Performance of tall wheatgrass (Thinopyrum ponticum, cv. 'Jose') irrigated with saline-high boron drainage water : Implications on ruminant mineral nutrition

Tall wheatgrass (Thinopyrum ponticum, cv. 'Jose') (TWG) has been identified as a salt-tolerant forage that has acceptable nutritional value and shows considerable promise for reducing saline drainage volumes in California's San Joaquin Valley (SJV). The SJV drainage water also contains high concentrations of boron (B), selenium (Se) and molybdenum (Mo), which may affect the production potential and mineral nutritional value of the forage. A greenhouse study was conducted with soil-filled pots using irrigation waters that varied in B (0.7-20 mg L-1) and sodium-sulfate dominated salinity (0.5-20 dS m-1), a quality characteristic of the SJV, with a constant background of 0.5 mg L-1Se and Mo. Our experiment confirms that TWG is very tolerant to salinity. For example forage production in treatments with irrigation water with an ECw of 10 dS m-1 (ECe of16.8 dS m-1) was 74% of those irrigated with non-saline water. We found that the stable carbon isotope fractionation in shoot tissue was a good cumulative stress indicator of the crop as the discrimination value (A) decreased with increased salinity and reduced shoot biomass. Moreover, this forage crop is extremely tolerant to B, tolerating up to 20 mg L-1 in the irrigation water without a significant reduction in cumulative biomass. Tissue B concentrations increased with increased B to values above 2000 mg kg-1 dry matter (DM). However as salinity increased within a particular B treatment, tissue B decreased. Forage quality, from a ruminant mineral nutrition perspective, raised concerns. Forage samples contained high levels of B, Se, Mo and sulfur near or above the recommended maximum tolerable level (MTL). Although ruminant grazing of pastures containing forage of this quality is not recommended, it has considerable potential as a forage supplement if rations can be controlled. Livestock consuming forage of this quality should be monitored for signs of B, Se and sulfur toxicity and molybdenum-sulfur induced copper deficiency.