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Crop Science 40:1317-1324 (2000)
© 2000 Crop Science Society of America

SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

White Clover Seed Production

III. Cultivar Differences under Contrasting Management Practices

R.B. Medeirosa and J.J. Steinerb

a Bolsista da Capes, Dep. de Plantas Forrageiras e Agrometeorologia, Faculdade de Agronomia, UFRGS, CP. 776, 91.501-970, Porto Alegre, Brazil
b National Forage Seed Production Research Center, USDA-ARS, 3450 SW Campus Way, Corvallis, OR 97331 USA

steinerj{at}ucs.orst.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Information is needed to determine the optimal combinations of agronomic practices for white clover (Trifolium repens L.) seed production in humid temperate marine climatic conditions. Effects on seed yield were determined for two stand ages (first and second seed year of production), six cultivars varying in leaf size (ladino large-leaf type: Canopy, California Ladino, Osceola, and Regal; and intermediate leaf-type: Louisiana S-1 and White Dutch), grown with and without spring herbage removal, and with and without supplemental irrigation. The experiment was arranged in a modified split-split-split-plot design with four replications. First year seed was harvested in 1997, and first and second year seed was harvested in 1998. Intermediate-leaf size cultivars reached initial bud and flower stages earlier than large-leaf types, but there were no differences between the two leaf types at the time of seed harvest. The number of flowers produced early in the reproductive period for nonirrigated plants was highly correlated with seed yield. Supplemental irrigation delayed flower maturity, but herbage removal did not. First seed year yields were greater than second seed year yields for all cultivars except Osceola, which had similar yields both seed years. Supplemental irrigation only increased seed yields of Canopy. In first seed year stands, White Dutch was the only cultivar that did not recover lost seed yields due to herbage removal when supplemental irrigation was applied. Herbage removal and supplemental irrigation treatments generally did not increase white clover seed yields under the conditions of this experiment, so maximal yields were generally achieved with minimal management inputs.

Abbreviations: GDD, growing degree days • TAGP, total aboveground phytomass


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
PACIFIC NORTHWEST GRASS SEED cropping systems have historically relied on disturbance practices, large quantities of purchased agricultural chemicals, and open-field burning to establish new crops, control diseases and weeds, and dispose of postharvest residues to achieve high seed yields with good seed quality. Many of these historic systems have been continuous grass seed monocultures, and thus have lacked rotation crop diversity. In western Oregon, white clover grown for seed is one of several minor crops that can provide rotation crop options for grass seed production systems on poorly drained soils. For the production of economic amounts of good quality seed, alternative conservation systems are needed to manage forage and turfgrass seed crops in the absence of open-field burning and with reduced chemical inputs.

White clover is a broadly adapted perennial forage legume that grows under a wide range of climatic conditions. Many cultivars are grown throughout the United States for herbage, but seed production is limited to regions within the western states (Steiner, 1994). The climatic conditions in western Oregon are similar to those found in western Europe and New Zealand, but are very different from the arid Mediterranean-like conditions found in the irrigated Great Central Valley of California (Bailey, 1996) where white clover seed is also produced. Approximately 1200 ha of seed are produced in California (J. Reich, 1998, personal communication) and 600 ha in western Oregon (E. Edminster, 1998, personal communication). Historic production in California and Oregon were {approx}13000 and 10000 ha, respectively (Scullen, 1952). The ladino large-leaf type cultivars California Ladino, Canopy, Osceola, and Regal are typically grown for seed in California, but can also be grown in Oregon. The intermediate leaf-type cultivars Louisiana S-1 and White Dutch are typically grown in Oregon, but not in California. White clover seed yields in western Oregon are highly variable and are often low (Oliva et al., 1994b). Soil water content, herbage removal time, and stand age are primary agronomic factors that can affect seed yield potential.

White clover has indeterminate growth from stolon tips that form either secondary stolons or flowers (Thomas, 1961). Intermediate-leaf size cultivars generally produce more flowers than large-leaf ladino types (Pederson, 1995). Herbage removal by grazing or haying prior to or during early flowering is a common agronomic practice for some perennial and annual forage legume seed crops (Rincker and Rampton, 1985; Rincker et al., 1988; Steiner, 1994). Spring herbage removal by sheep (Ovis aries L.) from white clover fields can also provide secondary income to seed growers and has weed control benefits. However, the actual impact of herbage removal on white clover seed production in Oregon is not known.

Removal of white clover leaves can decrease starch concentration in stolons (Gallangher et al., 1997) and may adversely impact seed yield and seed yield components (Zalesky, 1970; Clifford, 1979; Marshall et al., 1993). In Great Britain it was reported that herbage removed from 3 wk after bud initiation to just before bud emergence did not affect seed yield components (Marshall et al., 1993). However, when grazing was continued past early flowering, seed yields could be reduced (Marshall and Hides, 1990). The effect of herbage removal on seed yield is influenced by climatic conditions that differ between production years, so it is difficult to develop definitive guidelines. Even though complex relationships between cultivars, stand age, and production years are known for red clover (T. pratense L.) grown for seed under irrigation in central Washington (Dade, 1966) and generally nonirrigated conditions in western Oregon (Steiner et al., 1995), such interactions have not been determined for white clover seed production.

Adjusting soil water content by properly timed water applications during the reproductive period can reduce vegetative growth and favor reproductive development, resulting in higher seed yields (Zalesky, 1966; Clifford, 1986; Bullita et al., 1988; Danyach-Deschamps and Wery, 1988; Oliva et al., 1994b). Maximal seed yield of Osceola white clover was obtained in western Oregon when a single water application was delayed until 68% of the available soil water was used by the crop (Oliva et al., 1994b). However, in a second seed year stand, manipulation of soil water content had no effect compared with an nonirrigated control because of the production of a dense mass of stolons that limited flower production (Oliva et al., 1994b). Second seed year production is desirable to amortize establishment costs across two production years, rather than producing a single seed crop. This is important when the seed crop is spring-established since no income is realized for 16 mo from initial planting time.

This research was done to determine the optimal production strategies using combinations of commonly used agronomic practices to produce white clover seed as a 2-yr crop component of a grass seed rotation system suited for a humid temperate marine climatic region. This experiment investigated the response of six white clover cultivars to stand age, spring herbage removal, and supplemental irrigation. No other reported research from any white clover seed production region has investigated the effects of different stand ages compared within the same production year or the interactions of multiple agronomic production practices.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
The study was initiated in autumn of 1996 at the Oregon State University Hyslop Research Farm near Corvallis, OR, on a Woodburn silt loam (fine-silty, mixed, mesic Aquultic Agixeroll). The six cultivars were (i) intermediate-leaf types: Louisiana S-1 (Hollowell, 1958) and White Dutch (common ecotype grown in western Oregon); and (ii) ladino large-leaf types: California Ladino (Cal/West Seeds, Woodland, CA), Canopy (Cal/West Seeds), Osceola (Baltensperger et al., 1984), and Regal (Johnson et al., 1970).

The experiment was arranged in a modified split-split-split-plot design with four replications. The main plots were stand age (first- and second-year seed crop). The main plots were divided into two subplot supplemental irrigation treatments. The irrigation treatment subplots (without and with) were assigned at random within the stand age main plots. The six cultivar sub-subplots were arranged in four randomized complete blocks, with each cultivar planted in 3 m wide by 5 m long plots. The herbage management treatments (without and with removal) were applied in a lengthwise strip fashion to one-half of the 3-m-wide cultivar plots across all four blocks. It was assumed that the strip herbage removal effect was random and independent of cultivar plot position across the four blocks. Analyses of variance were done to test the treatment effects for contrasts of stand age (second and first seed year from planting in 1996 and 1997, respectively) and first seed year harvests in 1997 and 1998.

The cultivars were planted into rows 30 cm apart using {approx}4 kg ha-1 of seed on 14 September in 1996 and 1997. Approximately 20 mm of water per application was applied by high-pressure overhead sprinklers just after seeding and as needed during the following 20 d for crop establishment. Crop culture management followed common commercial practices for western Oregon. Annual grasses and broadleaf weeds were controlled in winter 1997 and 1998 with propyzamide [3,5-dichloro-N-(1,1-dimethyl-2-propynyl)benzamide] and paraquat dichloride (1,1'-dimethyl-4-4'-bipyridinium dichloride) at rates of 2.2 and 0.6 kg ha-1, respectively.

The application of agronomic practices between the two study years was based on accumulated growing degree days (GDD) using the formula:

(1)
with Tbase = 10°C.

Four honey bee (Apis mellifera L.) hives were placed adjacent to the experiment area at the beginning of flowering of the intermediate type cultivars (28 April in both years), when 46 and 42 GDD had accumulated, respectively. Honey bee activity was considered adequate for pollination based on visual inspection and previous experience.

The herbage was removed with a commercial forage chopping machine to a height of 5 cm on 27 May (186 GDD) in 1997 and 2 June (151 GDD) in 1998. Supplemental irrigation water amounts totaling 114 and 110 mm were applied in three applications by overhead sprinklers on 7, 10, and 14 July in 1997 and on 11, 13, and 15 July in 1998. The time of application was based on a soil water content of 680 g kg-1 (Oliva et al., 1994a, 1994b). The soil water status in 1997 was estimated from six randomly selected plots of the cultivar Osceola using neutron attenuation as described by Oliva et al. (1994a, 1994b) and using a Trase 6050XI Time Domain Reflectometer (Soil Moisture Equipment, Santa Barbara, CA) in 1998.

In the first seed crop year (1997), the number of flower flowers in four 0.1-m2 random samples per plot were counted on 8 and 20 June, 3 and 18 July, and 1 Aug. 1997. The flowers were counted if the florets had become brown-colored. Pearson correlation coefficients (Snedecor and Cochran, 1980) were used to determine the umbel sampling date that was most significantly associated with seed yield for the nonirrigated and irrigated treatments. The number of flowers counted on 1 August (681 GDD), when peak flowering was estimated to occur, was used to compare treatment differences. In the second seed year (1998), the flowers counted at peak flowering time on 21 July for nonirrigated treatment plots and 5 August for irrigated plots (516 and 722 GDD, respectively) were used to compare the flowering capacity of the cultivars. The flowers in the 1998 irrigated treatments were counted {approx}2 wk after the nonirrigated plots because of delayed maturity due to the irrigation application. Flowers were also counted in all plots prior to harvest in 1998.

When 80% of the flowers of a cultivar were mature and prone to shatter, the plants were swathed using a self-propelled mower in early morning to avoid inflorescence shattering. Nonirrigated plots were harvested on 11 August (795 GDD) and 12 August (761 GDD), and the irrigated plots on 26 August (963 GDD) and 28 August (914 GDD), in 1997 and 1998, respectively. A 4-m-long sample of the mowed plant material was gathered by hand, put in burlap bags, and dried at ambient temperature for 5 d on a clothes line. Twenty-four hours before threshing, the bags were placed in a forced-air oven and dried at 32°C to constant weight. The harvested material was weighed, the seeds threshed from the plant material, and the seeds cleaned and weighed. Total aboveground phytomass (TAGP) was the weight of the harvested material minus seed yield. One thousand-seed weight from each plot harvested in 1998 was determined. To measure reproductive efficiency, the TAGP was divided by the number of flowers at the time of harvest.

All variables were tested by analysis of variance and Fischer's protected least significant difference test (Snedecor and Cochran, 1980) was used to test seed yield, phytomass, and flower density differences (Tables 1 and 2) . The analysis of variance was also determined for 1000-seed weight for seeds harvested in 1998 (Table 1). Using the seed yield means for the average cultivar effect and the means for the irrigation x herbage removal x cultivar interaction, correlation coefficients were calculated for the differences in seed yield between first and second seed year seed production measured in different years (first year in 1997 with second year in 1998) and in the same year (first and second year in 1998). All differences reported are significant at P <= 0.05, unless otherwise stated.


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  		Table 1 Analysis of variance for four white clover seed production practices and their interactions on first and second seed year seed yield, seed yield components, and aboveground phytomass of six cultivars grown near Corvallis, OR, in 1998*

 

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  		Table 2 Analysis of variance for four white clover seed production practices and their interactions on first seed year seed yield and yield components of six cultivars grown near Corvallis, OR, in 1997 and 1998

 

   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Climatic Difference among Years
In both years, plant growth increased rapidly in mid April when the average daily ambient temperatures reached 10°C (Table 3) . Monthly precipitation patterns were slightly different for the 2 yr. In 1997, total rainfall was lower than in 1998, but June precipitation was greater in 1997 than in 1998 (76 and 25 mm, respectively; Table 3). The number of GDD accumulated by June was greater in 1997 than in 1998, but by the time of seed harvest at the end of August, only 45 more total GDD were accumulated in 1997 than in 1998.


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  		Table 3 Monthly weather conditions during plant growth in 1997 and 1998 near Corvallis, OR{dagger}

 
The intermediate-leaf cultivars reached initial bud and flower stages earlier than ladino large-leaf types (Table 4) . However, there were no differences among cultivars by the time of overall crop maturity that affected harvest time. In both years, the time to seed maturation for harvest was {approx}2 wk later for irrigated than nonirrigated plants (963 and 795 GDD in 1997; and 914 and 761 GDD in 1998, respectively) (Table 4). In 1997, the earlier in the season that the number of mature flowers produced were counted and correlated with seed yield, the greater the coefficient value (Fig. 1) . Later counts were not as effective due to continual flower production throughout the growing season, most of which had not produced mature seed at the time of harvest. Thus, final seed yield among the nonirrigated cultivars was most highly correlated with the number of flowers counted on 8 June ( ; P <= 0.001), which was 12 d after the time of herbage removal. When irrigated, the highest correlation between seed yield and the number of flowers was on 3 July ( ; P <= 0.001). The correlation between number of flowers produced and seed yield was generally lower for the irrigated compared with nonirrigated treatments. Similar patterns of reproductive maturation as affected by supplemental irrigation applications were reported for the cultivar Osceola by Oliva et al. (1994b).


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  		Table 4 Crop development for six white clover cultivars grouped into intermediate and ladino large-leaf types, grown in 1997 and 1998 near Corvallis, OR

 


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  		Fig. 1 Pearson correlation coefficients for the number of white clover flowers with seed yield at five sampling times for six white clover cultivars grown near Corvallis, OR, in 1997

 
Herbage removal did not significantly affect reproductive maturity because flowering was reestablished within 4 d after herbage removal (data not shown). Therefore, no adjustment for harvest time was needed due to herbage removal management differences. In general, these results indicate that enhancing a flush of early-season flowers is important for achieving maximal seed yields under these growing conditions. This differs from seed production in central California under obligatory irrigated conditions where multiple flushes of flowering are controlled by multiple water applications to achieve maximal seed yields (Marble et al., 1970).

Cultivar Differences and the Effects of Stand Age on Seed Yield
This report was the first to demonstrate the effects of different stand ages compared within the same production year on white clover seed yield. Other researchers have also shown that first seed year stands produced more seeds than second seed year stands, but the stand age comparisons were made in different years of production (Marshall et al., 1993; Oliva et al., 1994a, 1994b), which may have been confounded with weather conditions unique for the production year. With the variable year-to-year weather commonly found in regions with humid temperate marine climatic conditions, understanding the seed yield stability of cultivars produced with different management practices is important to help ensure dependable economic returns to producers. In this study, the average seed yield reduction of the six cultivars for second seed year stands (1998) compared with first seed year stands in the prior year (1997) and the same year (1998) were correlated ( ; P <= 0.05). This suggests that the average response of cultivars to stand age can be evaluated in different production years and does not require evaluation of different-aged stands in the same production year.

In 1998, all cultivars except Osceola (no seed yield difference) had greater first seed year than second seed year yields (Table 5) . Most notably, the first seed year yield of intermediate-leaf adapted ecotype White Dutch was nearly twice that of the second seed year stand, with a 46% reduction compared with the first production year. The other intermediate leaf-type cultivar, Louisiana S-1, had a similar first seed year yield as the large-leaf ladino types. The second seed year yield reductions for ladino large-leaf type cultivars California Ladino, Canopy, and Regal were 19, 36, and 27%, respectively, compared with the first seed year. There was no effect of stand age on 1000-seed weight (Table 1), and White Dutch had a greater 1000-seed weight than all other cultivars (0.71 mg for White Dutch and 0.66 mg for the average of the other five cultivars; P <= 0.001) (data not shown).


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  		Table 5 Effect of stand age on seed yield of six white clover cultivars grown near Corvallis, OR, in 1998

 
We speculate that the reason for the general seed yield reduction for most cultivars in the second year compared with the first seed year of production was due to the greater ratio of vegetative material to number of flowers in the second compared with the first seed year stands. The ratio of vegetative material to flower density was 1:21 and 1:15 (± 2.7) in the first and second seed year stands, respectively. The partitioning of photosynthate between reproductive and nonreproductive sinks could reduce the overall efficiency of reproduction. Fewer flowers and greater nonreproductive plant material in the second year than first year stands suggests that less photosynthate may be available for reproductive development, particularly for number of flowers. Similarly in New Zealand, seed yields have been increased by reducing vegetative growth though limiting available soil P to the plants (Clifford, 1987). Achieving high white clover seed yields depends on reducing vegetative development that otherwise occurs at the expense of reproduction.

Effects of Supplemental Irrigation and Herbage Removal
For the conditions of this experiment and with noted exceptions, maximal seed production for all cultivars was generally achieved without supplemental irrigation or herbage removal. First seed year stands in 1998 had highest seed yields without herbage removal and without irrigation (Table 6) . The same response for first seed year production was seen in 1997, except the resulting seed yield loss due to herbage removal could be compensated for with irrigation (Table 7) . The compensation response may have resulted from additional water from irrigation being available for plant regrowth following herbage removal in 1997. Available soil water is utilized for early-season canopy development and supports the initial phase of flowering. Recovery following herbage removal requires additional soil water to be depleted to first reestablish the canopy and then support reproduction. In this experiment, the single water application timing and amount treatment compared with the nonirrigated control and used in conjunction with herbage removal produced conditions similar to those reported in earlier research that resulted in maximal white clover seed yield (Oliva et al., 1994b).


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  		Table 6 Effect of irrigation and herbage removal treatments on the seed yield in 1998 of two different-aged stands of white clover grown near Corvallis, OR

 

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  		Table 7 The effect of irrigation and herbage removal treatments on first seed year seed yields of white clover harvested in 1997 and 1998 near Corvallis, OR

 
Second seed year stands also produced more seeds without herbage removal, but seed yield was increased when irrigation was done in the absence of herbage removal (Table 6). Different water application amounts and timing did not affect second seed year seed production when herbage was removed, compared with the nonirrigated control plants (Oliva et al., 1994b). In this experiment, the loss of seed yield due to herbage removal in the second production year was recovered by irrigation, but the {approx}60 kg ha-1 seed yield increase was offset by the partial budget cost of irrigation ({approx}$135 ha-1). This differs greatly from the significant increase in economic return that resulted from irrigation in the second seed year of production for red clover grown in the same region (Oliva et al., 1994c). Birdsfoot trefoil (Lotus corniculatus L.), like white clover, also has reduced seed yields in second and third seed years of production and was best managed without irrigation when grown under similar environmental conditions (Garcia-Diaz and Steiner, 2000). However, canopy was the only cultivar that had a positive seed yield response to supplemental water application (Table 8) . The general adverse effect of herbage removal on seed yield agrees with reports from Europe and New Zealand (Zalesky, 1970; Clifford, 1979; Marshall et al., 1993). The response of defoliated white clover to irrigation is not known for these other regions.


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  		Table 8 Effect of irrigation treatment on the seed yield of six white clover cultivars grown near Corvallis, OR, in 1998

 
Unlike the correlation between average cultivar first and second seed year differences for prior year and same seed year seed yields, there was no association for the irrigation x herbage removal x cultivar treatment interaction means ( ; P <= NS). The exceptions to the general use of minimal input management were due to interactions among irrigation and herbage removal treatments with stand age (stand age x irrigation x herbage removal; first and second seed crop measured in 1998) and first seed crop crop year (stand age x irrigation x crop year; 1997 and 1998). These kinds of interactions make it difficult to identify specific conditions where white clover seed yield can be increased using irrigation and herbage removal inputs and demonstrate that comparisons of different stands age in the same production year should be considered. This has not been the approach generally used by other researchers.

Treatment Effects on Flowering
The response of flower production to the agronomic treatments explained some of the seed yield differences due to the treatment combination differences. The supplemental irrigation and herbage removal treatments affected flower production, but high flower density was not necessarily associated with high seed yields. The two intermediate-leaf cultivars produced more flowers than the ladino-types, regardless of irrigation or herbage removal treatment (Table 9) . In addition to Canopy, the ladino-type cultivars California Ladino and Osceola produced more flowers when irrigated than without irrigation. However, only with Canopy did the greater number of flowers result in higher seed yields (Table 9). Despite Osceola having the greatest increase of all cultivars for number of flowers produced when supplemental water was applied (52%) (Table 9), seed yield was unaffected. Osceola and the other large-leaf ladino type cultivars generally produced more flowers under irrigation. The number of Osceola flowers and seed yield were unaffected by stand age (Table 5), and like the other ladino large-leaf types, was unaffected by herbage removal, regardless of irrigation treatment (data not shown). The number of flowers produced by Osceola in this experiment was the same as reported by Oliva et al. (1994b).


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  		Table 9 Effect of irrigation treatment on the number of flowers produced in 1998 for first and second seed year stands of six white clover cultivars grown near Corvallis, OR

 
Conversely, the intermediate-leaf types only produced more flowers when herbage was removed. First seed year stands of the intermediate leaf cultivars produced the same number of flowers in both 1997 and 1998 (Table 10) . However, the ladino large leaf-type cultivars produced more flowers in 1997 than in 1998. The ladino large leaf-type cultivars, which are not as prolific flowering as intermediate leaf-types, may have produced more flowers due to the greater number of heat units received during flowering in May and June in 1997 than in 1998, as well as the greater amount of precipitation received from June to August in 1997 (Table 3).


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  		Table 10 Effect of year of first seed-year of production on the number of flowers produced by six white clover cultivars in 1997 and 1998, near Corvallis, OR

 
These results suggest that seed yield components in addition to the number of flowers produced (e.g., seeds per floret or number of florets per flower) may have been affected by herbage removal and soil water availability. Irrigated plants produced greater 1000-seed weights than nonirrigated plants (0.70 and 0.63 mg, respectively; P <= 0.001). Oliva et al. (1994b) reported that seed weight was increased in irrigated plants in the first seed year of production, but not in the second. Plants that did not have herbage removed had greater 1000-seed weights than those with herbage removed (0.67 and 0.66 mg, respectively; P <= 0.01). Defoliation has been shown to decrease starch concentration in white clover stolons (Gallangher et al., 1997), which may limit the capacity of the plants to fill seeds and thus accounted for the decreased 1000-seed weights from plants that had herbage removed.


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Results from this experiment suggest that under humid temperate marine climatic conditions as found in western Oregon, white clover cultivars generally can be expected to produce maximal seed yields by not irrigating and not mechanically removing late-spring herbage. Seed producers can also anticipate reduced seed yields in the second seed year of production. The second seed year seed yield decline of different cultivars can be determined in different production years and does not require comparison of different-aged stands in the same year. However, identification of specific conditions that may increase white clover seed production, such as irrigation and herbage removal inputs interacting with different stand ages, requires comparisons of different-aged stands in the same production year.

The intermediate-leaf type cultivars produced more flowers than the ladino large leaf-types, but only White Dutch had higher seed yields than the large leaf-types. The number of flowers produced by the plants could be manipulated using the contrasting irrigation and herbage removal treatments, but the imposition of these treatments generally did not increase seed yields, compared with the untreated controls. Generally, there was no increase in seed yield using herbage removal and irrigation inputs. Supplemental irrigation tended to result in late-season flower production and delayed harvest time and only increased seed yields when used in combination with herbage removal in a first seed year stand in 1997. However, the partial budget value of the seed yield increase was offset by the input cost of irrigation. Mechanically removed white clover herbage is generally not used for livestock feed, but sheep grazing fees may offset some of the cost of irrigation. These strategies for white clover seed production under humid temperate marine climatic conditions differ greatly from those used in the irrigated arid central California region.


   ACKNOWLEDGMENTS
 
The authors thank W. Gavin, K. Reid, and B. Robinson for technical support while conducting this research.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Oregon Agric. Exp. Stn. Technical Paper no. 11539.

Received for publication October 4, 1999.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 





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