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but results from other regions show improved WUE with this
<br />irrigation method. Putnam et al. (2005) showed significant water
<br />savings from partial season irrigation of alfalfa in the Klamath
<br />Basin and Sacramento Valley of California with few long -term
<br />impacts on alfalfa stands. However, for arid climates and sandy
<br />soils, summer termination of irrigation can reduce alfalfa stands
<br />and biomass yields after irrigation is resumed (Ottman et al.,
<br />1996). More information is needed about partial season irriga-
<br />tion in the Great Plains and Intermountain West regions.
<br />Management Factors Affecting
<br />Alfalfa Water -Use Efficiency
<br />Variety
<br />While it is expected that alfalfa varieties would vary widely in
<br />QCTUE, there is little evidence to support this. A Utah line- source
<br />irrigation study evaluated WUE of alfalfa varieties 'Ladak',
<br />Washoe, and'Mesilla' with fall dormancy ratings of three, five,
<br />and seven, respectively (Retta and Hanks, 1980). The 2- yrstudy
<br />found no difference in biomass yield or water use among variet-
<br />ies. A line- source irrigation study in Texas (Undersander, 1987)
<br />examined WUE of alfalfa varieties `Vangard', 'Cody', `Zia', and
<br />'Dawson', all with varying dormancy characteristics and diverse
<br />genetic backgrounds. Results showed no significant differences in
<br />WUE among the varieties for any level of irrig,ation. Hattendorf et
<br />al. (1990) conducted a line - source irrigation study in Washington
<br />comparing water use and biomass yield of alfalfa varieties 'Vernal',
<br />'Vernema', and 'CUF 101' with fall dormancy ratings of two, four,
<br />and nine, respectively. No differences in biomass yield or water use
<br />were found. A line- source irrigation study conducted in California
<br />(Grimes et a1.,1992) evaluated WUE of alfalfa varieties 'CUF
<br />101', ` Moapa 69', and 'WL 318' with fall dormancy ratings of 9,
<br />8,and 3, respectively. In the California environment, the semi -
<br />dormant'WL 318' had a slightly higher WUE than the other two
<br />varieties during the cool spring conditions During the hot sum-
<br />mer conditions, the less dormant CUF 101 and Moapa 69 varieties
<br />had higher WUE. However, there was no significant difference
<br />amongvarieties when WUE was evaluated on a total season
<br />basis. These studies indicated that alfalfa varieties with varying
<br />dormancy traits and genetic backgrounds did not differ greatly in
<br />total- season WUE. Varieties with different fall dormancy may not
<br />relate to the suitability of the variety under deficit irrigation.
<br />Less is known about the potential role of alfalfa variety
<br />on partial season irrigation. It has been reported that alfalfa
<br />varieties that have cold tolerance and winter hardiness also
<br />have drought tolerance because both conditions desiccate plant
<br />cells (Jung and Larson, 1972). Drought and cold tolerance
<br />may be linked to small cell size. If advantages in WUE during
<br />particular periods of the growing season do exist, selecting
<br />varieties that possess drought tolerance and are more dormant
<br />may be best for stand survival during dry periods in a partial
<br />season irrigation system. More studies are needed to evaluate
<br />alfalfa varieties under partial season irrigation in different envi-
<br />ronments. It is thought that a more fibrous root system may be
<br />more efficient in extracting soil water than a dominant tap root
<br />system. However, it should be noted that numerically high fall
<br />dormancy ratings tend to be correlated with tap - rooted alfalfa
<br />varieties and low Fall dormancy ratings are correlated with
<br />fibrous- rooted alfalfa varieties (Smith, 1993).
<br />Table 1. Average total season biomass yield, evapotranspi-
<br />ration (ET), and water -use efficiency (WUE) from studies
<br />of alfalfa under variable irrigation in the Great Plains and
<br />Intermountain West of the United States.
<br />Author Location Treatment Yield ET WUE
<br />Mg ha l cm Mg ha i cm l
<br />Daigger et al., NE full irrigation 11.5 151.7 0.08
<br />1970
<br />Bauder et al.,
<br />1978
<br />Retta and
<br />Hanks, 1980
<br />ND dryland
<br />deficient
<br />optimum
<br />excessive
<br />average
<br />UT line source
<br />average
<br />Sammis, 1981 NM line source
<br />average
<br />Carter and MN high
<br />Sheaffer, med. high
<br />1983t med. low
<br />dryland
<br />average
<br />Undersander, TX line source 13.5 76.2 0.18
<br />1987$ average 22.6 113.0 0.20
<br />16.4 91.7 0.18
<br />173 93.6 0.19
<br />Wright, 1988 ID full irrig. 14.7 94.2 0.16
<br />Bogler and TX average na§ na 0.17
<br />Matches, 1990
<br />Smeal et al., NM line source
<br />1991 average
<br />Overall averages
<br />All authors all all
<br />All authors all full irrigation'.
<br />All authors all defict irrigation
<br />All authors all dryland
<br />5.8 33.9 0.17
<br />9.7 60.2 0.16
<br />10.3 64.5 0.16
<br />10.8 68.6 0.16
<br />9.2 56.8 0.16
<br />7.2 38.3 0.19
<br />8.0 37.6 0.21
<br />9.3 47.7 0.20
<br />10.7 55.4 0.19
<br />11.9 57.4 0.21
<br />12.5 61.6 0.20
<br />9.9 49.6 0.20
<br />6.0 66.7 0.09
<br />8.3 77.6 0.11
<br />10.0 78.5 0.13
<br />11.5 90.9 0.13
<br />12.3 92.5 0.13
<br />13.1 103.8 0.13
<br />14.3 109.4 0.13
<br />15.0 117.5 0.13
<br />17.0 124.5 0.14
<br />19.6 135.2 0.15
<br />22.1 145.7 0.15
<br />13.7 104.0 0.13
<br />7.4 32.6 0.23
<br />7.0. 29.9 0.23
<br />5.5 26.4 0.21
<br />2.1 17.9 0.12
<br />5.5 26.7 0.21
<br />3.0 45.8 0.07
<br />6.0 63.1 0.09
<br />8.7 78.5 0.11
<br />12.3 91.3 0.13
<br />15.1 104.9 0.14
<br />14.8 106.7 0.14
<br />12.5 93.7 0.13
<br />10.3 83.4 0.12
<br />12.8 88 0.16
<br />16.6 91. 0.19
<br />11.1 80 0.17
<br />6.0 39 0.14
<br />t Harvests 3 and 4 only reported.
<br />t. Maximum yield and ET only reported.
<br />§ na, not applicable.
<br />Harvest Timing
<br />Harvest timing has been found to influence alfalfa WUE. A
<br />study conducted in New Mexico from 1981 to 1987 by Smeal
<br />et al. (1991) evaluated the WUE response of alfalfa in relation
<br />to the accumulation of growing degree -days (G,) within each
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