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Forages

Trade-Offs in Forage and Seed Parameters of Annual Medicago and Trifolium Species in North-Central Texas as Affected by Harvest Intensity

^a Texas A&M Univ. Agric. Exp. and Ext. Cent., 1229 North U.S. Hwy. 281, Stephenville, TX 76401
^b Texas A&M Univ. Agric. Res. Stn., Beeville, TX. 78102

^* Corresponding author (j-muir{at}tamu.edu)

Received for publication December 8, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The trade-off between early spring harvest of cool-season annual forage legumes and summer seed production may shed light on subsequent yearly stand regeneration. Sixteen annual clover and medic cultivars or locally collected accessions were seeded in the autumn of 1999 at Stephenville, TX, on a Windthorst (fine, mixed thermic Udic Paleustalf) fine sandy loam and allowed to self-reseed in the autumn of 2000 and 2001. During the first 2 yr, three harvest treatments were imposed: harvest at 5 cm from crown whenever branches reached 10 cm from crown, harvest at 10 cm when branches attained 15 cm, and a single April harvest. Forage yields showed a year x species interaction (P = 0.001) and ranged up to 6 Mg ha^–1 yr^–1. Plants harvested in April only were consistently more productive than the 5-cm harvest and yielded nearly 4.9 Mg ha^–1 in Year 1. Forage acid detergent fiber and crude protein concentrations reflected entry x harvest x year interactions (P < 0.05). Seed yields varied considerably among entries, and all the annual clovers and most medics were able to self-reseed regardless of harvest treatment, showing promise for self-reseeding, sustainable pasture systems in the southern Great Plains.

Abbreviations: ADF, acid detergent fiber • CP, crude protein • DM, dry matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ALTHOUGH A WORLDWIDE interest in forage legumes often focuses on perennials (Carlier and Machiels, 1998), cool-season annuals such as medics and clovers have been identified as more promising for use in self-reseeding pastures where rainfall is limited and summer temperatures high (Holt and Weaver, 1981; Ocumpaugh et al., 1998; Muir, 2000; Walsh et al., 2001). Many have become widely naturalized in the southern Great Plains (Diggs et al., 1999), indicating adaptation to climate, soils, herbivory, and pathogens. They are recommended for both domesticated livestock (Ball et al., 1996) and wildlife (Gee et al., 1994).

Climate, especially temperature and rainfall, has a strong effect on annual legume development and reproduction (Cocks, 1995; Butler et al., 2002). Annual medic species in long growing seasons in southern Australia, for example, took 10 to 70 d longer to flower than in Wyoming, USA (Crawford et al., 1989; Walsh et al., 2001). Some medics that complete their life cycle in early summer in the south-central USA (Ocumpaugh et al., 1998) will grow throughout the summer and flower in the fall in north-central USA (Zhu et al., 1996). Forage nutritional value and seed production is likewise affected by climate and soil (Cocks, 1995).

More information is needed, however, on the appropriate management of naturalized and cultivated annual legume stands. Although seed production is essential for stand persistence (Muir et al., 2001) and can be affected by harvest regime (Muir and Reed, 1998) and climate (Cocks, 1995), little is known about the trade-off between forage and seed production as affected by plant harvest height, particularly as herbage removal influences stand regeneration in subsequent years. The objective of this study was to examine the effects of harvest height on seed production and season-long forage production and nutritive value of sixteen self-reseeding winter annual forage legumes.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The experiment was performed during the fall–winter growing seasons of 1999–2000 and 2000–2001, with a final forage harvest in 2002 at Stephenville TX (32°15' N, 98°12' W; altitude 395 m). The soil was a Windthorst fine sandy loam (pH 6.0, 8 mg P kg^–1, 247 mg K kg^–1, 1751 mg Ca kg^–1, and 529 mg Mg kg^–1), and no fertilizers were applied throughout the trial in an effort to emulate local soil conditions. The site was tilled in November 1999 before seeding and sprayed with clethodim {(E)-2-[1[(3chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} at 85 g a.i. ha^–1 and ammonium salt of imazethapyr {±2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid} at 28.4 g a.i. ha^–1 in December of both 1999 and 2000 to control grasses and broadleaf weeds. In addition, carbaryl (1-naphthyl N-methylcarbamate) at 1.7 kg a.i. ha^–1 was applied in March of both 2000 and 2001 to control herbivory by larval alfalfa weevil (Hypera postica) and twelve-spotted cucumber weevil (Diabrotica unclecimpunctata howardi).

Rainfall in the first season of the trial was low and totaled 332 mm from October 1999 to May 2000 (Table 1). Irrigation (overhead sprinklers) totaled 89 mm for the season and was applied in months that had less than 50% the long-term average rainfall. This resulted in a total of 421 mm moisture for the 1999–2000 winter growing season compared with a 449-mm 30-yr precipitation average for the same period. Rainfall the second (695 mm) and third seasons (565 mm) was greater and well distributed so that no irrigation was applied.

Seedling counts in a 0.09 m² that had not been disturbed in April of the previous year were taken in December of 2001, the third growing season after planting, from each subplot. The harvest treatments were not applied during Year 3 to measure the residual effect from harvests the previous 2 yr. Forage yields were measured in all subplots in a single 2002 late-April harvest.

Representative forage subsamples from each harvest of Years 1 and 2 were ground through a sheer mill equipped with 1-mm screen and analyzed for acid detergent fiber (ADF) and N concentrations. The fiber component was measured by methods defined by Van Soest and Robertson (1980). Total N concentrations were measured with a modification of the aluminum block digestion procedure of Gallaher et al. (1975). Sample weight was 1.0 g, digest used was 5 g of 33:1:1 K₂SO₄:CuSO₄:TiO₂, and digestion was conducted for 2 h at 400°C using 17 mL of H₂SO₄. Nitrogen in the digestion fluid was determined by semiautomated colorimetry (Hambleton, 1977) using a Technicon Autoanalyzer II (Technicon Industrial Systems, Tarrytown, NY). Nitrogen concentration was multiplied by 6.25 and reported as crude protein (CP) (Van Soest, 1994). Concentrations of these plant components are reported as season-long weighted averages for each subplot.

As they matured, seeds were collected from a 0.3- by 0.3-m area outside the 1.0-m² portion of each subplot used for measuring forage yield during the spring of the first and second years. This was done to avoid having artificial seed dispersal from influencing subsequent forage yields although seeds were returned to their 0.09 m² of origin in the respective subplots. These collections occurred in April through May, depending on the maturity of each entry. Seed yields from April-only subplots therefore represent the seed production potential of the entries where no forage harvest removal occurred. Collected seedpods were then dried, broken down in a Forsberg drum lined with sandpaper, and the seeds removed by hand to estimate seed yield per square meter.

Plots were arranged in a randomized complete block design (three replications), and clipping heights were superimposed on these in a split-plot arrangement. Year (1 and 2), entry, and harvest regime were used as independent variables in the model and analyzed for interactions. Year was included in the model despite the fact that plots were seeded the first year and self-reseeded the second year since seedling numbers were not considered to be limiting either year. Residual forage yields the third year (2001–2002) were not compared to previous years since harvest treatments were not imposed during this final season. Results were submitted to an analysis of variance with Duncan's multiple range test (P < 0.05) used to separate multiple (>2) means in the case of significant effects.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Forage Yield
Initiation of measurable forage production (Table 2) differed among species and was affected by origin of entries of the same species. Ball clover (Trifolium nigrescens Viv.) and crimson clover (T. incarnatum L.) had earlier harvestable production (February) than the rose clover (T. hirtum All.) (March). Among the annual medics, ‘Armadillo’ burr (Medicago polymorpha L.), ‘Jemalong’ barrel (M. truncatula Gaertn.), ‘Estes’ button [M. orbicularis (L.) Bartal.], and BEBLK black (M. lupulina L.) all had measurable yields in February. The BEFLK burr, Stephenville spotted burr [M. arabica (L.) Huds.], and BEUKT burr medics had measurable production only in April. The more precocious entries would likely be better suited for early-season grazing. A blend of early and late-maturing types would provide a longer season of high quality grazing and likely enhance the N contribution to the pasture.

There was an entry x year interaction (P = 0.001) for total yearly forage yields (Table 3). Jemalong barrel and BEDEV little burr [M. minima (L.) Bartal.] medics both produced over 5800 kg forage ha^–1 the first year, a contrast to less than 1300 kg ha^–1 produced by the Stephenville spotted medic and BEFLK burr medic during the same year. ‘George’ black medic, although able to produce 4.1 Mg ha^–1 the season of planting compared with an average 2.2 Mg ha^–1 yr^–1 in a north-central USA study (Zhu et al., 1996), was the lowest-producing entry the second year, despite having a medium growth and development period (Walsh et al., 2001) and was outyielded by BEBLK black medic both years. During the second year, ‘Dixie’ crimson clover outyielded all other entries, including Armadillo burr medic, the second most productive, by 12%, an indication of strong re-establishment potential reported by Cassida et al. (2000). These two entries, along with six others, increased production from Year 1 to Year 2, perhaps as a result of earlier seedling establishment and improved rainfall patterns during the second year. In contrast, the barrel, little, and black medics decreased in productivity in Year 2 despite greater precipitation, perhaps explained by suboptimal seedling numbers due to hard seed or slow rate of breakdown of hard seed common in legumes (Rolston, 1978).

View larger version (49K): [in this window] [in a new window]   Fig. 1. Forage yield of cool-season annual legumes at Stephenville, TX, during 2 yr harvested once in April only at 10-cm height throughout the season and 5-cm height throughout the season (pooled over 16 entries; harvest regime x year interaction P = 0.01; columns within the same year headed by different letters differ at P < 0.05 according to Duncan's multiple range test).

Rose clover, the entry with consistently high ADF concentrations, has been reported elsewhere as high in this fiber component (Muir, 2000). In contrast, black and button medics have lower ADF concentrations relative to the burr medics (Muir, 2000), perhaps as a result of late-season development. Muir and Reed (1998) reported ADF concentrations as low as 157 g kg^–1 forage for burr medics and 196 g kg^–1 forage for crimson clover when these were harvested earlier (January through March) before seed set.

Acid detergent fiber concentrations tended to be high for rose clover regardless of harvest regime and for both crimson clovers in the 10-cm and April-only harvests. Acid detergent fiber in some of the burr medics such as the BEFLK, BECOM, and Armadillo was greater in the first-year 5-cm harvest compared with the second year because less moisture and later autumn establishment during the first year provided less leaf for early spring insect leaf predation that, in turn, appeared to increase the proportion of stem component. In contrast, Year 2 ADF concentrations, when autumn stand establishment occurred earlier and greater precipitation resulted in greater leaf production, were lower in the 5-cm harvests of these entries.

Forage Crude Protein
There was an entry x harvest x year interaction (P = 0.02) for forage CP concentrations (Table 5). Concentrations were invariably greatest in the 5-cm harvest and either lower or comparable in the other two harvests. Crude protein was particularly low in the April-only harvested forage of the crimson and rose clovers both seasons, within the range (134 g kg^–1) reported for crimson clover by Lloveras and Iglesias (2001). These entries appeared to have more stem in harvested material due to upright growth habits, and this was compounded, in the case of crimson clover, by early maturation. A decline in crimson clover leaf/stem ratio has been implicated in declining nutritive value with maturity (Lloveras and Iglesias, 2001).

The CP values measured in this trial were comparable to those reported for other trials (Muir, 2000; Muir et al., 2001; Walsh et al., 2001). In contrast, CP values reported by Muir and Reed (1998), who started harvesting medics and clovers earlier (January) and ceased harvesting sooner (March) in the season, ranged over 300 g kg^–1 forage.

Seed Yield
Although annual legume seed production is often correlated to climate, especially rainfall (Cocks, 1995), seed yields were similar in both years of the study (Fig. 2; P = 0.001). Low seed yields for some of the entries did not necessarily translate into fewer seedlings the year following the trial (Table 6). BEBLK black medic had the greatest seed yield, over 1700 kg ha^–1 yr^–1 when averaged over years and harvest regimes. Seed yields for some species were greater than those reported in other trials with the same species (Muir et al., 2001; Dear et al., 2002). George black medic was the only entry, among the lower seed producers, whose forage yields the second season may have been negatively affected by low first-season seed yields. Seed yields of all entries the first year exceeded their recommended seeding rates, as reported elsewhere for annual medics and clovers (Cassida et al., 2000), but viability, predation, and hard seed may have contributed to less than 100% germination at the beginning of the second and third year in the case of George black medic.

View larger version (24K): [in this window] [in a new window]   Fig. 2. Seed yield of 16 self-reseeding, cool-season annual legumes at Stephenville, TX (pooled over three harvest regimes and two year; columns with the same letter do not differ according to Duncan's multiple range test; see Table 2 for complete names).

Third-Season Residual Stand
December seedling counts and April forage yield of the third year following a season without harvest treatments indicated no measurable (P > 0.10) residual effects of different defoliation treatments during the previous 2 yr. December seedling counts ranged from over 3300 per m² for BEBLK black medic and ball clover down to 200 to 600 for Estes and Stephenville button medic, George black medic, or BEUKT and BEFLK burr medics (Table 6). These two groupings reflected seed yields in the previous 2 yr although efficiency of seedling establishment from average yearly seed production (seedling/seed) ranged from a low of 1.0% for ball clover up to 15.2% for Armadillo burr medic, lower than the 27 to 54% reported for Armadillo in a previous study (Muir et al., 2001). Seedling numbers were generally within the range of those reported in the literature for cultivated stands or greater than those reported for naturalized stands of annual cool-season legumes (Rumbaugh and Johnson, 1986; Wagner and Spira, 1994; Muir et al., 2001).

Greater yield of seeds for entries in Years 1 and 2 (Fig. 2) did not necessarily translate into greater forage yield the third year after planting (Table 6). The clover entries were among the greatest forage producers the third year following two seasons when, except for ball clover, they did not produce greater seed yields than some medics. Relatively low rates of hard seed in crimson clover compared with annual medics and other clovers (Smith, 1993; Hoveland and Evers, 1995) did not appear to explain this lack of correlation, however, since crimson clover seedling numbers were not greater than some of the medic seedling numbers. BEBLK black medic and ball clover (extremely small seed) both produced high seed yields during the trial as well as the greatest number of seedlings the third year following two seasons of harvest treatments. Prolonged low temperatures reaching down to –12°C during a 7-d period in March 2002 severely damaged all medic seedlings late in Year 3 (data not shown) and affected spring forage yields (Fig. 1). Crimson clover has been identified as frost tolerant compared with some medics, especially barrel (Brandsaeter et al., 2002), and both the crimson clovers and ball clover produced over 5 Mg DM ha^–1 yr^–1 despite the cooler temperatures. Among the medics, the two button entries outyielded all others by nearly 1 Mg ha^–1 yr^–1 in the single April harvest of Year 3.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Seed yields and seedling counts support the conclusion that these annual forage legumes, as a group, tolerated herbage removal with little negative effect on stand persistence. Stand management decisions can therefore be made based on nutritional needs of animals, with less concern for seed production and stand regeneration in subsequent years. Further research, however, is needed to determine how much cutting or grazing (both intensity and interval) these plants will tolerate and still produce sufficient seed to replenish soil seed banks and whether this differs from actual grazing pressure in mixed grass and legume pastures.

Forage nutritive value, by contrast, was negatively affected if plants were allowed to accumulate forage throughout the growing season without some herbage removal. Nutritive value of the clovers especially declined when no regrowth was stimulated, and entries that produced the majority of their forage late in the season showed less severe nutritive value decline. With over 5 Mg forage yr^–1 produced by some of the entries, there appeared to be good potential for considerable yields of good nutritive value in regions with climates similar to the Cross Timbers of Texas and Oklahoma.

The range in forage or seed production and seasonal growth patterns observed within these 16 cool-season annual forage legumes, many already naturalized in the south-central USA, indicated that there is likely an appropriate entry for most production systems. This would include forage growth patterns for early and or late grazing; heavy seed producers for game birds, even under severe forage harvest pressures; and substantial production of high quality forage for herbivores, both domesticated and wild.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS 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 Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Muir, J. P. Articles by Butler, T. J. Search for Related Content PubMed Articles by Muir, J. P. Articles by Butler, T. J. Agricola Articles by Muir, J. P. Articles by Butler, T. J. Related Collections Forage Management Other Forage Crops