The 2008 Joint Meeting of the Society for Range Management and the America Forage and Grassland Council. |
Wednesday, January 30, 2008 - 10:00 AM
Mineral Content of Cattle Diets under Two Grazing Systems
Merilynn Hirsch, Department of Agronomy and Horticulture, University of Nebraska, 357 Keim Hall, Lincoln, NE 68503
Mineral content of diets of cattle grazing rangeland in 2005 and 2006 was compared between short duration (SDG) and deferred rotation (DR) grazing systems. The study was conducted at the University of Nebraska-Lincoln Barta Brothers Ranch in the eastern Sandhills. Two replications of an 8-pasture SDG system and a 4-pasture DR system were grazed by cow-calf pairs at about 55 AUD/ha from May 15 to October 15. The SDG pastures were grazed in 3 cycles through the grazing season and the DR pastures were grazed only once. Diet samples were collected by esophageally-fistulated cows in both systems throughout the 2005 and 2006 grazing seasons. Ashing and colorimetry sampling methods were used to estimate phosphorus (P), copper (Cu), and zinc (Zn) content of the diets. Mineral content of diets did not differ (P>0.1) between the 2 grazing treatments. Content of P and Cu in diets was greater (P<0.1) in 2006 than in 2005. Zinc content of diets did not differ (P>0.1) between years. Phosphorus content of diets declined from 0.25% in late May to around 0.14% in late July when it leveled off for the remainder of the season in both years. Copper content of diets remained between 4 and 7 ppm throughout the grazing season; whereas Zn content tended to decline through the first half of the growing season before returning to levels similar to the early part of the growing season (20 ppm) in both years. The differences in timing and frequency of grazing in the 2 grazing systems did not appear to affect mineral content of diets. Mineral content of diets during the growing season generally was above lactating cow requirements. Supplementation of diets with P, Cu, or Zn in the Nebraska Sandhills during the growing season is not justified based on the results of this study.