Forage Production and Persistence of Alfalfa

in Dryland and Reduced Irrigation Systems

Alfalfa (Medicago sativa L.) is a very important hay and grazing crop in the Texas Rolling Plains. Most of alfalfa in this region has been traditionally produced under irrigation.

Rationale

    Alfalfa is an important hay crop in the Texas Rolling Plains. Most of alfalfa in this region has been traditionally produced under irrigation. Recent increases in energy prices, concerns about sufficient supply of surface water, and declining water tables due to repeated extreme drought periods may affect the profitability of irrigated alfalfa crop in the next few years. Water use requirements of alfalfa are directly related to the length of the growing season, which averages for 221 days at Vernon, TX. The average annual precipitation in the Texas Rolling Plains varies from 483 mm in the west to 725 mm in the east. The amount of irrigation required to achieve the maximum potential yield during this period is about 584 mm (Kizer, 1991). Alfalfa may require 101 to 254 mm of water to produce a ton of forage DM (Stichler, 1997; Trostle, 2003). In situ water is supplied by pre-season soil water and effective rainfall during the growing season. Alfalfa's water demand peaks in July at the level of about 7 mm per day, which would require a well capacity of about 90 dm3 min-1 ha-1 (Kizer, 1991).

    The mean annual temperature for Vernon, TX for the last 10 years (1996-2005) period was 17.48°C, 0.65°C higher than the 100-year long-term average annual mean temperature (16.83°C). At the same time, average annual precipitation for 1996-2005 decreased by 33 mm when compared to the 100-year long-term average (653 mm) (NOAA, 2006).

Objectives

    Our research initiated in 2002 addresses a critical concern of farmers: a reduction in profitability of alfalfa crop as a result of declining productivity and stand longevity caused by repeated severe drought. The objective of this project was to determine the role of variety fall dormancy (FD) on productivity of alfalfa in dryland and reduced irrigation systems.

Methods

Alfalfa varieties evaluated in this study and their FD ratings are listed in Table 1. Plots (1.8 by 7.6 m) were planted on 11 November 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) in two concurrent, adjacent experiments representing the dryland and reduced irrigation systems. In each experiment, the experimental design was a completely randomized block replicated 4 times (dryland) or 3 times (reduced irrigation). Initial soil pH was 6.9 with 20 ppm phosphorus (P) (low range) and 300 ppm potassium (K) (high range). Prior to planting, plots were fertilized with 84 kg ha^-1 P₂O₅ and 32 kg ha^-1 nitrogen. Each spring (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. Seeding rates were 22 or 13 kg ha^-1 for the dryland and reduced irrigation systems, respectively. Plants were defoliated to 5-cm height at 10% bloom (approximately every 28-d in the reduced irrigation system). Forage yield was estimated from manually harvested from a 0.5 m^-2 area randomly selected at each harvest. The rest of each plot was defoliated using a shredder and forage was removed. Cumulative forage yield was defined as a sum of DM yield from each harvest during a growing season. In the reduced irrigation system, water was supplied during April to October each year (flood irrigation) to meet the long-term annual precipitation for each month, if needed. Irrigation was applied within 1 to 5 days after defoliation, considering actual precipitation for the previous 15 d and forecasted possibility of rainfall for the following 15 d.

Table 1. Alfalfa varieties and their fall dormancy ratings

† Fall dormancy: very dormant (1-3); moderately dormant (4-6), nondormant (7-8).

           Cumulative forage DM yield, crude protein (CP), in vitro DM digestibility (IVDMD), and total phenolic concentration were analyzed using Mixed Procedure (SAS, 1999). Replications were considered random, whereas year and FD rating were considered fixed factors. Mean separation was performed using the protected least square means (LSMEANS) procedure at P < 0.05.

Results    

    Precipitation during the growing season (April to October) varied in each year, from 622 mm in 2002, 259 mm in 2003, 396 mm in 2004, and 475 mm in 2005 (Fig. 1). Irrigation varied from 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.

Figure 1. Precipitation and supplemental irrigation of alfalfa to meet the average monthly precipitation for April-October during 2002-2005 at Vernon, TX.

Cumulative DM Yield

   In the dryland system, FD had no effect on alfalfa cumulative DM yield that varied from 5.58 to 6.01 Mg ha^-1, regardless of year (Fig. 2A). Alfalfa production declined each year, regardless of FD (Fig. 2B), and the interaction between year and FD was not significant for cumulative DM.

Figure 2. Function of FD and cumulative DM yield in the dryland system, averaged for 2002-2005 (A), and cumulative DM yield of alfalfa as a function of growing season, averaged over FD groups (B). Vertical bars indicate ± 1 s.e.

    In the reduced irrigation system, there was an interaction between year and FD for cumulative DM yield. In 2002 (first growing season), cumulative DM yield was significantly lower than in the subsequent years 2003, 2004 and 2005, and was not affected by FD, ranging from 7.82 Mg ha^-1 to 9.55 Mg ha^-1 (Fig. 3). In the subsequent years, all varieties within each FD group produced similar cumulative DM yields (Fig. 3). In 2003 and 2004, varieties with FD ratings of 5-8 produced significantly more (18.47 to 21.68 Mg ha^-1) DM yield than varieties with FD ratings of 1-4 (15.36 to 17.65 Mg ha^-1). In 2005, varieties with FD ratings of 4-8 produced more cumulative DM yield (18.83 to 19.86 Mg ha^-1) than varieties with FD ratings of 1-3 (15.36 to 16.98 Mg ha^-1). 

    These results illustrate that high alfalfa production (about 20.18 Mg ha^-1) may be achieved in semiarid environments of the Texas Rolling Plains by choosing varieties with FD ratings of 7-8 and applying supplemental irrigation to meet the long-term monthly average during the growing season. Cumulative DM yield in the reduced irrigation system, averaged for 2003-2005, was about 1.6 times greater than average alfalfa DM yield in Texas in 2005 (Texas Agricultural Statistics Service, 2006). Fall dormancy explained 93% of the variation in DM yield between variety means (averaged over 2002-2005 growing seasons) in the reduced irrigation system. The average DM yield penalty was 0.63 Mg ha^-1 for each unit decrease in FD.

Forage Yield Distribution

    The effect of FD on forage yield distribution throughout a growing season is compared between varieties with contrasting FD ratings of 1-4, 5-6, and 7-8, representing high, moderate, and low FD, respectively. In the dryland system, FD had no effect on forage yield distribution, with harvest season ranging from April to June (Fig. 4). In the reduced irrigation system, varieties with FD ratings 1-4 produced less forage than varieties with FD ratings 7-8 at each harvest during April to November, while varieties with FD ratings 5-6 were intermediate (Fig. 4).

Figure 4. Forage DM yield distribution of alfalfa varieties with FD ratings of 1-4 (very dormant), FD ratings of 5-6 (moderately dormant), and FD ratings of 7-8 (nondormant) in dryland and reduced irrigation systems. Data represent average values for 2002-2005 growing seasons. Vertical bars indicate ± 1 s.e.

    High productivity of varieties with high FD ratings (low dormancy) is a result of early growth initiation in the spring and late growth arrest in the autumn. These varieties also have higher productivity during the growing season than varieties with moderate (FD ratings of 5-6) dormancy commonly grown in the southern Great Plains or very dormant (FD ratings of 1-4) varieties grown in the northern states and selected for exceptional drought tolerance.
 

Forage Quality

    Crude protein concentrations in dryland alfalfa forage were affected by FD and there was an interaction between year and harvest season. Alfalfa varieties with FD ratings of 1-4 had greater CP concentration (26.95 to 27.41%) than varieties with FD ratings of 5-8 (25.76 to 26.60%), regardless of year and harvest season (Fig. 5A). In each year, CP concentration was greater in forage harvested in April than in June (Fig. 5B). Forage harvested in April or in June had greater CP in 2004 than in 2002 and 2003.

Figure 5. The function of alfalfa FD on total CP concentration, averaged for 2002-2004 and spring and summer harvest (A), and interaction between year and harvest season, averaged for FD groups (B). Vertical bars indicate ±1 (A) and 1 (B) s.e.

    In the reduced irrigation study, CP concentration in alfalfa forage was affected by a three-factorial interaction of year, FD, and harvest season. Except for forage harvested in June 2003, CP concentration declined as FD rating increased (Fig. 6). In 2002 and 2003, CP concentrations were higher in forage harvested in October than April or June. In contrast, forage harvested in October of 2004 had less CP than forage harvested in April and June.

Figure 6. The function of year, harvest season, and FD on CP concentration in alfalfa forage in the reduced irrigation system. In 2002, R2 values for April (black -), June (---), and October (gray -) are 0.64, 0.20, and 0.63, and the regression equations are y = -0.2536x + 26.003, y = -0.3061x + 26.298, and y = -6404x + 35.329, respectively. In 2003, R2 values are 0.61, 0.00, and 0.75, and the regressions equations are y = -0.4564x + 32.909, y = -0.0039x + 25.734, and y = -0.51.59x + 35.943, respectively for April, June, and October. In 2004, R2 values are 0.70, 0.88, and 0.87, and y = -0.4935x + 29.565, y = -0.4686x + 30.262, and y = -0.5139x + 27.443, respectively for April. June, and October. Vertical bars indicate ± 1 s.e.

    Fall dormancy explained 87% of the variation in CP concentration between variety means (averaged over 2002-2005 growing seasons) in the dryland system, with average CP penalty of 0.28% for each unit decrease in FD. In the reduced irrigation system, FD explained 86% of the variation in CP concentration between variety means (averaged over 2002-2005 growing seasons), with average CP penalty of 0.39% for each unit decrease in FD.

    The IVDMD index, similar to CP concentration, was affected by FD, and there was an interaction between year and harvest season in the dryland alfalfa. The IVDMD tended to decline (R² = 0.53) in forage as FD rating increased (Fig. 7A). Alfalfa varieties with FD rating of 7 had lower IVDMD (80.52%) than varieties with other FD ratings (81.41 to 82.17 %), regardless of year and harvest season. In 2003, spring forage had greater IVDMD than summer forage (Fig. 7B), regardless of FD. In 2002 (first growing season), spring forage had lower IVDMD than in the following years. Summer forage in 2003 had lower IVDMD than in 2002 and 2004.

Figure 7. The effects of FD (A) and an interaction between year and harvest season (B) on IVDMD of alfalfa forage in the dryland system. Vertical bars indicate ±1 (A) or 1 (B) s.e.

    In the reduced irrigation system, IVDMD was affected by an interaction of year and harvest season, and an interaction of harvest season and FD. In each growing season (2002 to 2004), IVDMD of forage harvested in October was greater than that harvested in April and June, regardless of FD (Fig. 8A). In 2003, IVDMD of forage harvested in April was greater than forage harvested in June, while the IVDMD in April and June of 2002 and 2004 were similar. The IVDMD declined as FD ratings increased, especially in forage harvested in October (R² = 0.87) and April (R² = 0.75) when compared to forage harvested in June (R² = 0.68), regardless of year (Fig. 8B). 

Figure 8. The function of year and harvest season (A), and FD and harvest season (B) on IVDMD of alfalfa forage in the reduced irrigation system. In B, R2 values for April. June, October are 0.75, 0.68, and 0.87, and the regression equations are y = -0.3137x + 83.838, y = -0.3677 + 81.956, and y = -0.4043x + 87.728, respectively. Vertical bars indicate 1 (A) or ± 1 (B) s.e.

    Fall dormancy explained 53% of the variation in IVDMD between variety means (averaged over 2002-2005 growing seasons) in the dryland system, with average IVDMD penalty of 0.13% for each unit decrease in FD. In the reduced irrigation system, FD explained 90% of the variation in IVDMD between variety means (averaged over 2002-2005 growing seasons), with average IVDMD penalty of 0.34% for each unit decrease in FD.

Relation Between Fall Dormancy and Phenolic Concentration in Forage

    Total phenolic concentration was determined in alfalfa varieties grown under reduced irrigation in August 2004 (3rd growing season). Fall dormancy affected total phenolic concentration in forage. Varieties with FD ratings of 1-3 had greater phenolic concentrations (6551 to 7417 µg g-1 DM) than varieties with FD ratings of 6-8 (4862 to 5354 µg g-1 DM , and varieties with FD ratings of 4-5 were intermediate (6039 to 6209 µg g-1 DM ) (Fig. 9). Although phenolics play an important role in cell protection from oxidative stresses, such as drought, their involvement in drought tolerance of alfalfa has not been studied to date. In this study, total phenolic concentrations were greater in drought tolerant varieties with FD ratings of 1-3 than varieties with FD ratings 4-8. The production of phenolic compounds, therefore, may be considered as indicator of drought tolerance in alfalfa.

Water Use Efficiency

    Water use efficiency (WUE) was not statistically evaluated due to the experimental design of this study; however, WUE could be calculated and presented here as a trend. Alfalfa needed 2.8 times more water to produce 1 Mg ha^-1 of forage DM in the first growing season (2002) than in the following growing seasons (2003-2005). There was a strong relationship between FD and WUE in each year (Fig. 10). Varieties with higher FD ratings were able to use available water more efficiently than varieties with lower FD rating. With exception for 2002, varieties with FD ratings of 1-3 needed 36.4 mm water to produce 1 Mg ha^-1 of forage DM, while varieties with FD ratings of 7-8 needed 28.8 mm water, e.g., about 21% less. The WUE calculated from these data, except for the first growing season, was about 33%, 30%, and 18% greater for varieties with FD ratings of 7-8 (low dormancy), when compared to varieties with FD ratings of 5-6 (moderately dormant), and varieties with FD ratings of 1-4 (very dormant), respectively, than the lower limit of WUE (43 mm) reported by Trostle (2003). This roughly translates to about 30% less irrigation water to produce 1 Mg DM ha^-1 from varieties with FD ratings of 7-8 than the minimum amount of water presently recommended. Reduced irrigation requirements of alfalfa varieties with FD ratings of 7-8 would represent considerable savings of energy use necessary to maintain profitable forage production.

Conclusions

    Results of this study confirm that FD has a significant effect on yield and quality of alfalfa. Supplemental irrigation to meet at least the long-term monthly averages during the growing season (April to October) warrants high DM yields in semiarid environments of the Texas Rolling Plains, especially when selecting varieties with FD ratings 7-8. Forage quality factors (CP, IVDMD) are also highly influenced by FD, and are inversely related to alfalfa productivity potential. Selection of a variety with a particular FD rating may be a difficult decision to producers once the trade-off between yield and quality is considered. Other economic factors affecting profitability of alfalfa crop must be also considered (Putnam et al., 2005). Under dryland conditions, FD seems to be less important and alfalfa productivity declines in each growing season due to summer drought that severely limits plant growth in this environment.

References:

Kizer, M. 1991. Alfalfa irrigation. Chapter 6. In: Alflaf Production and Pest Management in Oklahoma. Circular E-826. Oklahoma Cooperative Extension Service.

Putnam, D.H., S. Orloff, and L.R. Teuber. 2005. Strategies for balancing quality and yield in alfalfa using cutting schedules and varieties. In Proc. 35th California Alfalfa and Forage Symp., Visalia, CA, 12-14 December, 2005 [Online]. Available at http://alfalfa.ucdavis.edu/+symposium/proceedings/asdf/alf_symp/2005/05-237.pdf. University of California, Davis.

Stichler, C. 1997. Texas Alfafla Production. Texas Agricultural Extension Service B-5017. College Station, TX.

Trostle, C. 2003. Texas High Plains Supplement to Texas Alfalfa Production. Texas Cooperative Extension Bulletin B-5017. Lubbock, TX.

Cooperation:

Dr. Twain Butler (The Samuel Roberts Noble Foundation, Ardmore, OK).

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