Differential Al tolerance among cultivars may be due to differences in the ability to absorb and retain more Mg when exposed to Al (5). The hypothesis that kinetic parameters of Mg2" uptake will be less affected by Al in Al-tolerant cultivars than in Al-sensitive cultivars remains to be tested.The objective of this study was to determine the influence of Al on the kinetics of net Mg2" uptake in two ryegrass cultivars differing in Al tolerance. MATERIALS AND METHODS Plant MaterialSeeds of two ryegrass (Lolium multiforum Lam.) cultivars, Wilo (relatively Al-sensitive) and Gulf (relatively Al-tolerant), were surface sterilized, germinated, and grown in complete nutrient solution in the growth chamber as described elsewhere (31).

Annual ryegrass (Lolium multiflorum Lam.) is an important forage crop in the southeastern USA, often grown on acid soils with potentially high levels of soluble Al. Effects of Al on growth and nutrient relationships in ryegrass are poorly understood. In this study four ryegrass cultivars were grown in a growth chamber in nutrient solutions at 0, 37, 74, 148, 296, and 592 µmol L−1 Al. The pH was initially adjusted to 4.2 and subsequent changes were monitored daily. More Al‐tolerant cultivars induced higher nutrient solution pH than more sensitive cultivars. For more Al‐tolerant cultivars, these pH changes were positively correlated with root and shoot tolerance indexes and negatively correlated with percent inhibition of Ca, Mg, K, and Na uptake. Root (shoot) tolerance index was defined as dry weight (wt.) of roots (shoots) grown with Al/dry wt. of roots (shoots) grown without Al. Changes in nutrient solution pH were effects rather than causes of differential Al tolerance, but once induced, differences in pH intensified cultivar differences in Al tolerance. An Al concentration of 37 µmol L−1 in solution significantly increased root Al concentrations, but very little Al was translocated to shoots. Higher shoot Al concentrations were found more often in the less Al‐tolerant cultivars Wilo and Urbana than in the more tolerant cultivars Gulf and Marshall. Larger root and shoot tolerance indexes, and lower percentages of inhibition of total Ca and Mg uptake were observed more often for more Al‐tolerant cultivars Marshall and Gulf than for more sensitive cultivars Wilo and Urbana. The average net influx of Ca and Mg was decreased where Al was added. Total uptake of K (quantity factor) was decreased by Al, while net K influx (intensity factor) increased for plants grown in the nutrient solution containing up to 74 µmol L−1 Al. Short exposure to Al increased net influx and total Na uptake. Longer exposure to higher concentrations of Al (at least 74 µmol L−1) inhibited both total uptake and net influx of Na. The root tolerance index was positively correlated with net influx of Ca, Mg, K, and Na. Increasing the nutrient solution Al level to 148 µmol L−1 greatly increased K/(Ca+Mg) ratio of shoots, thus increasing the grass tetany potential of the ryegrass forage.

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In recent years climatic fluctuations have been increasingly associated with seasonal variation in yield and quality of forage crops. Although considerable information is available concerning environmental influences on yield and quality of many temperate and tropical grasses, very little is known about these relationships in the major grasses grown in the southern U.S.A. The objectives of this study were to determine the relative influences of temperature (T) and photon flux density (L) on dry matter yield (DMY) and in vitro true digestibility (IVD) of four warm‐season perennial grasses grown at two soil moisture levels and to relate the differences in IVD to fiber component concentrations. Coastal and common hermudagrass [Cynodon dactylon (L.) Pers.], Pensacola hahiagrass (Paspalurn notatum Flugge), and dallisgrass (P. dilatatum Poir.) were grown in an artificial growth medium at 4 day/night temperatures (35/24, 32/21, 29/18, 26/15 C), four photon Aux densities (1,050, 840, 630, 420 µEm−2 sec−1 PAR) and two soil moisture levels in growth cabinets. After 14 and 21 days regrowth, DMY and IVD were measured. Regression equations were developed to predict DMY and IVD as functions of T and L at each soil moisture level and regrowth period. The study clearly showed that DMY of all grasses increased with increasing T and L. The imposed moisture stress sharply reduced DMY of dallisgrass (DAL) hut not of the other grasses. In all grasses IVD consistently decreased as T increased, resulting in maximum IVD at the lowest T. The effect of L on IVD ranged from positive to negative, depending on the grass and T. Maximum IVD values occurred at the high soil moisture (HM) level after 14 days regrowth. At the HM level maximum IVD of Coastal bermudagrass (COA), common hermudagrass (COM), and Pensacolabahia grass (BAH), respectively, was 5.7, 4.9, and 3.6 percentage units higher after 14 than after 21 days regrowth. In DAL at the HM level and in all grasses at the low soil moisture level, these differences due to age were only about one percentage unit or less. The Paspalum spp. averaged about seven percentage units higher in IVD than the Cynodon spp. Significant (P ≤ 0.01) negative correlations were obtained between IVD and previously reported concentrations of acid detergent fiber (ADF), cellulose, permaganate lignin (LIG), and silica in all grasses and concentrations of neutral‐detergent fiber in COA and COM. Although correlation coefficients were ≤ 0.50, hemicellulose concentration and IVD were positively correlated in all grasses. Strongest correlations were obtained between IVD and concentrations of ADF and LIG.

To minimize the incidence of grass tetany, winter pastures should be established on soils containing Mg-rich minerals, drainage should be improved on five-textured soils, legumes should be included in the sward and soil pH should be at least 5.5. Liming acid soils with dolomitic lime increases forage Mg by supplying Mg and by raising soil pH. Calcitic lime applications also can increase Mg availability to plants on soils with adequate Mg. Low rates of application of soluble Mg salts (less than 100 kg/ha of Mg) effectively increase Mg uptake from noncalcareous soils with low cation exchange capacity. Potassium levels in soils and plants should be kept in the lower range of recommended values. Nitrogen application should be regulated to provide the desired level of forage production. Nitrogen fertilizers, especially the nitrate form, stimulate plant Mg uptake if Mg is available in the soil. The most practical and cost-efficient method of supplementing dietary Mg intake is to provide free-choice Mg. Supplements must be palatable and placed in locations frequently used by cow herds. Including a high-energy feed in the supplement may at times increase its preventive effectiveness by increasing Mg absorption and reducing lipolysis. Regardless of the supplement formulation, Mg intake should be monitored on a regular basis, and formulation or management changes should be initiated if Mg consumption is below required levels. In severe grass tetany outbreaks, foliar application of Mg or administration of Mg via the drinking water may be warranted.