In situ solubility of selected macrominerals from common bermudagrass fertilized with different nitrogen rates and harvested on two dates

Minerals are essential nutrients for animals as primordial elements for structural tissues and metabolic reactions. Forages are considered an important source of minerals for ruminants, but forage mineral concentrations may vary across the growing season due to several factors such as forage species and fertilization levels. Common bermudagrass [Cynodon dactylon (L.) Pers.] growing on a poultry layer manure-amended site was fertilized with ammonium nitrate at four rates (0, 56, 112, and 168 kg N/ha) approximately 1 month before the first and third harvests (30 May and 18 August 2000, respectively) to evaluate those effects on forage concentration and ruminal disappearance of Ca, P, Mg, and K. Five crossbred ruminally cannulated steers (422±21.0 kg BW) were used to evaluate these forages in situ in a randomized complete block design with a 2×4 (harvest date×N fertilizationrate) factorial arrangement of treatments. Forage P content decreased (P<0.01) with increasing N fertilization on 18 August, while forage Mg increased (P<0.01) and forage Ca content tended (P=0.08) to increase with increasing N fertilization on both harvest dates. Approximately, 992 g/kg of the K disappeared immediately (fraction A) from the bags compared with approximately 821, 718, and 476 g/kg of Mg, P, and Ca, respectively. Fraction A, fraction B (the fraction disappearing at a measurable rate), and effective ruminal disappearance of Ca, P, Mg, and K were affected by linear or quadratic interactions between N fertilization and harvest date. Effective P disappearance increased linearly (P<0.05) on 30 May and decreased quadratically (P<0.01) on 18 August, and effective Mg disappearance increased linearly (P<0.01) on both dates with N fertilization rate. The intercepts of the regression of effective P, Mg, and K disappearance on N rate were greater (P<0.05) on 30 May than 18 August, but the mean values for each date were >830 g/kg. Rate of Mg disappearance responded quadratically (P<0.01) to N fertilization rate and was greater (P<0.05) on 18 August compared with 30 May. Therefore, N fertilization rates affected the concentration and ruminal liberation of macrominerals from bermudagrass, and effective mineral disappearance was greater than 700 g/kg in all instances.