Ruminal solubility of nitrogen and minerals from fescue and fescue-red clover herbage

Ruminal solubility of N, K, P, Ca, Mg and S were measured for three forages at six growth stages. The three forages were Kentucky 31 tall fescue (TF), Kenhy fescue (KN) and a red clover-tall fescue mixture (RC). Disappearance parameters were measured by means of the dacron bag technique using cannulated steers. Exposures were for 48 h; maximum extent of disappearance for all elements occurred before this time. High proportions (greater than 60%) of P, K and Mg were released from all three forages during the first 3 h of incubation, with small losses thereafter. Amounts of N and S released during the first 3 h ranged between 40 to 75%. Ca had the lowest initial disappearance. Forages differed (P less than .05) in extent of mineral disappearance after 3- and 48-h incubations and in rate of disappearance (Kd) of the potentially available, slowly solubilizing, fraction of N, Ca and S. Across growth stages, TF had generally the lowest extent and rate of disappearance. With increasing maturity, Kd for Mg, Ca and S decreased (P less than .05). For each element, Kd was not correlated with herbage concentration or initial (3 h) disappearance. Partial correlations between disappearance of N, K, Ca, Mg and S after the 48-h incubation and herbage concentration were significant. Solubilization values corrected for rate of passage (ERS) showed significant differences among forages for K, Ca and S. Average ERS values for S and K were highest for KN; RC herbage had higher ERS values for Ca. Increases in forage maturity lowered (P less than .001) ERS for all elements. The results indicate that for all growth stages, ruminal solubility and potential availability of N and minerals from the three forages was high. The rate of release differed among elements and may have affected efficiency of microbial fermentation. Although ruminal solubility of minerals from TF was generally lower than from the other two forages, the difference was not sufficient to explain fully the lower apparent availability observed in previous studies.