Late‐Season Management of Alfalfa in Irrigated Valleys of the Intermountain West

Yield and stand loss in irrigated alfalfa (Medicago sativa L.) in the Intermountain West are becoming a problem. The studies reported here evaluated the effect of late‐season management on total nonstructural carbohydrate concentration (TNC) in the roots and crowns of alfalfa and related these levels to yield and stand persistence. Three experiments were conducted at the Brigham Young University Agriculture Station near Spanish Fork, UT. The soil was a fine‐loamy, mixed mesic Calcic Argixerolls (Timpanogos loam). Treatments consisted of removing the fourth harvest from different plots incrementally, beginning 35 d prior to the mean first‐killing‐frost date (MKFD). Root samples were dug 2 to 3 wk after the MKFD. Four different alfalfa cultivars were used in these studies and all responded similarly to the treatments imposed. In old stands (Exp. 1 and 2), autumn TNC levels in the roots were significantly decreased by treatments in which the fourth harvest was removed during the critical period (2 to 6 wk prior to the MKFD). In the young stand (Exp. 3), removal of the fourth harvest during this period resulted in lower TNC concentrations in the roots during the third year of such management. First‐harvest and total dry matter yields were reduced in old stands if the fourth harvest was removed 2 to 6 wk prior to the MKFD (0.57–1.69 ton/acre). Removal of the final harvest during this period did not cause a reduction in first‐harvest yield in young stands (Exp. 3). The majority of yield loss occurred in the first harvest. Persistence in older stands was also negatively affected by harvesting during this period. Effects of intensive management can be reduced by prudent late‐season management that limits harvesting during the critical period to young stands, delaying the harvest until after the critical period, or harvesting during the critical period only once during the life of an alfalfa stand. Research Question Many producers and some technical advisors in the Intermountain West believe that reduced stand persistence associated with late‐season harvesting of alfalfa does not apply within the region. However, it has become evident in the past few years that stands of alfalfa in some areas lack persistence. This study addressed the hypothesis that reduced persistence is associated with intensive management systems used to produce high quality forage. Our objectives were to: (i) evaluate the relationship of late‐season management to total nonstructural carbohydrate (TNC) concentration in alfalfa roots and crowns, hay yields, and stand persistence and (ii) develop guidelines to minimize the effects of late‐season management on alfalfa stands. Literature Summary Beginning in the 1920s, researchers in the North Central USA showed that timing of cutting during September and October is critical in maintaining good stands of alfalfa. Cutting alfalfa in several consecutive years at immature stages of development lowered productivity and vigor of plants, favored weed encroachment, and accelerated winter and summer damage to stands. It was suggested that these deleterious effects were associated with depleted food reserves in the roots. Subsequent research established relationships between carbohydrates in the crowns and roots of alfalfa, stand longevity, and yield. Cutting alfalfa near l/10‐bloom is considered to be important to the accumulation of high carbohydrate reserves and the maintenance of plant vigor (high rates of growth in the spring and after each harvest) and productivity. Experience and reports in the literature suggest that cutting or grazing alfalfa near the mean fall‐frost date will not injure the stand. Though this practice is very different from defoliation during the critical period preceding frost, that distinction may not be made by growers. Consequently, alfalfa is often cut during the time considered in other temperate regions to be most critical in maintaining a vigorous, long‐lived stand (25 August through 1 October, depending on the location). Recent work in Montana showed that the last harvest during the growing season should occur more than 30 d prior to the long‐term mean fall‐frost date or after 30 September. If this guideline was not followed significant reductions in first‐harvest yield occurred the following year. Study Description This report includes results from three studies, all of which were on a Timpanogas loam soil. The first experiment was initiated in 1986 on 4‐yr‐old stands of ‘Apollo II’ and ‘Deseret’ alfalfa. The treatments consisted of removing the fourth harvests of the season at 2‐wk intervals beginning 19 August and ending 27 October. The second experiment was begun in 1987 on a 5‐yr‐old stand of ‘WL12’. Ten four‐harvest treatments were applied, with the final harvest removed at weekly intervals beginning 4 September and ending 6 November. The third experiment was established in 1989 on first‐production‐year stands of ‘Champ’ and Apollo II alfalfa. The last harvest of the season, in the four‐harvest systems, was applied between 3 September and the first killing frost of the season. Control treatments in each experiment consisted of the standard three‐harvest system used in the area. Applied Questions Is the total nonstructural carbohydrate (TNC) concentration in the crowns and roots of alfalfa related to late‐season management? Yes. Removal of the last harvest of the season during the critical period, 2 to 6 weeks prior to the mean killing‐frost date (MKFD, overnight low ≤28°F), resulted in consistent and significant reductions in root TNC concentration in the late fall. How did date of removal of the additional harvest affect yield? Using the area's MKFD as a reference point, time of removal of the fourth harvest was most critical if it occurred 2 to 6 wk prior to the MKFD. Removing the final harvest of the season during this critical period resulted in yield decreases ranging from 0 to 1.50 ton/acre per yr. Losses in yield were closely associated with reduced stand persistence and less rapid rates of spring growth. Harvesting within the 2‐wk period prior to the MKFD resulted in minimal losses. Harvesting after the MKFD resulted in no detrimental influences on the stand or on hay yield. How did stand age affect yield loss caused by harvesting during the fall critical period? As stands aged, response to intensive management (gaining one more harvest in a season) was more pronounced. For example, an additional harvest on newly established alfalfa resulted in no reduction in hay yields or stand losses, when compared with the three‐harvest control treatment. In a four‐year‐old stand (age at the beginning of the study) yield loss was 0.79 ton/acre per yr in the second year of such management and 1.25 ton/acre per yr during the third year. A 5‐yr‐old stand (age at the beginning of the study), in which the recovery period between the final harvest of the season and the MKFD was insufficient to allow replenishment of root carbohydrates resulted in losses ranging from 0.45 ton/acre per yr (first year) to 1.12 ton/acre per yr (second year). What are appropriate fall harvesting recommendations based on these studies? Harvesting alfalfa from 2 to 6 wk prior to the MKFD for the area is not recommended. Several things can be considered with respect to this recommendation. First, hay quality in the fall, because of the cooler temperatures, is maintained over fairly long periods, thus hay can be left standing through the critical period without diminishing quality significantly. Adhering to this principle will allow one to avoid the most critical times for deleterious effects of late‐season management. A complicating factor to be considered with this procedure is the extended period required in the late fall for drying the hay before baling. This problem, however, does not arise in systems where the forage is ensiled. Second, in older stands significant yield losses occurred in the second year in which the fourth harvest was removed during the critical period. Therefore, if alfalfa must be harvested during this time, minimal damage will be done by limiting the practice to only one year. This means that records should be kept and harvesting during the critical period should be rotated among alfalfa fields. Least affected are young stands, thus they might be preferentially harvested one time during the critical period without causing permanent damage. Alfalfa fields to be plowed and rotated to other spring‐planted crops would be prime candidates for such harvests. Third, from the literature it is evident that adequate periods of recovery between any of the harvests or between the last harvest and the MKFD during the season and high level of soil K fertility will minimize the negative effects on hay yields and stand persistence. Following these guidelines when making decisions on time of late‐season harvest will minimize damage to the stand and result in healthy, vigorous alfalfa growth the following year.