Water-stress effects on forage quality of alfalfa
Although water stress is recognized as the most important physical limitation to alfalfa yields, its effects on the nutritive quality of alfalfa have received comparatively little attention. The primary objective of this study was to characterize the responses of the nutritive quality of plant parts of alfalfa and its total herbage to water stress, occurring either throughout a growth cycle or at certain growth stages. In 1983 and 1984, alfalfa, grown in potometers, was subjected to eight irrigation treatments, and was cut at one of five harvest dates during a 7-week regrowth period. In one set of treatments, plants were irrigated to 112, 100, 88, 77, and 65% of field capacity; the other three treatments imposed water stress at one of three growth stages: late vegetative, bud, and flower;Water stress applied throughout the growth cycle reduced dry-matter yield by up to 50%, delayed plant maturity, and increased leaf-to-stem ratio (LSR). Total-herbage in-vitro digestible dry-matter (IVDDM) concentration increased by up to 7% with increasing water stress as a result of increased stem IVDDM concentration and increased LSR. Crude-protein (CP) concentration in stems increased by up to 10% while that of leaves decreased by up to 14% with increasing water stress; as a result, total-herbage CP concentration was unaffected by water stress. Cell-wall concentration (CWC) of both leaves and stems decreased with increasing water stress, causing total-herbage CWC to decrease by up to 11%. As a fraction of cell wall, lignin was unaffected, cellulose decreased, and hemicellulose increased, with increasing water stress. Effects of water stress on forage quality were generally significant even after accounting for differences in plant maturity. Most of the forage-quality differences could be related to a canopy-temperature based crop-water-stress index (CWSI);Stress at the bud and flower stages resulted in leaf wilting and leaf losses, causing a reduction in LSR by up to 40% and a deterioration in IVDDM and CP concentration of total herbage. Water stress at the vegetative stage did not significantly affect total-herbage forage quality. Because CWSI was insensitive to leaf losses caused by stress at the bud and flower stages, it did not significantly relate to total herbage forage quality although it did relate to quality of plant parts.