Agronomy Journal 92:1077-1080 (2000)
© 2000 American Society of Agronomy
ALFALFA
Morphological Development of Alfalfa Cultivars Selected for Higher Quality
Marvin H. Hall,
W.Scott Smiles and
Robert A. Dickerson
Dep. of Agron., Pennsylvania State Univ., University Park, PA 16802 USA
mhh2{at}psu.edu
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ABSTRACT
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Alfalfa (Medicago sativa L.) maturity at the time of harvest greatly influences forage quality. The objective of this research was to determine if observed quality differences between high quality and other alfalfa cultivars were due to differences in morphological development. Two high-quality cultivars (`WL 252HQ', `WL 322HQ') and two check cultivars (`5262', `5454') with similar fall dormancy ratings were established on a Murrill silt loam (fine-loamy, mixed, mesic Typic Hapludult) in the spring of 1996. During 1997, 1998, and 1999, each cultivar was hand-sampled weekly for 4 consecutive wk during the spring growth period and for 3 consecutive wk during the three subsequent growth periods. Morphological development and forage quality were determined for each alfalfa cultivar at each sampling. There were no interactions between growth period or sampling times and alfalfa cultivars for either alfalfa maturity or forage quality. Morphological development of the cultivars was not different at any of the sampling times. Averaged across all growth periods and sampling times, crude protein (CP) concentrations and in vitro dry matter digestibility (IVDMD) were 14 and 13 g kg-1, respectively, higher for the high-quality cultivars (ADF) and neutral-detergent fiber (NDF) concentrations were 16 and 15 g kg-1, respectively, lower. The data from this research indicates that the difference in quality between the high quality and traditional alfalfa cultivars is not due to differences in morphological development. Selecting for higher-quality alfalfa was successful and is not an artifact of inadvertently selecting for delayed development.
Abbreviations: ADF, acid-detergent fiber • CP, crude protein • IVDMD, in vitro dry matter digestibility • MSC, mean stage by count • NDF, neutral-detergent fiber • NIRS, near infrared reflectance spectroscopy
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INTRODUCTION
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RELATIVELY SMALL INCREASES in forage quality can improve animal performance and substantially reduce off-farm feed purchases. Forage breeders have traditionally used dry matter yield, winter survival, and disease resistance as the primary criterion for cultivar selection. More recently, near infrared reflectance spectroscopy (NIRS) has been used to rapidly and inexpensively determine forage quality and allow forage cultivar selection based on quality parameters (Coors et al., 1986; Huset et al., 1991; Sumberg et al., 1983). Since 1991, alfalfa cultivars that are selected for higher quality have been commercially available (Huset et al., 1991). However, the morphological or physiological reasons for the higher quality of these cultivars is not clearly understood. McQueen and Belanger (1994) reported no leaf to stem ratio differences between the high quality and traditional cultivars, but they later reported a greater stem digestibility and a greater proportion of leaves in the high-quality cultivars (Belanger and McQueen, 1995).
Alfalfa maturity plays a large role in the quality of harvested forage. The inverse relationship of advancing alfalfa maturity and declining forage quality is well established (Hintz and Albrecht, 1991; Sanderson, 1992; Sulc et al., 1997). Declining quality is associated with marked decreases in the quality of stems while little quality change occurs in the leaves (Albrecht et al., 1987; Barnes and Gordon, 1972; Buxton and Hornstein, 1986). The decrease in alfalfa quality associated with advancing maturity is strongly influenced by temperature and is greater in the summer than the fall (Christian, 1977; Griffin et al., 1994; Kalu and Fick, 1981, 1983).
Alfalfa cultivars with a greater fall dormancy tend to grow slower after cutting than less dormant cultivars; however, considerable growth-rate variability exists within a dormancy group (Busbice and Wilsie, 1965; Larson and Smith, 1963). In other words, alfalfa cultivars with similar fall dormancy ratings may grow and mature at different rates, resulting in forages of different quality when harvested on the same day of growth.
High-quality alfalfa cultivars are generally compared with traditional cultivars on the same calendar date. However, the stage of maturity at the time of sampling has not been reported with the quality data. The objective of this study was to determine if the observed differences in the quality of high-quality alfalfa cultivars were due to changes in morphological development.
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Materials and methods
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Two high-quality alfalfa cultivars (WL 252HQ and WL 322HQ) and two check cultivars not selected for high quality (5262 and 5454) were established on a Murrill silt loam into 1 by 5 m tilled plots at the Russell E. Larson Agricultural Research Center near Rock Springs, PA in April 1996. The two check cultivars had fall dormancy ratings that were identical to the high-quality cultivars (WL 252HQ = 2; WL 322HQ = 4). Throughout the experiment, the soil pH, available P, and exchangeable K were maintained above 6.5, and 78 and 360 kg ha-1, respectively. Dimethoate insecticide {0,0-dimethyl S-[N-(methylcarbamoyl)methyl] phosphorodithioate} was applied as needed to control potato leafhopper (Empoasca fabae Harris) during the study.
The experimental design was a split plot arrangement of a randomized complete block design with four replicates. The whole plots were four growth periods (before the first, second, third and fourth harvest), and the subplots were the four cultivars. In 1997, 1998, and 1999 alfalfa sampling was initiated when flower buds were first observed within each of the four growth periods. The average dates of sampling were 15, 22, and 29 May and 5 June for the first growth period; 22 and 29 June and 6 July for the second; 20 and 27 July and 3 August for the third; and 1, 8, and 15 September for the fourth. At sampling, alfalfa was hand-harvested 3 cm above the soil surface from two randomly selected 0.05-m2 areas within each subplot within a whole plot. Alfalfa herbage from the two areas within a subplot were combined, and maturity was immediately determined using the mean stage count (MSC) method described by Kalu and Fick (1981). The herbage was then dried in a forced-air oven at 60°C for 48 h and ground to pass through a 1-mm screen before undergoing quality analysis.
Forage quality was predicted using NIRS. In 1997, 80 samples were selected from all samples using the SELECT program described by Shenk and Westerhaus (1994) and analyzed for CP, ADF, NDF, and IVDMD. The CP concentrations were determined as Kjeldahl N x 6.25. The ADF and NDF concentrations were measured using the procedures of Goering and Van Soest (1970), and the IVDMD was determined using the methods described by Marten and Barnes (1980). These 80 samples were used to create a calibration equation to predict the constituents of all samples collected in 1997. In 1998 and 1999, 40 and 23 samples, respectively, were selected using the SELECT program and then chemically analyzed and added to the existing NIRS prediction equation to predict the quality of the samples collected in those years. The coefficients of determination (r2) exceeded 0.962 for all of the NIRS prediction equations.
The data were first subjected to an exploratory analysis to determine if the assumptions of analysis of variance held. The homogeneity of variance was tested using Hartley's F-max test (Milliken and Johnson, 1984). The data in each experiment were analyzed by year, and then variables of interest were analyzed across years. In the statistical model, growth period, cultivar, and growth period x cultivar were considered fixed variables while year, sampling time, and sampling time x cultivar were considered random variables. All statistical analyses used SAS Institute (1997) software. Tukey's multiple comparison procedure was used for mean separations. The differences reported in this paper are all at the P 
0.05 level of significance.
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Results and discussion
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Growing Conditions
Precipitation during the 1997 and 1998 alfalfa-growing seasons were within 40 mm of the previous 20-yr average. However, 1999 precipitation was 138 mm below average during May, June, and July. The lack of precipitation during these months slowed plant growth (visual observation) but had no effect on the relationship between alfalfa development and forage quality.
Average daily temperatures for the four growth periods in each year were within 0.3°C of the 20-yr average. Average daily temperatures during the first and fourth growth periods (14.0 and 17.8°C, respectively) were lower than during the second and third growth periods (19.5 and 20.6°C, respectively).
Morphological Development
The variances were homogeneous for the 3 yr of this study, so the development and quality data across all years were combined for analyses and presentation.
There were no interactions for morphological development between the cultivars and growth period or the cultivars and sampling times (Fig. 1)
. Averaged across all cultivars and sampling times within a growth period, morphological development was less during the first and fourth growth
periods compared with the second and third growth periods
at similar days of growth (Fig. 1). Averaged across all alfalfa cultivars and growth periods, the MSC increased from 2.6 to 4.1 from the first to the final sampling time, respectively. The morphological development of alfalfa in this study paralleled that reported by Sanderson and Wedin (1988) at similar days of growth.
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Fig. 1 Morphological development (MSC) of high-quality and check alfalfa cultivars during four growth periods. Each data point represents two cultivars over a 3-yr period. MSC is the mean stage by count, where a value of 3 indicates that the majority of stems have 1–2 nodes with buds, but no flowers are present. Cultivar means identified with an * are different at the P 0.05 level of significance
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The cultivars were not different from each other in morphological development at any sampling time during the study. The lack of differences in the development between cultivars was unexpected based on the previous findings of Busbice and Wilsie (1965), who found that the development rate of alfalfa was positively correlated to fall dormancy. It was anticipated that the alfalfa cultivars in this study with a fall dormancy of two would mature slower than those with a fall dormancy of four. Busbice and Wilsie (1965) did point out that there was considerable variability in the rate of alfalfa development and that alfalfa development could be isolated from the fall dormancy trait through selection. Perhaps the previously reported relationship has been weakened through the selection for higher yields within a dormancy group.
Forage Quality
There were no interactions between cultivars and growth periods, or cultivars and sampling times for any of the forage quality parameters that were measured in this study (Fig. 2)
. Averaged across all cultivars and sampling times within a growth period, the CP concentrations and IVDMD were 22 and 29 g kg-1 greater, respectively, during the first and fourth growth periods compared with the second and third growth periods, whereas the ADF and NDF concentrations were 16 and 15 g kg-1 lower, respectively. Greater alfalfa development during the summer growth periods compared with those in the spring or fall may account for the lower forage quality during the summer (Fig. 2). In addition, changes in cell-wall chemistry associated with higher temperatures may have also contributed to the lower quality of forage in the summer. (Griffin et al., 1994; Kalu and Fick, 1983; Sanderson and Wedin, 1988).
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Fig. 2 Nutrient content of high-quality and check alfalfa cultivars during four growth periods. Each data point represents two cultivars over a 3-yr period. CP is crude protein; ADF is acid-detergent fiber; NDF is neutral-detergent fiber; and IVDMD is in vitro dry matter digestibility. Cultivar means identified with an * are different at the P 0.05 level of significance
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The differences in forage quality between the check and high-quality cultivars occurred in all growth periods except the third, but this did not cause a significant interaction between the alfalfa cultivars and growth periods for forage quality (Fig. 2). This indicates that the higher forage quality of high-quality cultivars is a stable phenotypic trait throughout the growing season but is less pronounced during the third growth period. The lack of cultivar differences in forage quality during the third growth period may, again, be attributed to the higher average daily temperatures (>20°C) during that growth period, and its effect on cell chemistry (Griffin et al., 1994; Sanderson and Wedin, 1988).
Averaged over the four alfalfa cultivars and the four growth periods, forage quality declined from the first to the final sampling time (Fig. 2). The CP concentrations and IVDMD decreased by 28 and 43 g kg-1, respectively while the ADF and NDF concentrations increased by 35 and 46 g kg-1, respectively, from the first to last sampling time. Kalu and Fick (1983) reported similar declines in alfalfa quality associated with advancing morphological development as demonstrated in this study.
The high-quality alfalfa cultivars generally had higher quality than the check cultivars (Fig 2). Averaged over all growth periods and sampling times, the CP concentrations and IVDMD were 14 and 13 g kg-1 higher, respectively, for the high-quality cultivars than the check cultivars, whereas the ADF and NDF concentrations were 16 and 15 g kg-1 lower, respectively (Table 1)
. The higher quality of the high-quality alfalfa cultivars compared with the check cultivars in this research suggests that use of high-quality cultivars by farmers could reduce off-farm feed purchases and improve a farm's economic situation.
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Summary
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The alfalfa cultivars in this study that are marketed as high quality had higher quality than the other alfalfa cultivars in three of the four harvests. However, our results indicate that slower morphological development does not account for the higher quality of these cultivars. The findings of this research indicate that alfalfa quality prediction models based on alfalfa development may not be applicable to all cultivars. Revised prediction models for high-quality cultivars may be necessary. These finding also support previous reports that real improvements in alfalfa quality are possible through recurrent selection. Continued research is needed to elucidate the basis for the improved alfalfa quality, but a greater stem digestibility, a greater proportion of leaves, or both may ultimately account for this trait in the high-quality alfalfa cultivars.
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ACKNOWLEDGMENTS
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The authors thank Pioneer Hi-Bred International for financial support of this research. We also acknowledge the work of Amy Chemerys, deceased, in the quality analyses conducted for this research.
Received for publication February 1, 2000.
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TOP
ABSTRACT
INTRODUCTION
