Sulfur concentrations and in situ solubility of selected microminerals from common bermudagrass fertilized with different nitrogen rates and harvested on two dates

Common bermudagrass [Cynodon dactylon (L.) Pers.] growing on a poultry layer manure-amended site (layer manure was last applied in 1999) was fertilized with ammonium nitrate at four rates (0, 56, 112, and 168 kg N ha−1) at approximately 1 month before first and third harvests on 30 May and 18 August 2000 to evaluate forage concentrations of S and Mo, and concentrations and ruminal disappearance of Fe, Zn, and Cu. Five crossbred ruminally cannulated steers (422±21.0 kg BW) were used to evaluate ruminal disappearance of Fe, Zn, and Cu from these forages in situ in a randomized complete block design with a 2×4 (harvest date×N fertilization rate) factorial arrangement of treatments. Forage S concentrations decreased linearly (P=0.02) with increasing N application on both harvest dates, but Fe, Zn, Cu, and Mo concentrations did not differ (P≥0.24) across N fertilization rates. The intercepts for the regressions of S, Zn, Cu, and Mo concentrations on N fertilization rate were higher (P<0.05) on 30 May than on 18 August. Immediately soluble Zn and Cu fractions (fraction A) increased (P<0.05) with N fertilization on 18 August, and those intercepts were higher (P<0.05) on 30 May than on 18 August. The slowly degradable fraction (fraction B) of Fe increased (P<0.01) across both harvest dates, that of Zn and Cu decreased (P<0.01) on the 18 August harvest date, and fraction B of Zn increased (P=0.02) with N fertilization on the 30 May harvest date. The intercepts for B and C fractions (undegradable fraction) of Zn and Cu were higher (P<0.05) on 18 August than 30 May. Iron fraction C decreased (P<0.01) and the Fe degradation rate increased linearly (P<0.01) with N fertilization across both harvest dates, while fraction C of Zn and Cu decreased (P<0.01) on the 18 August harvest date only. Effective ruminal disappearance of Fe increased (P<0.01) quadratically on 30 May, while that of Zn and Cu increased linearly (P<0.01) with N fertilization rate on 18 August. The intercept for effective Fe disappearance was higher on 18 August than 30 May, while the Zn and Cu intercepts were greater (P<0.05) on 30 May than 18 August. Therefore, increasing N fertilization rate had little impact on forage Fe, Zn, Cu, or Mo concentrations but did affect partitioning of Fe, Zn, and Cu concentrations into soluble, degradable, and undegradable fractions.