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FORAGES

Productivity and Survival of Defoliated Wheatgrasses in the Rolling Plains of Texas

^a Texas A&M Univ., Texas Agric. Exp. Stn., P.O. Box 1658, Vernon, TX 76385
^b The Samuel Roberts Noble Foundation, P.O. Box 2180, Ardmore, OK 73402

^* Corresponding author (d-malinowski{at}tamu.edu)

Received for publication February 21, 2002.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
In the Texas Rolling Plains, cool-season perennial grasses may complement limited forage availability in March–May and October–December. In two experiments conducted at Vernon, TX, on a sandy loam soil (fine-loamy, mixed, thermic Udic Paleustalfs), we evaluated productivity and persistence of crested [Agropyron cristatum (L.) Gaertn. x A. desertorum (Fisch. ex Link) J.A. Schultes], hybrid [Elytrigia repens (L.) Nevski x Pseudoroegneria spicata (Pursh) L‘ve], intermediate [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey], pubescent [T. intermedium ssp. barbulatum (Schur) Barkw. & D.R. Dewey], and tall wheatgrass [T. ponticum (Podp.) Barkworth & D.R. Dewey] under combinations of 3- or 6-wk defoliation frequency at 7.5- or 15-cm. About 64% more herbage yield was harvested from all species at the 7.5- vs. 15-cm defoliation height in the first, but only from crested, hybrid, and pubescent wheatgrass in the second growing season. Frequent defoliation increased herbage by 22% in Experiment A or 37% in Experiment B only in the first growing season. Tiller survival increased with frequent defoliation in intermediate wheatgrass by 33% in Experiment A and up to 70% in Experiment B, but decreased by 52% in hybrid and by 33% in pubescent wheatgrass in Experiment A, and up to 50% in Experiment B. Lower nighttime soil temperatures increased tiller survival during summer in swards defoliated at the 7.5- vs. 15-cm height. Wheatgrass productivity increased under intensive or frequent defoliation in the first, but declined in several species in the subsequent growing season, making their potential to complement forage base limited.

Abbreviations: a.i., active ingredient • DM, dry matter • LAI, leaf area index

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
SEMIARID REGIONS of the Texas Rolling Plains are characterized by relatively mild winters and hot, dry summers often with prolonged drought (Kephart et al., 1995). In these environments, C₄ grasses are the dominant component of rangelands and improved pastures grazed by livestock during summer and fall (Weakley et al., 2000). High quality winter forage is based mostly on dual-purpose wheat (Triticum aestivum L.) pastures (Pinchak et al., 1996). The gaps in forage availability from early March (termination of grazing on dual-purpose wheat) until May (grazing on warm-season grass pastures) and from October until December (low quality forage from dormant warm-season grass pastures) may be filled by improved cool-season perennial grasses (Reuter et al., 1999). Their forage quality is generally better than that of dormant warm-season grasses (Kloppenburg et al., 1995). Therefore, perennial cool-season grasses may be considered as potential source of added value in beef production systems. Winter weather conditions are favorable for growth of most cool-season perennial grasses in the Texas Rolling Plains. Soil water deficits and high temperatures during summer months, however, reduce survival and significantly limit the number of potentially adapted species (Redmon, 1997).

Perennial cool-season grasses growing in semiarid environments with prolonged summer drought often have the ability to survive and recover rapidly once water becomes available in the fall (Kemp and Culvenor, 1990). Wheatgrasses native to the Great Plains meet this criterion, but are usually less grazing tolerant and have lower nutritional quality than wheatgrasses introduced from Europe and Asia (Asay, 1995). Responses of these C₃ grasses to defoliation intensity and frequency are not well understood at the margin of their survival range in C₄–dominated environments (Redmon, 1997). Our objectives were to determine productivity and persistence of several introduced wheatgrass species under a range of defoliation frequency and intensity combinations, and the relationship between defoliation regimes and soil temperature during summer drought on wheatgrass persistence.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Experiment A
Wheatgrass species were represented by CD II crested wheatgrass [Agropyron cristatum (L.) Gaertn. x A. desertorum (Fisch. ex Link) J. A. Schultes], NewHy hybrid wheatgrass [Elytrigia repens (L.) Nevski x Pseudoroegneria spicata (Pursh) Löve], Oahe intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D. R. Devey], Manska pubescent wheatgrass [T. intermedium ssp. barbulatum (Shur) Barkworth & D. R. Devey], and Jose tall wheatgrass [T. ponticum (Podp.) Barkworth & D. R. Devey]. Plots were planted in September 1998 near Vernon, TX, on a Miles fine sandy loam (fine-loamy, mixed, Thermic Udic Paleustalfs). Initial soil pH was 6.6, with adequate concentrations of P (20 mg kg^-1) and K (300 mg kg^-1). Before planting, plots were fertilized by deep placement of 45 kg N ha^-1 as liquid NH₄NO₃ and microelements. Seeds were planted with a Tye Pasture Pleasure & Stubble Drill (AgEquipment Group LP, Lockney, TX) at row spacing of 0.18 m in tilled soil at the seeding rate of 19 kg pure live seeds ha^-1. In March and May 1999 and 2000, all plots were broadcast fertilized with 45 kg N ha^-1 as (NH₄)₂SO₄. Broad-leaf weeds were controlled with metsulfuron {methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)-amino]carbonyl]-amino]-sulfonyl]benzoate} at the rate of 2.02 g a.i. ha^-1 in May 1999.

The experimental design was split-plot randomized block with four replicates. Whole plots were wheatgrass species were planted in strips with combinations of defoliation frequency and intensity imposed as subplots. A total of 112 subplots, 2 by 13 m in size, were defoliated between 17 April and 12 July 1999 (first growing season) and 27 April and 8 June 2000 (second growing season) by clipping at a height of 7.5 or 15 cm and at a 3- or 6-wk interval. The first harvest in each season was made when inflorescences emerged. Subplots were mowed with exception of an area of 0.25 m², which was manually clipped to determine dry matter (DM) of herbage yield. A section of each whole plot (2 by 3 m) was excluded from defoliation, and a part of this section (0.36 by 1 m) was then clipped, at ground level, only at the end of the growing season to determine aboveground biomass of undefoliated plants. In each year, biomass of undefoliated plants was harvested from a different section of the subplot to minimize carryover effects.

Herbage DM yield was defined as plant material harvested above the defoliation height. At the end of each growing season, a 0.25-m² section of each defoliated subplot was harvested to ground level for stubble determination. In each year, stubble was collected from a different section of a subplot to minimize carryover effects. Aboveground biomass was the sum of cumulative herbage DM yield and stubble DM below the defoliation height or, for undefoliated swards, herbage harvested at ground level at the end of the growing season. The number of live tillers (at least one green leaf present) was recorded before the onset of summer drought (July 1999 and July 2000) and after summer drought (September 1999 and December 2000) by counting tillers from a 0.6-m section of one row in the middle of each subplot. The percentage of leaf DM in herbage DM yield and stubble DM was determined to evaluate the effects of defoliation treatments on sward composition. Soil temperatures in crested and tall wheatgrass plots were monitored by 2-channel HOBO loggers (Onset Computer Corp., Bourne, MA) at a 5-cm depth between May and September in each growing season. The average daytime soil temperature was determined for the period between 0700 and 1900 h, and the average nighttime soil temperature for the period between 1900 and 0700 h.

Experiment B
Plots (72 in total; 2 by 11 m each) were established in September 1999 at a location adjacent to Exp. A. Defoliation treatments consisted of two defoliation intervals of 3 or 6 wk and at one height of 7.5 cm. This defoliation height was chosen because it increased tiller survival in Exp. A. Hybrid wheatgrass was omitted from the study, but all other species were the same as in Exp. A. The number of live tillers was recorded before the onset of summer drought in July 2000 (first growing season) and July 2001(second growing season) and after summer drought (November 2000 and October 2001). Soil temperature was not monitored in Exp. B.

Statistical Analysis
Data were analyzed using the MIXED procedure of the Statistical Analysis System (Littell et al., 1996). For each experiment, replications were considered random, whereas growing season, wheatgrass species, defoliation intensity (Exp. A), and frequency (Exp. A and B) were considered fixed factors. Percentage values of leaf DM in herbage yield and stubble were transformed by arcsin function before statistical analysis to ensure normal distribution. Mean separation was performed using the protected least square means (LSMEANS) procedure. Significance was declared at P < 0.05.

Soil temperature data (daily means) were analyzed using the MIXED procedure with measurements repeated in time for the period May to September in each growing season. Replications were considered random, while growing season, wheatgrass species, defoliation frequency, and intensity were considered fixed. Mean separation was performed using the LSMEANS procedure at P = 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Unusually severe drought occurred each summer. Average yearly precipitation for the experimental site is 653 mm with peaks in May and September, and average monthly temperatures reaching 26.6 to 28.4°C from June to August. Summer drought of 1999 was characterized by temperatures exceeding the long-term average for July (+0.5°C) and August (+2.6°C), and a severe precipitation deficit (-106.7 mm or 52%) from July until September, with some occasional rainfall events (Fig. 1A and 1B) . Unusual drought conditions continued during winter and early spring of 2000 as shown by higher than normal average monthly temperatures and precipitation deficits. Drought in 2000 was more severe than in 1999 because of trace amount of precipitation from July until October (deficit of 189 mm or 93%) and above normal temperatures in July (+0.6°C) and August (+2.9°C). From October 2000 until May 2001, precipitation was slightly above normal and average monthly temperatures were lower than the long-term average. During July–September 2001 precipitation deficit was 157 mm (88%) and average monthly temperatures were higher than the long-term average in July (+2.2°C) and August (+0.9°C).

View larger version (31K): [in this window] [in a new window]   Fig. 1. Average monthly temperature and precipitation for Vernon, TX, during 1999–2001.

View larger version (21K): [in this window] [in a new window]   Fig. 2. Cumulative herbage yield in response to defoliation frequency in the first and second growing season in (A) Exp. A and (B) Exp. B. Bars indicate 1 SE.

View larger version (20K): [in this window] [in a new window]   Fig. 3. Cumulative herbage yield of wheatgrass species in the first and second growing season in Exp. B. Bars indicate 1 SE.

View larger version (21K): [in this window] [in a new window]   Fig. 4. Stubble DM as a function of defoliation (A) frequency and (B) intensity in the first and second growing season in Exp. A. Bars indicate 1 SE.

Aboveground biomass was the result of an interaction of defoliation frequency and growing season in each experiment (Tables 1 and 2). More aboveground biomass was produced at 3 vs. 6-wk defoliation interval in the first growing season (Fig. 5A and 5B) . In the second growing season, aboveground biomass declined in response to frequent defoliation in Exp. A (Fig. 5A), but was not affected by defoliation frequency in Exp. B (Fig. 5B).

View larger version (40K): [in this window] [in a new window]   Fig. 5. Aboveground biomass in response to defoliation frequency in the first and second growing season in (A) Exp. A and (B) Exp. B. Bars indicate 1 SE.

All defoliation treatments reduced aboveground biomass when compared with undefoliated plants in Exp. A, regardless of growing season (Fig. 5A, data for defoliation intensity not shown). In contrast, aboveground biomass of wheatgrasses defoliated every 3 wk was not different from undefoliated plants in Exp. B in the first growing season (Fig. 5B). In the second growing season, however, defoliated plants produced less aboveground biomass than undefoliated plants.

Herbage Yield and Stubble Composition
The proportion of leaf DM in herbage yield was modified by a three-way interaction among wheatgrass species, defoliation frequency, and intensity in Exp. A (Table 1). The 3- vs. 6-wk defoliation interval increased the proportion of leaf DM in herbage in crested and intermediate wheatgrass clipped at the 7.5-cm height and in tall wheatgrass defoliated at the 15-cm height (Table 5). Under more intensive defoliation, crested, hybrid, and intermediate wheatgrass had more leaf DM in herbage than pubescent and tall wheatgrass when defoliated every 3 wk. When defoliated every 6 wk, hybrid wheatgrass had a greater proportion of leaf DM in herbage than crested, intermediate, and tall wheatgrass. Hybrid and pubescent wheatgrass had more leaf DM in herbage than other species when defoliated at the 15-cm height, regardless of defoliation frequency. In Exp. B, the proportion of leaf DM in herbage yield was determined by main effects of growing season, wheatgrass species, and defoliation frequency (Table 1). More leaf DM in herbage yield was produced by wheatgrasses in the first vs. second growing season (Fig. 6A) and in response to more frequent clipping (Fig. 6B). Crested wheatgrass had a greater proportion of leaf DM in herbage yield than other species, regardless of defoliation frequency (Fig. 6C).

View larger version (22K): [in this window] [in a new window]   Fig. 6. The proportion of leaf DM in herbage yield as a function of (A) growing season, (B) defoliation frequency, and (C) wheatgrass species in Exp. B. Bars indicate 1 SE.

View larger version (20K): [in this window] [in a new window]   Fig. 7. The proportion of leaf DM in stubble of wheatgrass species in (A) Exp. A and (B) Exp. B. Bars indicate 1 SE.

View larger version (16K): [in this window] [in a new window]   Fig. 8. The proportion of leaf DM in stubble as a function of defoliation frequency and intensity in Exp. A. Bars indicate 1 SE.

View larger version (30K): [in this window] [in a new window]   Fig. 9. The number of live tillers before summer drought in response to defoliation intensity in the first and second growing season in Exp. A. Bars indicate 1 SE.

View larger version (34K): [in this window] [in a new window]   Fig. 10. The number of live tillers before summer drought as a function of defoliation frequency and wheatgrass species in the (A) first and (B) second growing season in Exp. B. Bars indicate 1 SE.

View larger version (27K): [in this window] [in a new window]   Fig. 11. The number of regrowing tillers after summer drought as a function of defoliation frequency and wheatgrass species in (A) Exp. A and (B) Exp. B. Bars indicate 1 SE.

View larger version (24K): [in this window] [in a new window]   Fig. 12. The number of regrowing tillers after summer drought as a function of defoliation intensity and wheatgrass species in Exp. A in the (A) first and (B) second growing season. Bars indicate 1 SE.

View larger version (17K): [in this window] [in a new window]   Fig. 13. The number of regrowing tillers in wheatgrass species after summer drought in the first and second growing season in Exp. B. Bars indicate 1 SE.

In the first growing season, nighttime soil temperature was lower in crested vs. tall wheatgrass swards and not affected by defoliation frequency (Table 9). In contrast, crested wheatgrass defoliated every 3 wk had higher nighttime soil temperature than tall wheatgrass, and the difference between the species was not significant at the 6-wk defoliation interval. The 3- vs. 6-wk defoliation interval reduced nighttime soil temperature in crested, but increased in tall wheatgrass swards.

Regardless of growing season, defoliation frequency of 6- vs. 3-wk reduced nighttime soil temperature in crested wheatgrass swards defoliated at the 7.5- but not 15-cm height (Table 10). In contrast, nighttime soil temperature in tall wheatgrass was reduced in response to the 3- vs. 6-wk defoliation interval at the 15- but not the 7.5-cm height.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

Aboveground biomass was also greater in response to frequent defoliation in the first growing season in both experiments, but decreased in the second growing season in Exp. B. This indicates a carryover effect of initial defoliation and growing conditions on grass productivity in the following year (Casler and Goodwin, 1998). An inverse relationship between defoliation frequency and intensity for above- and belowground biomass was shown in several wheatgrass species (Stroud et al., 1985; Binnie and Chestnutt, 1991; McCaughey and Simons, 1996). Other studies, however, indicate frequent defoliation may stimulate herbage production in grasses because of a greater number of vegetative than reproductive tillers in subsequent regrowths (Cook and Stoddart, 1953; Olson and Richards, 1989). Wheatgrasses defoliated every 3 wk were usually in earlier developmental stages at each harvest date than those defoliated every 6 wk (visual observations, data not recorded), which was confirmed by a greater proportion of leaf DM in herbage and stubble of frequently defoliated plants. Baker and Hunt (1961) observed more herbage yield produced by intermediate and pubescent wheatgrass in response to intensive defoliation (5 cm), but tiller numbers increased in response to higher defoliation height (10 cm). This could affect herbage yield in a long-term period.

Precipitation during winter and spring in the second growing season of our experiments was lower than that during the first growing season; therefore, soil water deficits could interact with defoliation regimes affecting regrowth rates (Busso and Richards, 1993). For example, the availability of nitrogen might have been inadequate for growth of intensively defoliated plants, reducing herbage yield when compared with extensively defoliated plants (Guitian and Bardgett, 2000).

The 3- vs. 6-wk defoliation frequency increased leaf production above and below defoliation height. This indicates a faster reestablishment of photosynthetic area and suggests that wheatgrasses may tolerate frequent defoliation (Arredondo et al., 1998; Romo and Harrison, 1999). Several authors suggested the importance of a rapid reestablishment of photosynthesis rather than root reserves as the main source of C for regrowing shoots (Caldwell et al., 1981; Richards and Caldwell, 1985).

Drought tolerance of temperate grasses adapted to semiarid environments is related to the ability to produce a large number of tillers (Sutherland, 1994) and to summer dormancy of tillers (Biddiscombe et al., 1977). In our experiments, effects of defoliation intensity (Exp. A) or frequency (Exp. B) on the number of live tillers before and after summer drought were modified by growing conditions in each season and varied with wheatgrass species.

Defoliation intensity did not affect tiller number of any wheatgrass species before summer drought in the first growing season, but more intensive defoliation increased the number of regrowing tillers in crested and pubescent wheatgrass after summer drought. In Exp. B, more frequent defoliation increased tiller numbers in these two species before summer drought in the first, but not in the second growing season. Crested and pubescent wheatgrass are the most drought tolerant among species evaluated in our experiments (Currie and White, 1982; Frank, 1983) and their responses to defoliation intensity are typical for grazing-tolerant grasses (Cook and Stoddart, 1953; Angel, 1995). The beneficial effect of intensive defoliation on tiller number before summer drought was also present in the second growing season; however, defoliation intensity did not affect tiller survival during summer drought.

These contrasting responses to defoliation intensity could be due to a more severe and longer summer drought in the second (2000) when compared with the first (1999) growing season, and may involve complex interactions of defoliation regimes, C reserves, and drought severity (Volaire, 1994). Another reason might be differences in the rate of soil water loss by transpiration of grass swards in response to defoliation. In the first growing season, the last harvest was made on 12 July because of relatively adequate precipitation in May and June. Plants became dormant thereafter without significant regrowth. This might have reduced evapotranspiration and conserved water in the soil for a longer time in intensively defoliated swards because of reduced leaf area when compared with extensive defoliated swards (Baker and Hunt, 1961; Miller et al., 1990). In contrast, the last harvest in the second growing season was made on 8 June and followed by intensive rainfall events until the beginning of July. Under these conditions, plants continued with regrowth and accumulated a significant amount of herbage. After the onset of drought in July 2000, soil water was probably lost through high evapotranspiration, since no rainfall occurred until October. This apparently resulted in a high tiller mortality, especially in less intensively defoliated plants. Mohammad et al. (1982) found that herbage production and tiller survival of crested wheatgrass and Russian wildrye (Elymus junceus Fish) were greater in defoliated plants (40% defoliation) than undefoliated plants grown at field capacity [-0.03 MPa (-0.3 bar) soil water potential]. The authors questioned the common practice of withholding defoliation of grasses during drought as it had little effect on root growth, but it could reduce transpiration and increase tiller survival. To conserve soil water, herbage growth is removed before summer drought in Mediterranean grasslands (Volaire and Thomas, 1995); however, it is not practiced in grasslands of the Texas Rolling Plains.

Tiller survival was affected by interactions of defoliation frequency and wheatgrass species in both experiments, regardless of growing seasons. Less frequent defoliation increased the number of regrowing tillers in hybrid and pubescent wheatgrass, but decreased it in intermediate wheatgrass. Thus, the effect of defoliation frequency on tiller survival seems to be inconclusive and may also depend on the severity of summer drought (Busso and Richards, 1995). Our data show significant variation among wheatgrass species in tiller survival. Crested wheatgrass had the greatest, whereas tall wheatgrass the least number of regrowing tillers in Exp. A, regardless of defoliation regimes. This was consistent with results presented by Volaire and Thomas (1995) who suggested that tiller mortality was usually higher in late-maturing (i.e., tall wheatgrass) than in early maturing species (i.e., crested wheatgrass), and was related to lower root densities in late-maturing cool-season grasses.

Soil Temperature Effects
Soil temperature was not affected by defoliation regimes nor wheatgrass species during spring and early summer when soil water was not a growth limiting factor, and in late summer when soil moisture was severely depleted in all treatments. With the onset of drought in July 1999 (first growing season) and 2000 (second growing season), effects of defoliation regimes and wheatgrass species on soil temperature became significant. We suggest that responses of soil temperature in wheatgrass swards to defoliation regimes could be related to the amount and composition of stubble biomass left after the harvest before the onset of drought. A greater stubble biomass in swards defoliated at 15-cm height might have caused more shading of the soil surface, which reduced daytime soil temperature and increased nighttime soil temperature (Gianelle et al., 1998; Mika et al., 1998). In contrast, the effect of defoliation frequency on soil temperature was cumulative through the growing season up to the onset of summer drought as it affected sward composition. Swards defoliated at 3-wk intervals had a higher proportion of leaf DM than swards defoliated at 6-wk intervals; therefore, the expected LAI and leaf transpiration area were probably greater in the former swards. It is likely that swards defoliated at 3-wk intervals shaded the soil surface more efficiently than those defoliated every 6 wk. In a similar manner, swards defoliated at the 15-cm height (more biomass) provided more shade when compared with swards defoliated at the 7.5-cm height, reducing daytime and increasing nighttime soil temperature.

The differences in soil temperature between crested and tall wheatgrass may be related to soil–water–plant dynamics. With the onset of drought, grass species adapted to semiarid environments (e.g., crested wheatgrass) deplete soil water, and become dormant faster than less adapted grasses (e.g., tall wheatgrass) (Bittman and Simpson, 1987; Bleby et al., 1997; Wilman et al., 1997). Roots of crested wheatgrass are distributed mostly in the upper soil level (Caldwell et al., 1981), whereas tall wheatgrass has a deep root system (Bleby et al., 1997); therefore, water would be depleted from the upper soil level faster by crested than tall wheatgrass swards (Eissenstat and Caldwell, 1988; Bittman and Simpson, 1989). This could explain the higher daytime soil temperature in crested vs. tall wheatgrass swards in the second growing season when trace amounts of precipitation occurred during summer drought, and tall wheatgrass might have benefitted from water reserves in deeper soil levels. In the first growing season, however, daytime soil temperature was lower in crested than tall wheatgrass swards, except for the defoliation regime of 6-wk interval at the 15-cm height. Apparently, weather patterns in each growing season can modify responses of soil temperature to defoliation regimes differently in each wheatgrass species.

The relationship between defoliation regimes and soil temperature on tiller survival was not consistent in our experiment. In the first growing season, tiller survival of crested wheatgrass was greater in swards defoliated at the 7.5- vs. 15-cm height and tall wheatgrass did not respond to defoliation intensity. More tillers, therefore, survived in crested wheatgrass swards with higher vs. lower daytime, but lower vs. higher nighttime soil temperatures. Perhaps nighttime soil temperatures may affect tiller survival of some wheatgrass species more than daytime soil temperatures during summer drought. Higher nighttime soil temperatures accelerate plant respiration in dense grass swards when compared with short or loose swards and may have a more detrimental effect on plant survival than daytime soil temperatures (Feldhake et al., 1996).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
These experiments confirm that frequent or intensive defoliation has no detrimental effect on productivity of introduced wheatgrasses in the first growing season, but reduces persistence during summer drought and productivity in subsequent growing season. Species responses should be considered in managing wheatgrass pastures. In the first growing season, crested and pubescent wheatgrass may be defoliated more intensively without significant reduction in productivity and persistence, whereas tall wheatgrass should be defoliated less intensively.

Precipitation patterns during summer drought interact with defoliation regimes in determining the persistence of introduced wheatgrasses in semiarid environments of the Texas Rolling Plains. Defoliation regimes affected soil temperature during summer drought, suggesting nighttime soil temperature be related to tiller survival to a greater extent that daytime soil temperature. With some occasional precipitation during summer drought, all wheatgrass species could persist and their productivity could be maintained in the subsequent growing season. With unusually low precipitation during summer drought, wheatgrass persistence rapidly declined and productivity was low in the subsequent growing season. None of the evaluated wheatgrass species was adapted to prolonged and severe summer drought. In semiarid regions of the Texas Rolling Plains, introduced wheatgrasses are probably limited in their potential to complement forage base for dual-purpose wheat and perennial warm-season grass pastures.

	ACKNOWLEDGMENTS

 
We thank Michelle Armstrong, Kelly Barnett, Matt Angerer, and Scott Showers (Texas A&M Univ., Texas Agric. Exp. Stn., Vernon, TX) for technical support. We appreciate comments by Dr. David P. Belesky (USDA-ARS, Appalachian Farming Systems Res. Center, Beaver, WV) and Dr. David D. Briske (Texas A&M Univ., Dep. Rangeland Ecology and Management, College Station, TX) on an earlier version of the manuscript.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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