor fall TE applications are a viable alternative if 4 yr of production are considered. According to these results, a high priority should be given to creeping red fescue when allocating the area that is allowed for open-field burning each year.
The use of TE spring applications on flail plots might be considered as an alternative to open-field burning if shorter stand life is economically feasible. In addition, comparing continuous open-field burning with mechanical removal of post-harvest residue during the first 2 yr and burning after the second seed production year of creeping red fescue merits further investigation. The different types of response obtained for each year reported here set the basis for further research.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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