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SEED PHYSIOLOGY, PRODUCTION & TECHNOLOGY

Birdsfoot Trefoil Seed Production

II. Plant-Water Status on Reproductive Development and Seed Yield

^a Instituto Nacional de Investigaciones Forestales y Agropecuarias, CE Pabellon, Carretera Ags.-Zac. km. 32.5, Apdo. Postal 20, Pabellon de Arteaga, Ags. C.P. 20660, Mexico
^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 REFERENCES

 
Forage legume seed crop responses to water stress differ for each species, so a single optimal water management strategy is not applicable. The objectives of this study were to determine the effects of irrigation timing and replenishment amount on birdsfoot trefoil (Lotus corniculatus L.) reproduction and seed yield in the Willamette Valley of western Oregon, USA. Six treatments varying in water depletion percentage and replenishment amount were applied in 1994 and 1995 on a Woodburn silt loam soil (fine-silty, mixed, mesic Aquultic Argixeroll) near Corvallis. In 1996, only a low stress (LS) that met the weekly crop evapotranspirative demand and a non-irrigated control (C) treatment were investigated. In the first year of production, maintaining plants under low-stress conditions sustained flowering longer than with limited or no irrigation. Flowering was not affected by irrigation in the subsequent two production years. Total above-ground phytomass was correlated with the amount of irrigation water (r = 0.92). The C and all single application treatments had greater seed yields (SY) than the LS treatment in 1994. In 1995, all single application treatments had greater SY than the LS treatment. There was no difference between LS and C in 1995 and 1996. Umbel density and the number of seeds per pod were the primary determinants of total seed yield (r = 0.77 and 0.92, respectively). Optimal total seed production was achieved without supplemental irrigation under the humid temperate marine climatic conditions found in western Oregon.

Abbreviations: C, non-irrigated control treatment • CWSI, crop water stress index • DOY, day of year • ET_c, evapotranspiration • FAWU, fraction of available water used • LS, low stress treatment • SY, seed yield • and TAGP, total above-ground phytomass

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
BIRDSFOOT TREFOIL is a perennial, non-bloating forage legume used for pasture, hay, and silage in the midwestern and northeastern USA and eastern Canada. Annual U.S. certified seed production is estimated to be about 83 Mg from 830 ha (AOSCA, 1994) and the total area for seed production in North America estimated to be 4800 ha (Beuselinck, 1997). Seed yields have been reported to range from 50 to 560 kg ha^-1 (Seaney and Henson, 1970; McGraw and Beuselinck, 1983; White et al., 1987; Li and Hill, 1989), with 50 to 170 kg ha^-1 considered as average (Seaney and Henson, 1970; McGraw and Beuselinck, 1983; Winch et al., 1985). Though the wide range in seed yield is in part due to seed pod dehiscence and shattering before or during harvest, soil-water availability also may be a factor that constrains birdsfoot trefoil seed yields.

Research has been carried out on birdsfoot trefoil seed production and some of the most important limiting factors have been identified (Fairey, 1994). However, there is no information available on water management of birdsfoot trefoil grown for seed. Water management practices for maximal seed production of other forage legume seed crops are distinct from those for hay or pasture crops (Hutmacher et al., 1991; Steiner et al., 1992). In most crops, appropriate soil-water availability is needed to promote flower development, pollination, seed growth, and maturation. Forage legume species have distinct responses for water stress adjustment and there is no general water management strategy for all forage legume seed crops. Alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) seed yields can be optimized by limiting the plant vegetative growth by controlled water stress (Clifford, 1985; 1986; Steiner et al., 1992, Oliva et al., 1994c). Red clover (T. pratense L.), however, responds optimally when there is no or low water stress during the reproductive phase of growth (Oliva et al., 1994b). Optimal irrigation management also can differ between the year red and white clover seed crops are first established and successive years of seed production in western Oregon (Oliva et al., 1994a; 1994c, respectively), and by the quantity of water the soil profile receives during the winter when growing an alfalfa seed crop in central California (Steiner et al., 1992).

Birdsfoot trefoil flowering occurs from continuous shoot succession with new shoots replacing older shoots after appropriate conditions for flower induction are met (Li and Hill, 1988). Plants grown under low water-stress conditions may have delayed flowering, particularly when there is space competition between vegetative and reproductive structures (Li and Hill, 1989).

The most important birdsfoot trefoil seed yield component reported is the number of umbels per unit area (Albrechtsen et al., 1966; Bresciani and Frakes, 1973; Pankiw et al., 1977; McGraw et al., 1986; Stephenson, 1984; Li and Hill, 1988; 1989). Management practices that reduce the number of umbels per unit of area will ultimately decrease seed yield. When the number of umbels is not limited, yield components such as number of florets per umbel, number of seeds per pod, number of pods per umbel, and seed mass may influence seed yield.

The objective of this research was to determine the effects of irrigation timing and replenishment amount on birdsfoot trefoil reproductive development, seed yield, and yield components when grown in the Willamette Valley seed production region of western Oregon, USA.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
The experiment was conducted in 1994, 1995, and 1996 at the Oregon State University, Hyslop Field Laboratory near Corvallis on a Woodburn silt loam soil (fine-silty, mixed, mesic Aquultic Argixeroll). The experimental area was fumigated with methyl bromide (360 kg ha^-1) preceding seedbed preparation to evenly control weeds. Birdsfoot trefoil `MU-81' (Beuselinck and McGraw, 1986) was planted 30 Aug. 1993 into a level seedbed in rows 0.6 m apart and at a rate of 2.3 kg ha^-1. Water was applied with high-pressure overhead sprinklers just after planting and as needed during the following 22 d to establish the crop. Common agronomic practices for forage legume seed production including weed and insect control were used (Garcia-Diaz and Steiner, 1999). Four honey bee (Apis mellifera L.) hives were placed close to the experimental area each year at initial bloom time to ensure adequate pollination.

The plots were arranged in a randomized complete block design with four replications and six treatments in 1994 and 1995. Only two of the six treatments were used in 1996. Each plot was 4.5 m wide by 10 m long and was surrounded by a furrow with dikes to prevent lateral surface water movement if application rates exceeded the soil-water infiltration rate. The 1-m wide alleys at the ends of each plot were also diked.

A surface trickle irrigation system delivered water uniformly to each plot through 3.5 L h^-1 in-line, turbulent-flow emitters spaced 0.9 m apart in five plastic drip lines 60 cm apart and perpendicular to the planting rows. A distribution manifold consisting of a mesh filter, ball valve, residential water flowmeter, volumetric controller, and a pressure regulator allowed water to be applied to all four replications of each treatment at the same time.

For 1994 and 1995, five supplemental irrigation treatments were applied between the period from herbage clip-back to seed harvest. The treatments were (i) low-stress (LS), in which the soil-water content was maintained close to 100% field capacity (FC) by two or three water applications per week during the period from early-June until 3 wk before seed harvest; (ii) single water replacement to 50% FC when soil-water depletion was 30% (D30–F50); (iii) single water replacement to 100% FC when soil-water depletion was 30% (D30-F100); (iv) single water replacement to 50% FC when soil-water depletion was 60% (D60-F50); (v) single water replacement to 100% FC when soil-water depletion was 60% (D60-F100); and (vi) a non-irrigated control (C). In 1996, only treatments LS and C were evaluated.

The soil-water status of each treatment was monitored weekly from the time of herbage removal until seed harvest by neutron attenuation (Cuenca, 1988). The neutron attenuation measurements were calibrated using available published data for the same local soil conditions (Oliva, 1992). Readings were taken 48 h after each water application at depths of 0.45, 0.65, 0.95, 1.25, 1.55, and 2.00 m below the soil surface in all plots. The soil-water measurements were based on the number of hydrogen atoms in a 40-cm-diam sphere (Cuenca, 1988). Procedures for determining volumetric soil-water content, total available soil-water, and fraction of available water used (FAWU) are described in Garcia-Diaz and Steiner (1999).

Weather data were obtained from a monitoring station located 400 m from the trial site. Seasonal estimated crop evapotranspiration (ET_c ) was the sum of applied water, precipitation, and the change in soil-water content estimated by neutron attenuation (Cuenca, 1988). On the basis of the neutron attenuation measurement through the soil profile, it was assumed that no deep percolation occurred in 1994 and 1995. Drainage through the soil profile was presumed to have occurred in 1996 through root channels of plants that died during winter 1996 (Garcia-Diaz and Steiner, 1999).

To estimate the soil-induced, plant-water stress, a crop water stress index (CWSI) was estimated (Idso et al., 1981) with a Scheduler¹ (Plant Stress Monitor, Carborondum Co. Solon, OH) that measured crop canopy temperature, air temperature, and water vapor pressure deficit (Garcia-Diaz and Steiner, 1999). Five measurements were taken at least once a week in all plots from 24 June [day of year (DOY) 175] to 25 August (DOY 237) in 1994; from 17 July (DOY 198) to 25 August (DOY 237) in 1995; and from 5 July (DOY 186) to 22 August (DOY 234) in 1996 on clear, cloud-free days, and between 1200 to 1400 h. Measurements were taken 1 m from the top of the canopy at a 45° oblique angle facing northwest from both sides of the longer west-east axis of each plot.

Umbel and pod density were estimated weekly from six 0.1-m², random samples per plot from the time of first bud appearance until seed harvest. The pods were classified as (i) immature pods, pod color ranges from dark green to light green; (ii) mature pods, pod color is green-white to dark brown; and (iii) dehisced pods, pods begin to twist and dehisce and the seeds shatter (Winch et al., 1985). Peak flowering was defined as the time when the maximum umbel density was initially obtained.

Seed loss due to shattering was determined every Monday, Wednesday, and Friday by collecting shattered seeds from two plastic pans (60 by 40 by 10 cm) placed 10 cm above the soil surface between two rows and below the crop canopy of each plot. The shattered seeds were collected with a vacuum, cleaned, and weighed. Shattered seeds at harvest was calculated as the sum of all seeds collected from the time of initial seed shattering until harvest time.

Seed was harvested when most pods were light-tan to brown-colored, the number of mature pods had reached a maximum, and shattering had begun. Two subplots, 1 by 4 m, were harvested from both ends of each plot by a gas-powered mower in the early morning to avoid seed shatter losses. The plant material was collected by hand, bagged, and dried at 32°C for 1 d. Above-ground phytomass was weighed, the seeds threshed from the plant material, and the seeds cleaned and weighed. Harvested seed yield (SY) was the amount of non-shattered seeds at harvest time. Total above-ground phytomass (TAGP) was above-ground phytomass minus harvested SY. Harvest index (HI) was calculated by dividing the SY by TAGP. Total SY was the sum of harvested seed yield plus the accumulated shattered seed losses until harvest time. Seed yield components were estimated from mature umbels collected from six 0.1 m², random samples taken at harvest time. The number of pods per umbel were estimated from 20 random umbels per sample, number of seeds per pod from 40 random pods, and mean seed mass from four random samples of 200 seeds.

The effect of soil-water availability on the relative contribution of each seed yield components was determined by path-coefficient analysis (Fig. 1) combined over all the irrigation treatments (Oliva et al., 1994b, 1994d). This analysis quantifies the direct influence of one yield component upon another and allows the partitioning of the correlation coefficient into direct and indirect effects (Li, 1956; Dewey and Lu, 1959). Path-coefficient analysis has been previously used to determine seed yield component relationships among birdsfoot trefoil genotypes (Albrechtsen et al., 1966).

View larger version (16K): [in this window] [in a new window]   Fig. 1 Causal relationships of the path-coefficient analysis for birdsfoot trefoil seed production; six irrigation treatments were used. Doubled-arrowed lines indicate mutual associations that are measured by correlation coefficients (rij), and single-arrowed lines represent direct influence as measured by path-coefficients (Pij)

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Inflorescence Development
The dates of irrigation initiation for LS treatment were similar all 3 yr (June 8, DOY 160 in 1994 and 1995; and June 7, DOY 159, in 1996). The time of application of the single water replacements that were based on available soil-water amount thresholds was delayed in 1995 until soil-water depletion levels were similar to those of 1994. The soil-water content at the time of the irrigation initiation for LS treatment was different in the 3 yr because of different amounts of precipitation received during the spring prior to the water applications (Table 1) . The amount of available soil water that had been depleted at forage removal time was 16, 11, and 20% in 1994, 1995, and 1996, respectively. The amounts of precipitation received from March to May were 165, 290, and 315 mm, respectively.

View larger version (43K): [in this window] [in a new window]   Fig. 2 Number of flowers and crop water stress index until harvest time for six birdsfoot trefoil irrigation treatments in 1994 and 1995, and two treatments in 1996. Treatment codes are indicated on the upper-right corner of each graph. Arrows and vertical lines indicate time of irrigation application (not shown for LS treatment) and standard error of the mean (SEM), respectively. Data symbols without SEM have errors smaller than the size of the symbol

Seed Yield and Total Phytomass
Harvested SY was generally greater for all treatments in 1994 (the first year of production) than in 1995 and 1996 (580, 260, and 230 kg ha^-1, respectively; Table 2) . Seed yield reductions in the later two years may have been due to the plants developing successively more extensive root systems after the establishment year of growth and which resulted in more soil-water being available to support vegetative plant growth (Garcia-Diaz and Steiner, 1999). In 1994, all deficit irrigation treatments and the non-irrigated control had greater harvested SY than the LS treatment. In 1995, all single irrigation treatments had greater harvested SY than the LS treatment. There were no differences between the LS and non-irrigated control in 1995 and 1996.

View larger version (24K): [in this window] [in a new window]   Fig. 3 Total seed yield (harvested plus shattered seeds) as a function of fraction of available water used (FAWU) at time of irrigation for six birdsfoot trefoil irrigation treatments in 1994, and two treatments in 1996. Symbols indicate: LS (, , ) and C (+, *, open plus sign) for 1994, 1995, and 1996, respectively. Symbols indicate: D30–F(50, 100) (, •), and (, ); and D60–F (50, 100) (, ) and (, ) for 1994 and 1995, respectively. Broken line between two irrigation treatments in 1996 is for identification purposes and not for interpolation

View larger version (18K): [in this window] [in a new window]   Fig. 4 Total above-ground phytomass (TAGP) as a function of average estimated crop evapotranspiration (ETc) for six birdsfoot trefoil irrigation treatments in 1994 and 1995, and two treatments in 1996. Symbols indicate: LS (, , ) and C (+, *, open plus sign) for 1994, 1995, and 1996, respectively. Symbols indicate: D30–F(50, 100) (, •), and (, ); and D60–F (50, 100) (, ) and (, ) for 1994 and 1995, respectively

View larger version (21K): [in this window] [in a new window]   Fig. 5 Total above-ground phytomass (TAGP) as a function of average seasonal crop water stress index (CWSI) for six birdsfoot trefoil irrigation treatments in 1994, 1995, and two treatments in 1996. Symbols indicate: LS (, , ) and C (+, *, open plus sign) for 1994, 1995, and 1996, respectively. Symbols indicate: D30–F(50, 100) (, •), and (, ); and D60–F (50, 100) (, ) and (, ) for 1994 and 1995, respectively

View larger version (18K): [in this window] [in a new window]   Fig. 6 Relationship of seasonal estimated crop evapotranspiration (ETc) and amount of water applied for six birdsfoot trefoil irrigation treatments in 1994 and 1995. Symbols indicate: LS (, , ) and C (+, *, open plus sign) for 1994, 1995, and 1996, respectively. Symbols indicate: D30–F(50, 100) (, •), and (, ); and D60–F (50, 100) (, ) and (, ) for 1994 and 1995, respectively

Seed Yield Components
The water management treatments affected the expression of seed yield components differently in the first two production years (Table 4) . Over all treatments, plants grown under conditions that increased ET_c (Garcia-Diaz and Steiner, 1999) reduced umbel density (r = -0.53; P 0.05). In addition, the number of pods per umbel was negatively associated with seed yield (r = -0.75; P 0.01). These results suggest that conditions that are conducive to increasing the number of pods per umbel are not advantageous for increasing the number of seeds produced within each pod. It may be that birdsfoot trefoil pods cannot support all fertilized ovules beyond a threshold level of seed set during the seed maturation period. This has been suggested as a mechanism that limits the number of seeds produced per pod in red clover (Clifford and Scott, 1989). The number of seeds per pod has been shown to be a significant determinant of seed yield differences among birdsfoot trefoil genotypes (Bresciani and Frakes, 1973).

In 1995, the LS treatment also had the lowest umbel density and number of seeds per pod, but also had the lowest seed mass of all treatments (Table 5). With the most delayed water application time and the greatest water replenishment amount (D60–F100), umbel density, and seed mass were optimized. The lower water replenishment amount (D60–F50) resulted in the greatest number of seeds produced per pod (Table 5), but total SY was unaffected by any of the treatments (Table 2). As determined by path analysis, the only significant direct effect was the negative association with pods per umbel with total SY, which differs from the results observed in 1994 (Table 4). Seed yield also was correlated with the number of umbels per unit area and seed mass, but no other indirect effects were observed with the other seed yield components.

In 1996, the number of pods per umbel was greater in the LS than non-irrigated control, whereas the other seed yield components were unaffected (Table 5). Maintaining the plants under low water stress conditions reduced the number of seeds produced per pod in 1995 and 1995, but not in 1996. Path analysis was not employed in 1996 because only C and LS irrigation treatments were examined.

In conclusion, optimal seed production was achieved in western Oregon by not irrigating this crop. Similarities in total amounts of seeds produced among different irrigation treatments after the first year of production probably were due to soil water being depleted from greater depths in the second and third years of production, which masked the effects of the different water application times and replenishment amounts. The negative birdsfoot trefoil seed crop response to irrigation differs significantly from the positive response for alfalfa, red clover, and white clover grown for seed. First seed-year seed yields were greater and later seed-year yields similar to average seed yields reported by others for traditional birdsfoot trefoil seed production regions (Seaney and Henson, 1970; McGraw and Beuselinck, 1983; White et al., 1987; Li and Hill, 1989). These results are applicable to environments with humid temperate marine climatic conditions as found in western Oregon and much of central and northern Europe.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 
Oregon Agric. Exp. Stn. Technical Paper No. 11,393.

¹ The use of trade names in this publication does not imply endorsement of the products named nor criticism of similar ones not mentioned.

Received for publication August 28, 1998.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (3) Citing Articles via Google Scholar Google Scholar Articles by Garcia-Diaz, C.A. Articles by Steiner, J.J. Search for Related Content PubMed Articles by Garcia-Diaz, C.A. Articles by Steiner, J.J. Agricola Articles by Garcia-Diaz, C.A. Articles by Steiner, J.J.