Leaf Senescence and Seasonal Decline in Nutritional Quality of Three Temperate Forage Grasses as Influenced by Drought
- S. Bittman ,
- G. M. Simpson and
- Z. Mir
Growth of cool season grasses in the northern Great Plains occurs mainly in the spring, and summer pasture consists of mature, quiescent, and drought-affected vegetation. This study was conducted to determine drought effects on leaf'stem ratio, leaf senescence, and nutritional quality of three grasses, ‘Parkway’ crested wheatgrass [Agropyron cristatum (L.) Beauv. spp. pectinatum (Bieb.) Tzvel.], ‘Magna’ smooth bromegrass (Bromus inermis Leyss.), and ‘Prairieland’ Altai wildrye [Leymus ungustus (Trin.) Pilger] commonly grown on the northern Great Plains. The grasses were grown in the field on deep black soil (Meifort silty clay, Typic Cryoboroll) at Melfort, Saskatchewan under two moisture regimes: plots subjected to drought (DRY) were covered by rain out shelters starting at the end of May; and irrigated (IRR) plots were watered when soil water potential averaged −0.07 MPa. Altai wildrye maintained its quality through the season (as indicated by N, P, in vitro digestibility, and lignin) better than the other species because it had the lowest proportion of stems and least leaf senescence. Crested wheatgrass had the lowest quality from June to August because it had the greatest leaf senescence and highest proportion of stems. Water deficit (DRY plots) increased the rate of seasonal decline in N and P concentration probably by increasing leaf senescence. However, drought reduced the seasonal decline in digestibility by reducing the rate of increase in acid detergent fiber and lignin. Drought increased leaf senescence more in floral than in nonfloral tillers, suggesting that leaf senescence was controlled hormonally. Because leaf senescence was lower in nonfloral than in floral tillers, the quality of midsummer pasture may be improved by introducing cultivars having a high proportion of nonflowering tillers.
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