Mineral elements concentration and carbohydrate trends in woolly loco Astragalus mollissimus

Of all the poisonous plants, none perhaps is considered more destructive than the genus Astragalus by virtue of its broad geographic range. In the Big Bend region of the Trans-Pecos resource area, ranchers have suffered extensive livestock losses as a consequence of poisoning, particularly from woolly loco Astragalus mollissimus, known also as locoweed, purple loco and Texas loco. A knowledge of the growth requirements and general characteristics of the plant as well as livestock manipulation to avoid losses has been considered essential in adjustment of grassland management. A project was, therefore, initiated to study both the trends of the mineral element concentrations and total non-structural carbohydrates (TNC) in relation to the phenological development of the plant.

During the 1984-85 growing season, 10 plants from the Clay Miller Ranch in Presidio County, 15 miles WSW of Valentine, Texas were collected monthly for measurement of total non-structural carbohydrates (TNC), crude protein (CP), amino nitrogen (-NH^), phosphorous (P), calcium (CA), magnesium (Mg), potassium (K), zinc (Zn), selenium (Se), manganese (Mn), iron (Fe), and sodium (Na). Plants were dissected and differentiated into-roots and crown for TNC analysis, and shoots (foliar) for mineral analysis. On each sampling date, the phenology of the plant wás recorded.

Levels of CP^-NH^, and P (expressed as percent of dry weight) were generally highest after the period of seedling emergence and reached its peak in the early part of January. The values ranged from 12.70-17.lU, 2.04-2.74%, and 0.13-0.23% for CP,-NH2 and P, respectively. The lowest value was recorded in early summer after the plant had flowered. Levels of K, Ca, Mg, Mn, Na and Zn followed the same trend of CP,-NH2 and P having the lowest value after the plant had flowered. The Fe content peaked significantly during flowering but soon declined with maturity. On the other hand, the Se level fluctuated with flowering and maturation.

Changes in phenological development were complemented by corresponding changes in concentration of TNC. The plant began translocating TNC in both above (crown) and below (roots) ground biomass after it became reproductive in late December to the end of January. At this phenological stage, it is assumed that the plant used much of the available carbohydrate recharge for flower initiation, induction and seed production, hence, a decrease in the TNC both on roots and crown was noted. The TNC was generally highest when the plant was vegetative. The TNC concentration declined after flower initiation, reaching its lowest at full seed production. The % TNC in roots and crown ranged from 9.4-25.6% and 4.9-22.7%, respectively.