NITROGEN AND MINERAL-COMPOSITION OF AUTUMN-GRAZED PASTURE

Grazing management in autumn can influence the botanical composition a nd productivity of a sward. Cycling of nutrients as a result of grazin g livestock activity and variable canopy growth rates may influence mi neral nutrient supply and demand in a dynamic canopy. An experiment wa s conducted to determine the influence of autumn grazing practices on the growth and composition, including minerals in terms of ruminant re quirements, of a grass/legume sward. Paddocks were established and thr ee replicates grazed by growing lambs for 30-, 60-, or 90-d intervals beginning in late summer. Herbage samples were collected at the beginn ing of the grazing interval and at the end of each interval (closing d ate). Herbage mass, and nitrogen (N), phosphorus (P), potassium (K), m agnesium (Mg), calcium (Ca), and sulfur (S), as well as copper (Cu) an d zinc (Zn) were examined in terms of the influence of sampling date, closing date, year, and the interaction of these factors from stockpil ed and grazed canopies. Soil mineral composition was determined as wel l. Concentrations of all minerals declined with increasing soil depth and P, Na, Mg, and Ca increased in soil over the course of the experim ent. Soil N concentration was reflected in the pattern of herbage grow th in autumn. In general, closing date had no influence on herbage min eral composition and concentrations were within the recommended levels for a range of livestock. Phosphorus was the exception and concentrat ions in herbage were low in terms of requirements for high producing l ivestock such as lactating dairy cattle. Uptake or mineral reallocatio n within the plant remained constant during the autumn growth interval , since mineral yields were stable as growth rates declined in 1991 an d increased when growth rates were stable in 1992. Mineral related nut ritional problems in grazed mixed-species pasture, would most likely b e a function of mineral bioavailability or interactions, rather than l ow concentrations in the herbage.