2011-08-04_ENFORCEMENT - C1981008 (4)

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100 80 E E 60 E O N E g '� se d 40 20 0 Author Daigger et al., 1970 Undersander, 1987 Wright, 1988 Smeal et al., 1991 Overall average y = 1.28x - 28.52 R 0.94 20 40 60 80 Relative ET (% of maximum) Fig. 2. A relative alfalfa water production function from studies with variable irrigation in the Great Plains and Intermountain West of the United States. The figure relates average season long evapotranspiration (ET) as a percentage of the maximum ET within the same study to biomass yield as a percent of the maximum biomass yield within the same study. growth cycle. This study compared biomass yield per harvest to transpiration and growing degree -days with all other variables held constant. Aboveground biomass yield to transpiration ratio decreased as growing degree -day accumulation increased between the levels of450 G (bud initiation) and 700 G (full bloom) in a very linear fashion. Smeal et al. (1991) suggested that this reduction in biomass yield per unit of transpiration may have been due to partitioning of dry matter into crowns, roots, and reproductive structures rather than new leaves and sterns. This assumption was based on the previous work of Smith (1960) who evaluated establishment and management of alfalfa and Brown et al. (1972) who studied energy accumulation and utilization in plants. The accumulation of aboveground growth follows a sigmoid pattern, with growth rates declining after about the bud stage. Since the canopy has achieved full cover by this time ET has reached a maximum and continues at a near - constant level until the bloom state, while growth rates are declining. This Table 2. Effect of harvest interval on alfalfa biomass yield, evapotranspiration (ET), and water -use efficiency (WUE) from studies in the Great Plains and intermountain West of the United States. Yield, ET, and WUE were averaged over irrigation treatments and years. Location NE TX 1D NM all • Treatment full irrigation line source full irrigation line source all 100 Harvest interval 2 3 1 2 3 4 5 2 3 1 2 3 4 I 2 3 4 5 G L` 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 y = -1E-05x= + 0.0026s - 0.0331 • = 0.881 0 20 40 60 80 100 120 140 160 • ET (cm) Fig. 3. Alfalfa water -use efficiency (WUE) relationship to evapotranspiration (ET) from two deficit irrigation studies in New Mexico (Sammis, 1981; Smeal et al., 1991). suggests that the WUE of alfalfa may be increased by harvesting on a 400 to 450 G schedule rather than the 550 G schedule recommended by Selirio and Brown (1979). However, Smeal et al. (1991) also noted that harvesting alfalfa at the prebud stage (400 G for any period of time will reduce plant vigor and decrease stand longevity as shown by Smith (1972) and Robin- son and Massengale (1968). Growing degree -day accumulation for each harvest certainly has some impact on alfalfa WUE, but it is likely that the risk of reduced stand longevity would not justify harvesting at an early growth stage to improve WUE. Stand Age Stand density and vigor decline over time in alfalfa. Stands can persist for 7 to 10 yr under normal conditions with many reported exceptions. Stand density is important for production and is linked to WUE in alfalfa ( Smeal et al., 1991). When corn - paringWUE over time, it was found that WUE was least in the establishment year and was maximized at year 5 and remained constant afterward. This may be due to a difference in CO, par- titioning as the plant matures. Studies done by Thomas. and Hill (1949) and Brown et al. (1972) found that over a 2 -yr period, an Yield Mghai 5.0 32 3.3 5.4 4.5 4.5 3.8 3.2 6.7 5.0 3.4 4.7 2.7 2.4 1.7 CM 54.5 40.6 56.6 18.2 29.9 24.3 19.0 25.0 37.5 31.9 28.4 33.6 20.9 18.0 19.2 ET WUE Mg ha l cml 0.09 0.08 0.06 0.29 0.15 0.19 020 0.13 0.18 0.16 0.12 0.14 0.13 0.13 0.09 0.18 0.13 0.13 0.15 0.13 48 Agronomy Journal • Volume 103, Issue 1 • 2011