Variation in mineral concentration and grass tetany potential among Russian wildrye accessions

Grass tetany or hypomagnesemic tetany in cattle (Bos taurus) is caused by a n imbalance of K, Ca, and Mg in the diet. Indications of grass tetany range from reduced milk yield or weight gain to severe convulsions and death. Th e risk of grass tetany dramatically increases when the K/(Mg + Ca) ratio of forage exceeds 2.2, especially for dams during early lactation. Russian wi ldrye [Psathyrostachys juncea (Fisch.) Nevski], a valuable forage species, has ratios well above this level. Our objectives were to determine the mine ral concentration and ratio values for 65 accessions of Russian wildrye to select germplasm sources with low tetany ratio and to determine the effects of year, location, and their interactions with accessions. Seedlings of ea ch accession and two checks, Syn A and Mankota, were established in replica ted space-plant nurseries at Logan, UT, Mandan, ND, and Swift Current, Sask atchewan, Canada. Years-within-location effects generally produced the larg est variance component, while the accession variance was larger than locati on x accession and location x accession X year interaction variances for K, Ca, Mg, K/(Mg + Ca) ratio, and Reduced Tetany Potential (RTP) index. Selec tion for these traits in Russian wildrye germplasm will require multiple ye ars to characterize adequately accessions, breeding lines, or synthetics. T he K/(Ca + Mg) ratio of the accessions tested ranged from 2.2 to 3.0 when a veraged across sites and years for V4 growth stage. A similar range of rati o values and ranking of the accessions was observed at the E2 growth stage. The three tetraploid accessions evaluated were among the five accessions w ith the highest tetany ratios. Previously reported forage yield and seed yi eld means were significantly correlated with K, Mg, and N concentrations an d K/(Ca + Mg) ratio. The RTP index was not correlated with forage yield, se ed yield, or N concentration. Therefore, selection in Russian wildrye shoul d be based on increased RTP index rather than K concentration or K/(Ca + Mg ) ratio to avoid concomitant unintentional selection of reduced forage yiel d and seed yield.