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Published in Agron J 99:621-629 (2007)
DOI: 10.2134/agronj2006.0056
© 2007 American Society of Agronomy
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Alfalfa

Supplemental Irrigation and Fall Dormancy Effects on Alfalfa Productivity in a Semiarid, Subtropical Climate with a Bimodal Precipitation Pattern

D. P. Malinowskia,*, W. E. Pinchaka, B. A. Krampa, H. Zuob and T. J. Butlerc

a Texas Agric. Exp. Stn., P.O. Box 1658, Vernon, TX 76385
b Beijing Research and Dev. Ctr. for Grass and Environment, Beijing Academy of Agric. and Forestry Sci., Beijing 100097, P.R. China
c The Samuel Roberts Noble Fdn., 2510 Sam Noble Pkwy., Ardmore, OK 73401

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

Received for publication February 21, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Alfalfa (Medicago sativa L.) is an important hay crop produced under rain-fed conditions or various levels of irrigation in the southern Great Plains of the USA. The objective of this study was to determine the role of fall dormancy (FD) on productivity and forage nutritive value of alfalfa in rain-fed and supplemental irrigation systems in a semiarid, subtropical climate with a bimodal pattern of precipitation. Cultivars with FD ratings of 1 to 3 (dormant), 4 to 6 (moderately dormant), and 7 to 8 (nondormant) were planted in November 2001 on a Miles fine sandy loam (fine-loamy, mixed, Thermic Udic Paleustalfs) near Vernon, Texas in two adjacent, randomized, complete-block experiments representing rain-fed and supplemental irrigation systems. In the supplemental irrigation system, water was supplied during April to October to meet the long-term average monthly precipitation. During 2002–2005, plants were defoliated to 5 cm height at 5 to 15% bloom in all cultivars. In the rain-fed system, FD had no effect on productivity (5.6–6.0 Mg ha–1). Nondormant cultivars produced higher dry matter yield (20.2 Mg ha–1) than dormant cultivars (15.9 Mg ha–1) under supplemental irrigation. Moderately dormant cultivars were intermediate (18.6 Mg ha–1). Forage nutritive value was greater in the spring (rain-fed) or spring and autumn (supplemental irrigation) than in the summer. Weather patterns and harvest time interacted with FD in determining forage nutritive value under supplemental irrigation. Cultivars with FD ratings of 5 to 8 may be used in similar environments of the southern Great Plains to maximize productivity of alfalfa.

Abbreviations: a.i., active ingredient • CP, crude protein • DM, dry matter • FD, fall dormancy • IVDMD, in vitro dry matter digestibility


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ALFALFA (Medicago sativa L.) is an important hay crop in the southern Great Plains of the USA, particularly in the Texas Rolling Plains, High Plains, and Trans-Pecos areas, which are characterized by semiarid and subhumid subtropical climates (Holt, 1980). Texas producers harvested about 735480 Mg of alfalfa from 60750 ha in 2005, with an average yield of 12.8 Mg ha–1 (Texas Agricultural Statistics Service, 2006). In 2004, these producers received an average price of $145 Mg–1 and generated more than $112 million in farm-gate receipts statewide.

The intensity of irrigation varies greatly for alfalfa in the southern Great Plains from no irrigation (rain-fed) to full irrigation (1000 mm yr–1). Recent increases in energy prices, concerns about sufficient surface water supplies, and declining water tables due to repeated extreme drought periods might affect the future profitability of irrigated alfalfa crops (Bevers, 2001). The average annual precipitation in the Texas Rolling Plains, a geographic region of the southern Great Plains, varies from 483 mm in the west to 725 mm in the east, with climate classification ranging from semiarid subtropical (west) to subhumid subtropical (east) (Larkin and Bomar, 1983).

Water use requirements of alfalfa are directly related to the length of the growing season, which may be longer than the period of 221 frost-free days at Vernon, TX (Texas Rolling Plains, Wilbarger County). For example, in parts of Oklahoma, the amount of supplemental irrigation required to achieve the maximum potential yield during the growing season was about 584 mm (Kizer, 1991). Alfalfa is drought tolerant, using up to 65 to 75% available soil water before transpiration decreases (Sheaffer et al., 1988b). In situ water is supplied by preseason soil water and effective rainfall during the growing season. The peak water demand of alfalfa in July averages about 7 mm d–1, which would require an irrigation capacity of about 90 L min–1 ha–1 (Kizer, 1991).

Research was initiated in 2002 to address the concern of local producers that the reduced profitability of alfalfa might result from declining productivity caused by repeated severe drought periods (USGCRP, 2000). The mean annual temperature for Vernon, TX, for 1996 to 2005 was 17.5°C, which was 0.7°C higher than the 100-yr average annual mean temperature (16.8°C). The average annual precipitation for 1996 to 2005 decreased by 33 mm when compared with the 100-yr average (653 mm) (NOAA, 2006). The historic bimodal pattern of rainfall distribution with peaks in May and September may also be changing, with rain events becoming less predictable and more variable in volume (Nielsen-Gammon et al., 2005).

There are several criteria producers in the southern Great Plains need to consider while selecting alfalfa cultivars, such as yield potential, disease and insect resistance, fall dormancy (FD), and winter hardiness. These characteristics determine stand persistence and productivity. Fall dormancy of alfalfa cultivars characterizes regrowth potential in the autumn in response to decreasing temperatures and day length (Teuber et al., 1998). Fall dormancy of alfalfa ranges from 1 to 11, representing a range from dormant (minimal fall regrowth) to nondormant (maximum fall regrowth) cultivars. Alfalfa cultivars with lower FD ratings are adapted to colder regions, whereas cultivars with higher FD ratings are suited for warmer climates (Melton et al., 1988; Putnam et al., 2005). Fall dormancy, however, may not always be related to winter hardiness (Weishaar et al., 2005). The expression of fall dormancy results from the combined effects of short days and cool temperatures (McKenzie et al., 1988). Nondormant cultivars maintain a higher growth rate later in the autumn and resume growth earlier in the spring and after harvest than dormant ones. In certain environments, some moderately dormant and nondormant cultivars have higher annual forage yield than dormant cultivars (Brummer et al., 2002).

The climate of the southern Great Plains, with relatively mild winters and hot and dry summers, and a bimodal precipitation pattern with peaks in May and September may be favorable for the cultivation of alfalfa with lower FD. Such nondormant cultivars may make better use of the extended growing period and the available soil water in the spring and autumn, resulting in a greater annual forage yield than that of dormant or moderately dormant cultivars (Brummer et al., 2002). Results of current research on alfalfa cultivar performance in Oklahoma (Caddel and Prater, 2005) showed that the most adapted and productive cultivars had FD ratings of 4 and 5. In New Mexico (Artesia, flood irrigation), most adapted and productive cultivars had FD ratings ranging from 6 to 9 (Lauriault et al., 2004).

The objective of this study was to determine the role of FD on productivity and forage nutritive value of alfalfa in a rain-fed system and with supplemental irrigation to meet the long-term average precipitation during April to October (defined here as a growing season) in an environment representative of semiarid/subhumid environments of the U.S. southern Great Plains.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Plant Material and Cultivation
Two concurrent, adjacent experiments representing the rain-fed and supplemental irrigation systems were planted on 11 Nov. 2001 on a Miles fine sandy loam (fine-loamy, mixed, Thermic Udic Paleustalfs) near Vernon, TX (34°09' N, 99°20' W; elevation 370 m). Alfalfa cultivars evaluated in this study and their FD ratings are listed in Table 1. The seedbed was conventionally tilled flat with borders formed around the supplemental irrigation test for flood irrigation. Seed inoculated with Dormal PLUS Rhizobia inoculant (Urbana Laboratories, St. Joseph, MO) were broadcasted on plots (1.8 by 7.6 m), and plots were raked and culti-packed. Seeding rates were 13 and 22 kg ha–1 for the rain-fed and supplemental irrigation systems, respectively. These seeding rates were based on recommendations for nonirrigated and irrigated systems on sandy soils (Shroyer et al., 1998).


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  		Table 1. Cultivars, breeding organizations, and fall dormancy (FD) ratings of alfalfa cultivars evaluated at Vernon, TX, during 2002–2005.

 
Initial soil pH was 6.9, with 20 mg kg–1 phosphorus (P) and 300 mg kg–1 potassium (K). These P and K soil levels were considered low and high range concentrations, respectively, for calculating fertilizer amount in alfalfa plantings (Koenig et al., 1999). Before planting, plots were fertilized with 84 kg ha–1 P2O5 and 32 kg ha–1 nitrogen. Each March, plots were fertilized with P, K, and boron (B) at 67 kg ha–1 P2O5, 117 kg ha–1 K2O, and 2.2 kg ha–1 B, respectively. Broad-leaf weeds (mainly careless weed, Amaranthus spp.) were controlled with 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 62 g active ingredient (a.i.) ha–1. Winter annual grasses (Bromus spp.) were controlled with fluazifop-p-butyl R-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoate at 259 g a.i. ha–1 in early spring each growing season. Plots were treated with Malathion [0,0-dimethyl-S-(1,2-dicarbethoxyethyl) dithiophosphate] at 1.4 kg a.i. ha–1 to control occasional cowpea aphid (Aphis craccivora Koch) infestations during June to August. No control was needed for alfalfa weevil (Hypera postica Gyll.).

Supplemental Irrigation System
In the supplemental irrigation system, water was supplied from April to October each year (flood irrigation) to meet the difference between actual precipitation and long-term average precipitation for each month. Irrigation was applied to all plots at once within 1 to 5 d after harvest, around the 15th of each month. To calculate the amount of supplemental irrigation, the actual precipitation for the first 15 d of the month; the forecast possibility of rainfall (www.noaa.com) for the subsequent 15 d of the month; and the expected, long-term average precipitation for this particular month were considered. For example, the actual precipitation during 1 to 15 May 2004 was 14 mm, the long-term average monthly precipitation for May was 97 mm, and forecast possibility of rainfall for 16 to 31 May 2004 was 50% below normal. Supplemental irrigation was calculated as a difference between long-term average for May (97 mm) and the actual precipitation for the first 15 d of May (14 mm), with no correction for expected precipitation during 16 to 31 May. If during the second half of a month the forecast possibility of precipitation was normal or above normal, we reduced the amount of supplemental irrigation by 50% of the expected precipitation and made necessary corrections during the next irrigation cycle if the precipitation amount was different from the forecast (Fig. 1 ).


Figure 1
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  		Fig. 1. Monthly precipitation (2002–2005), long-term average monthly precipitation (1904–2005), and supplemental irrigation of alfalfa to meet the average monthly precipitation at Vernon, TX, during April to October. No precipitation occurred in November and December 2005.

 
Harvest Schedule, Forage Sampling, and Analyses
Plants were harvested to 5 cm height when the average maturity across cultivars was 10% bloom. All cultivars were harvested on the same date. At each harvest, maturity varied among cultivars from 5 to 15% bloom. In the supplemental irrigation system, the first harvest was on 3 May 2002, 7 Apr. 2003, 14 Apr. 2004, and 21 Apr. 2005. There were total of five harvests in 2002, eight harvests in 2003 and 2004, and seven harvests in 2005. Rain-fed plots were harvested on 3 May, 11 June, and 16 July 2002; 16 Apr., 29 May, and 27 June 2003; 14 Apr., 11 May, and 10 June 2004; and 19 Apr., 25 May, 19 July, and 18 Aug. 2005. Forage biomass produced after the last harvest was marginal in the rain-fed system because of insufficient precipitation during July to September and induction of dormancy in late autumn. Any regrowing forage biomass was left in the rain-fed system for the next year. This remaining forage biomass did not contribute to the first harvest of the subsequent growing season. Forage yield was estimated from a manually harvested 0.5-m2 area randomly selected at each harvest. Above-ground biomass of the rest of each plot was rotary mowed, raked, and removed. Forage samples for dry matter (DM) yield determination and nutritive value were transported to the lab within 30 min after harvesting and oven-dried at 55°C for 48 h. Samples were ground to pass a 1-mm screen for nutritive value analyses. Crude protein (CP) and in vitro dry matter digestibility (IVDMD) were determined in forage samples harvested in April, June, and October in the supplemental irrigation system or in April and July in the rain-fed system during 2002–2004. The CP was determined by the Kjeldahl digestion procedure (AOAC, 1990). The IVDMD of dried forage samples was determined with the DAISY II System using an in vitro filter bag technique (Holden, 1999). In this analytical method, sodium sulfite was used in the filter water only (Hunt et al., 1995). This technique estimates true in vitro digestibility (Belyea et al., 1999), which can yield slightly greater estimates than IVDMD estimates using the two-stage technique (Holden, 1999).

Statistical Methods
In each experiment, the experimental design was a randomized, complete block replicated four times (rain-fed system) or three times (supplemental irrigation). Each experiment was analyzed separately. All variables were analyzed using Procedure Mixed of SAS (SAS Institute, 1999). For annual DM yield, replications and cultivars were considered random, whereas growing seasons and FD ratings were considered fixed effects. Alfalfa productivity usually declines in each subsequent growing season (Sheaffer et al., 1988b; McGraw and Nelson, 2003), especially in rain-fed systems (Kallenbach et al., 2002). Growing season effects on alfalfa crop, therefore, were considered fixed (Sheaffer et al., 1998). For the 4-yr total forage DM yield, replications were considered random, whereas FD ratings were considered fixed effects. For seasonal forage DM distribution, CP, and IVDMD, plots (i.e., replications within cultivars) were considered random, whereas growing seasons, harvest months, and FD ratings were considered fixed effects. Growing seasons and harvest months were considered fixed effects for seasonal forage DM distribution, CP, and IVDMD because of potential effects of environmental variables during a growing season on these parameters. Harvest months within growing seasons were treated as repeated measure and plots (i.e., replications within cultivars) as subjects of the REPEATED command syntax of the Procedure Mixed analysis. Mean separation for each variable was performed using the least square means procedure of Procedure Mixed. Significance of differences between means was declared at P < 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather Patterns and Irrigation Amounts
Precipitation during the growing season (April–October) varied in each year (622 mm in 2002, 259 mm in 2003, 396 mm in 2004, and 475 mm in 2005) (Fig. 1). These amounts represented +25%, –48%, –20%, and –5% departure, respectively, from the long-term average precipitation (498 mm). Irrigation amounts were 183 mm in 2002, 310 mm in 2003, 147 mm in 2004, and 152 mm in 2005; thus, available water (precipitation + irrigation) during the growing season was 805 mm in 2002, 569 mm in 2003, 544 mm in 2004, and 628 mm in 2005 (Fig. 1). The temperature range during the course of the study was considered normal for this environment (Fig. 2 ), which was classified as USDA Hardiness Zone 7. Average annual minimum temperature range for this zone is –15.0 to –17.7°C. The lowest temperatures during 2002–2005 were –10°C on 25 Dec. 2002 and on 25 and 26 Feb. 2003 and –11°C on 25 and 26 Dec. 2004. The highest single-day temperature was 43°C on 8 Aug. 2003.


Figure 2
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  		Fig. 2. Mean monthly temperature (2002–2005) and long-term average monthly temperature (1904–2005) at Vernon, TX.

 
Annual and Total Dry Matter Yield
In the rain-fed system, annual DM yield was affected by growing season. The effects of FD rating and the interaction between growing season and FD rating were not significant. Annual DM yield declined in each consecutive growing season, as reported in other studies (Sheaffer et al., 1988a; Kallenbach et al., 2002; McGraw and Nelson, 2003). Averaged for FD ratings, the greatest annual DM yield (6.2 Mg ha–1) occurred during the first growing season (2002) and declined to 5.6 Mg ha–1 in 2003 and 2004 and 5.2 Mg ha–1 in 2005. These results were similar to findings reported by Holt (1980) and Takele and Kallenbach (2001), who demonstrated declining productivity and persistence of alfalfa as a response to repeated soil water deficits for extended periods in summer, conditions that are also common in the southern Great Plains. Alfalfa productivity was similar among FD ratings and ranged from 5.2 to 6.0 Mg ha–1 on average for the four growing seasons. Severity of drought stress at this location might have resulted in induction of drought-related dormancy in all cultivars regardless of their FD rating (Williams and Boschma, 1996).

In the supplemental irrigation system, we found a significant growing season by FD interaction for annual DM yield. All cultivars produced about 50% less annual DM yield in the first growing season when compared with subsequent growing seasons (Table 2). Possible reasons for this could be a precipitation deficit (80%) and above-normal (2°C) mean monthly temperature in December 2001. Such conditions might have resulted in moisture deficits in the soil profile that affected alfalfa establishment by reducing seedling survival (Tesar and Marble, 1988) because the plots were not irrigated in the autumn of 2001. On the other hand, alfalfa forage yields are usually lower in the seeding year than in the subsequent years (Tesar and Marble, 1988) due to competition, diseases, and insect or winter injury. In 2004, FD ratings of 4 to 5 produced less annual DM yield than in 2003 or 2005, and FD 7 produced less DM yield in 2005 than in 2003 or 2004. Other FD ratings maintained similar productivity during 2003–2005. Fall dormancy did not affect annual DM yield in the first growing season (Table 2). In 2003, FD ratings of 7 to 8 produced about 11% more annual DM yield than FD ratings of 5 to 6 and 31% more than FD ratings of 1 to 4. In 2004, FD ratings of 5 to 8 had a similar annual DM yield, which was 20% greater than that of FD ratings of 1 to 4. Cultivars with FD ratings of 4 to 8 produced 16% higher annual DM yield than FD ratings of 1 to 3 in 2005. These results illustrate that high alfalfa productivity (about 19.7 Mg ha–1) may be achieved in semiarid and subhumid environments of the Texas Rolling Plains by choosing cultivars with FD ratings of 5 to 8 and applying supplemental irrigation to meet at least the long-term monthly average precipitation during April through October.


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  		Table 2. Interaction of growing season and fall dormancy (FD) rating on alfalfa annual dry matter yield in the supplemental irrigation system. Data are least square means of three replications and one to five cultivars representing each FD rating, excluding FD 2.

 
Total DM yield during the course of the study (four growing seasons) is presented here to describe the effects of FD rating on alfalfa productivity during the expected average lifetime of a stand in the targeted environment, which is 4 yr (Dr. C. Trostle, Texas Agricultural Extension Service, Lubbock, TX, personal communication). In the rain-fed system, FD rating had no effect on total DM yield (Fig. 3 ). In the supplemental irrigation system, total DM yield was similar from FD 1 to 3, whereas it increased 10% in FD 4 and 21% in FD 5 to 8, respectively, over FD 1 to 3.


Figure 3
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  		Fig. 3. Effects of alfalfa fall dormancy (FD) rating on total dry matter (DM) yield in the rain-fed and supplemental irrigation systems. Vertical bars indicate 1 SE. Data are the means of four replications (rain-fed system) or three replications (supplemental irrigation) with one to five cultivars representing each FD rating, excluding FD 2.

 
Seasonal Forage Dry Matter Distribution
Distribution of forage DM during a growing season was affected by harvest month in the rain-fed system. Most of the annual DM yield (39%) was harvested in April, whereas forage DM production in May and June was similar and contributed 30% (May) and 31% (June) to the annual DM yield. Fall dormancy had no effect on distribution of forage DM under rain-fed conditions. In the supplemental irrigation system, harvest month and FD rating determined forage DM distribution. Forage DM harvested in April accounted for 18% of the annual DM yield, followed by forage DM harvested in May, June, and July (14, 15, and 15%, respectively); August (11%); and September, October, and November (9, 10, and 8%, respectively). Averaged over all harvests, FD ratings of 5 to 8 produced about 11, 16, and 26% more forage DM than FD ratings of 4, 3, and 1, respectively, during the growing season. Forage production of FD 3 did not differ from that of FD 1 and FD 4. The greater forage DM production of FD ratings of 5 to 8 than that of FD ratings of 1 to 4, averaged for 2002–2005 growing seasons, is presented in Fig. 4 . The decline in alfalfa forage production with the progress of the growing season was typical for climates with hot and dry summer and wet winter-spring seasons (Fick et al., 1988; Lazaridou and Vrahnakis, 2000) and is known as "summer slump."


Figure 4
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  		Fig. 4. Forage dry matter distribution of alfalfa cultivars with fall dormancy (FD) of ratings 1 to 4 and FD ratings of 5 to 8 as a function of harvest month in the supplemental irrigation system. Vertical bars indicate ±1 SE. Data are the means of three replications with one to five cultivars representing each FD rating (excluding FD 2) and four growing seasons. DM, dry matter.

 
This study showed that productivity of alfalfa cultivars in this environment may be increased by applying supplemental irrigation to meet the monthly precipitation during a growing season. The use of moderately dormant (FD 5–6) or nondormant (FD 7–8) cultivars may further increase annual forage production with supplemental irrigation. Caddel and Prater (2005) reported that the majority of best-yielding alfalfa cultivars commonly cultivated in Oklahoma had an FD rating of 4. In this study, FD ratings of 5 to 8 produced more forage than an FD rating of 4 at each harvest during the growing season, resulting in the greatest annual and total forage production. This is an important finding because nondormant cultivars have not been recommended for this environment. Putnam et al. (2005) reported a similar relationship of increased productivity with greater FD rating for irrigated alfalfa grown in California. In their study, the greatest effect of FD rating on forage productivity was in the spring and autumn.

Recent objectives of alfalfa breeding programs emphasize developing less fall dormant cultivars with greater forage production in autumn while maintaining high winter hardiness (Volenec et al., 2002). In this study, nondormant alfalfa cultivars with FD ratings of 7 to 8 were not killed by winter temperature in the range of –11 to –10°C. Some temporary freeze damage occurred on the leaves of cultivars with FD ratings of 7 to 8 (data not presented) after exposure to such low temperatures in late February 2003, when they were actively growing. Lower temperature (–12.7°C) occurred for the last time at the experimental location in December 1996; thus, the winter minimum temperatures were higher for the past 10 yr than the average low temperatures for this hardiness zone (–15.0 to –17.7°C). Contemporary studies illustrated that FD may not always be genetically related to winter hardiness (Brummer et al., 2002; Weishaar et al., 2005); however, nondormant (FD ratings of 7–9) alfalfa cultivars are not winter-hardy in the upper Midwest (Brummer et al., 2000). In this environment, low winter temperatures have usually short duration, in most instances lasting for several hours during the night, followed by significant warming up during the day. At this location, cultivars with FD ratings of 7 to 8 seem to tolerate winter temperatures.

Forage Nutritive Value
Alfalfa maturity plays an important role in determining forage nutritive value (Marten et al., 1988; Putnam et al., 2005). In this study, we carefully scheduled harvests based on flowering rate so that the average percentage of blooming flowers was about 10% (Sheaffer et al., 1988a); however, the range varied among cultivars with different FD ratings from 5 to 15% bloom. Not all cultivars grew at the same rate in this study, and differences in plant maturity at each harvest might have accounted for differences in forage nutritive value with regard to FD rating (Hintz and Albrecht, 1991; Sulc et al., 1997). For example, differences in growth rates, even within an FD group (Larson and Smith, 1963), could affect the ratio of stem vs. leaf material (Albrecht et al., 1987; Marten et al., 1988). Busbice and Wilsie (1965) reported that morphologic development of alfalfa was positively correlated with less pronounced FD. In this study, therefore, cultivars with lower FD ratings were most likely maturing slower than cultivars with higher FD ratings.

Crude protein concentrations in rain-fed alfalfa forage were affected by a relation between growing season and harvest month, with FD rating as the main effect. Forage harvested in April had a greater CP concentration than forage harvested in June each year (Fig. 5 ). In 2004, CP concentrations in April (254 g kg–1) and June (237 g kg–1) were greater than those in 2003 (241 and 207 g kg–1, respectively) and 2002 (237 and 228 g kg–1, respectively). Higher CP concentrations in April 2004 may be related to lower temperatures in the spring of 2004 when compared with temperatures in the springs of 2002 and 2003 (Fig. 2). The significant precipitation deficit in the spring of 2003 could affect CP concentrations in forage harvested in June when compared with forage harvested in June of 2002 and 2004. Alfalfa with FD ratings of 1 and 4 had the highest CP concentrations (240–241 g kg–1), whereas FD ratings of 5, 7, and 8 had the lowest CP concentration in forage (227–232 g kg–1). Cultivars with FD ratings of 3 and 6 had an intermediate CP concentration (234 g kg–1). Observed CP levels among all FD ratings were above the CP requirements of all classes of beef cattle (NRC, 1996).


Figure 5
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  		Fig. 5. Interaction between growing seasons and harvest months on CP concentration in alfalfa forage in the rain-fed system. Vertical bars indicate 1 SE. Data are the means of four replications and one to five cultivars representing each FD rating, excluding FD 2.

 
In the supplemental irrigation system, CP concentrations were determined by a three-way interaction among growing season, harvest month, and FD rating. In the first growing season (2002), CP concentrations were higher in October than in April or June (Fig. 6 ). The effect of FD rating was inconsistent in April, whereas FD ratings of 7 to 8 had lower CP concentrations than FD ratings of 1 to 3 in June and October. In 2003, forage harvested in June had the lowest CP concentration, and there was no effect of FD on CP concentration. In contrast, April and October harvests had similar CP concentrations in forage, and FD ratings of 6 to 8 had lower CP concentrations than FD ratings of 1 to 4. In 2004, CP concentration in forage harvested in October declined when compared with previous seasons and was lower than that measured in forage harvested in April and June. The effect of FD was similar at April, June, and October harvests. Fall dormancy ratings of 7 to 8 had lower CP concentrations than FD ratings of 1 to 4.


Figure 6
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  		Fig. 6. Function of growing season, harvest month, and fall dormancy (FD) rating on crude protein (CP) concentration in alfalfa forage in the rain-fed system. Vertical bars indicate 1 SE. Data are the means of four replications and one to five cultivars representing each FD rating, excluding FD 2.

 
The IVDMD of alfalfa grown in the rain-fed system was affected by an interaction between growing season and harvest month, and FD had no effect. Forage harvested in April had a greater IVDMD than forage harvested in June only during the exceptionally dry and hot growing season of 2003 (Fig. 7 ). The IVDMD of April forage increased in 2003 and 2004 (by 18 and 12 g kg–1, respectively) when compared with that in 2002 (769 g kg–1). Forage harvested in June had a lower IVDMD in 2003 (728 g kg–1) when compared with 2002 (767 g kg–1) and 2004 (780 g kg–1) and an IVDMD higher in 2004 than 2002.


Figure 7
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  		Fig. 7. Interaction between growing seasons and harvest months on in vitro dry matter digestibility (IVDMD) of alfalfa forage in the rain-fed system. Vertical bars indicate 1 SE. Data are the means of four replications and one to five cultivars representing each FD rating, excluding FD 2.

 
Similar to CP concentration, IVDMD of alfalfa forage grown under supplemental irrigation was affected by a three-way interaction among growing season, harvest month, and FD rating. In 2002 and 2004, the IVDMD values were higher in October than in April or June, but IVDMD of forage harvested in June 2003 was lower than in April or October (Fig. 8 ). The effect of FD rating on IVDM was highly dependent on growing season and harvest month. Cultivars with higher FD ratings (7–8) had lower IVDMD than cultivars with lower FD ratings (1–3) in forage harvested in October 2002, April 2003, and June 2004; the differences were not significant at other harvest dates.


Figure 8
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  		Fig. 8. Function of growing season, harvest month, and fall dormancy (FD) rating on in vitro dry matter digestibility (IVDMD) of alfalfa forage in the supplemental irrigation system. Vertical bars indicate 1 SE. Data are the means of three replications and one to five cultivars representing each FD rating, excluding FD 2.

 
Forage nutritive value was generally higher in April (rain-fed system) or April and October (supplemental irrigation) than in June. This agreed with Christian (1977) and Kalu and Fick (1983), who found that high temperatures in the summer decreased alfalfa forage nutritive value when compared with the autumn. This phenomenon was associated with increased accumulation of lignin in cell walls at higher summer temperatures (Sanderson and Wedin, 1988). In a study by Hall et al. (2000) conducted in Pennsylvania, CP and IVDMD were greater in spring and autumn (May and September) than in summer (June and July), reflecting similar trends as in this study. Results on FD effects on forage nutritive value observed in this study agreed with Putnam et al. (2005). In general, cultivars with lower FD ratings (1–4) produced forage with higher nutritive value than cultivars with higher FD ratings (7–8). The preponderance of published research on alfalfa nutritive value is from mechanically harvested sun-dried hays. Our average, nutritive value estimates are about 3 (CP) or 50 (IVDMD) g kg–1 greater than averages commonly reported; however, they are within the range of values reported for hand-harvested samples at 5 to 7 cm height (Belyea et al., 1999).

Results of this study and those presented by Julier and Huyghe (1997) and Putnam et al. (2005) suggest a trade-off between productivity and nutritive value of alfalfa forage with regard to the selection of cultivars based on their FD rating for a particular environment. We do not suggest that choice of alfalfa cultivars for this and similar environments should be based solely on FD. Alfalfa cultivars may differ widely in productivity and other characteristics even within an FD class (Larson and Smith, 1963); however, recent studies indicate that within-cultivar variation in such traits as forage yield and quality may be as high as among-cultivars variation (Julier et al., 2000). We declared cultivars effects to be random for the purpose of the statistical analysis of this study. Our results suggest general trends with regard to FD effects on alfalfa productivity and forage nutritive value in this semiarid, subtropical climate with a bimodal pattern of precipitation.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results of this study show that FD was unrelated to alfalfa productivity in a rain-fed system in a semiarid/subhumid environment of the southern Great Plains. Severe water deficits and high temperatures during summer were superseding factors limiting alfalfa growth and resulting in declining forage yield in each consecutive growing season. Other characteristics than FD (e.g., resistance to pests and diseases, abiotic stresses, and type of management) may be more important for producers who chose alfalfa cultivars for rain-fed systems in similar environments. Supplemental irrigation to meet the long-term monthly precipitation during April to October resulted in three to four times greater annual forage yields when compared with the rain-fed system. In this environment, cultivars with FD ratings of 5 to 8 had similar productivity, and no decline in forage yield occurred during the four growing seasons. Forage nutritive value (CP and IVDMD) declined late in the growing season or during periods with high temperatures and severe soil water deficits in summer, even with supplemental irrigation. The significance of changes in forage nutritive value may be of limited practical importance considering the narrow range of CP and IVDMD values with regard to FD rating.


   ACKNOWLEDGMENTS
 
We appreciate cooperation of the seed companies, research institutes, and producers who donated alfalfa seed for this experiment. We thank Matt Angerer and Doug Fulford (TAES Vernon, TX) and Ashley Bain and Josh Arismendez (summer student workers) for help with conducting the experiments and analyzing plant material. This research project was funded by the Texas taxpayers.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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J. P. Muir, T. J. Butler, R. M. Wolfe, and J. R. Bow
Harvest Techniques Change Annual Warm-Season Legume Forage Yield and Nutritive Value
Agron. J., May 7, 2008; 100(3): 765 - 770.
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