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

Abstract

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 treatment of the following three irrigation treatments. (I) Water desalinated by reversed osmosis, aimed at simulating a fresh water source, with a salinity 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 particular irrigation system of secondary effluent, EC 6.59 and 9.78, for the 1st and 3rd cuts, respectively. The middle 1-m wide stripe along the entire length of each plot was harvested, weighed and sampled. Freeze-dried ryegrass samples were ground and analyzed. Water-soluble carbohydrates (WSC) content was 3–6 times higher in the 1st cut forage than in the 3rd cut, and unlike neutral detergent fiber (NDF) concentration, responded positively to the increase in water salinity. During that period, the plants counteracted the high osmotic pressure of the salinated secondary effluent, by accumulating more WSC, on the expense of cell wall (CW) synthesis. The 3rd cut was apparently not sensitive to water quality with respect to WSC and NDF contents. During that period, osmotic accommodation was attained apparently by accumulating more sodium in the plants. Organic matter yield was negatively affected by salinity, but partly compensated by an increase in the in vitro organic matter digestibility from 70.9 to 83.2 and from 73.5 to 78.1 in the 1st and 3rd cuts, respectively. The NDF monosaccharide composition was unaffected by salinity treatment. The proportion of ferulates involved in bridging was ∼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 1st cut, followed the pattern of NDF, and was negatively related to digestibility. Such relationship between cinnamic acids content and digestibility was not found in the 3rd cut. The lack of consistency in this regard, poses some doubt as to the role of cinnamic acids as CW degradation obstacles.