Effect of water salinity on the composition and in vitro digestibility of winter-annual ryegrass grown in the Arava desert

Winter-annual ryegrass (Lolium multiflorum, Lam.) was sown on a sandy soil in the southern Arava desert of Israel and watered by drip irrigation. The experimental design was that of a random block including four plots per tre atment of the following three irrigation treatments. (I) Water desalinated by reversed osmosis, aimed at simulating a fresh water source, with a salin ity expressed as electrical conductivity (EC) of 1.64 and 1.44 dS m(-1) (dS : deci siemens). (II) Secondary effluent from the sewage treatment plant of the city of Eilat, EC = 3.20 and 4.24 dS m(-1). (III) Salinated secondary effluent, produced by pumping a concentrated NaCl solution into a particula r irrigation system of secondary effluent, EC 6.59 and 9.78, for the 1st an d 3rd cuts, respectively. The middle 1-m wide stripe along the entire lengt h of each plot was harvested, weighed and sampled. Freeze-dried ryegrass sa mples were ground and analyzed. Water-soluble carbohydrates (WSC) content w as 3-6 times higher in the ist cut forage than in the 3rd cut, and unlike n eutral detergent fiber (NDF) concentration, responded positively to the inc rease in water salinity. During that period, the plants counteracted the hi gh osmotic pressure of the salinated secondary effluent, by accumulating mo re WSC, on the expense of cell wall (CW) synthesis. The 3rd cut was apparen tly not sensitive to water quality with respect to WSC and NDF contents. Du ring that period, osmotic accommodation was attained apparently by accumula ting more sodium in the plants. Organic matter yield was negatively affecte d by salinity, but partly compensated by an increase in the in vitro organi c matter digestibility from 70.9 to 83.2 and from 73.5 to 78.1 in the 1st a nd 3rd cuts, respectively. The NDF monosaccharide composition was unaffecte d by salinity treatment. The proportion of ferulates involved in bridging w as similar to 10% in the forage of the 1st cut and raised to 20-40% in that of the 3rd cut. Salinity effect on cinnamic acids content in the Ist cut, followed the pattern of NDF, and was negatively related to digestibility. S uch relationship between cinnamic acids content and digestibility was not f ound in the 3rd cut. The lack of consistency in this regard, poses some dou bt as to the role of cinnamic acids as CW degradation obstacles. (C) 2001 E lsevier Science B.V. All rights reserved.