Submitted to: Agricultural Water Management
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/11/2004
Publication Date: 11/20/2004
Citation: Grattan, S.R., Grieve, C.M., Poss, J.A., Robinson, P.H., Suarez, D.L., Benes, S.E. 2004. Evaluation of salt-tolerant forages for sequential water re-use systems: III. Implications for ruminant mineral nutrition. Agricultural Water Management. 70:137-150. Interpretive Summary: Use of saline drainage water for crop irrigation has been proposed as a strategy to reduce drainage volumes and conserve good quality water. However, drainage effluents in the San Joaquin Valley of California (SJV) are a unique mixture of salts of sodium, sulfate, chloride, magnesium, and chloride as well as other constituents such as the potentially toxic trace elements, selenium (Se) and molybdenum (Mo). In this year-long study, ten forages were grown in greenhouse sand cultures irrigated with solutions prepared to simulate typical SJV drainage waters containing low concentrations of Se and Mo. The forages were four legumes (2 alfalfa varieties, two trefoil species) and six grasses (two paspalum varieties, kikuyugrass, bermudagrass, tall wheatgrass, alkali sacaton). At each sequential harvest herbage was analyzed for content of the major ions, but only at selected early harvests for copper (Cu), Se and Mo. Mineral ion concentrations in the forages were compared with nutrient ion requirements for ruminants, principally dairy and beef cattle. Forage-selenium was low and would not constitute a hazard to ruminants fed on these crops. The study revealed, however, several instances where ion concentrations or interactions in the plant may affect nutrient bioavailability to ruminants. For example, high potassium in the legumes would probably reduce magnesium availability to the point that the animal would risk magnesium deficiency. Likewise, the low Cu to Mo ratio in most of the forages may cause Cu deficiency in ruminants. These imbalances can easily be corrected by monitoring the livestock and supplying mineral supplements as needed. The major risk to ruminants, however, is that sulfur (S) concentrations in the forages were close to or exceeded the maximum tolerable limit. Dietary S above 3 to 4% dry weight is toxic in itself and also decreases the availability of essential micronutrients to ruminants. The three-way interaction of Cu, Mo and S is of particular importance. These preliminary results indicate that these forages irrigated with saline-sodic waters typical of those present in the SJV should be used judiciously in the ruminant diet.
Technical Abstract: Reuse of drainage waters is an attractive management option that has been proposed for many irrigated agricultural areas. In California's San Joaquin Valley (SJV), however, drainage effluents are not only saline, but may also contain potentially toxic trace elements such as selenium and molybdenum. Crop suitability for reuse systems depends on the influence the sodium sulfate-dominated waters have on biomass production, plant sustainability, and mineral elements that are critically important for forage quality. Ten promising forage crops were grown in greenhouse sand cultures irrigated with synthetic drainage waters dominated by Na2SO4 with an EC of either 15 or 25 dS/m each containing 500 micrograms/L Se and Mo as Selenate and Molybdate Plant material was analyzed several times for mineral content and selected trace elements that may have a profound influence on ruminant health. Trace element concentrations indicate Se toxicity is of little concern, but that high concentrations of both Mo and S in the herbage will likely pose a problem regarding Cu deficiency in ruminants. Similarly, high K/Mg and K/(Ca+Mg) ratios in many of the legume and grass forages, respectively, indicate that there may be potential for development of sub-normal Mg levels (hypomagnesaemia) in ruminants. However, each of these disorders can be avoided or corrected with dietary supplements. The greatest concern regarding ruminant nutrition is sulfur toxicity. Sodium-sulfate dominated drainage waters will likely elevate forage S concentrations to levels that could potentially cause excessive sulfide concentrations in the rumen and lead to serious neurological disorders affecting animal health.