Exploring seasonal variations in the mineral profile of rotationally grazed tropical pastures on commercial beef and dairy farms in Jamaica

Abstract

This study explored seasonal variation in the mineral profile of rotationally grazed pastures supporting beef and dairy cattle on commercial farms in Jamaica. While beef and dairy cattle are primarily reared on pasture in Jamaica with minimal supplementary feeding, the mineral profile of Jamaican pastures is largely unknown. Grass samples were collected by hand plucking from three (3) dairy and two (2) beef representative farms during the dry (January - March), intermediate (May - July) and wet seasons (September - November) following a stratified random sampling scheme. Laboratory analyses were conducted to determine concentrations of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), Zinc (Zn), iron (Fe) and manganese (Mn). Nitrogen concentration was significantly impacted by season (p < 0.05) and was highest in the wet season (18.4 – 27.6 g/kg DM) and lowest in the dry season (10 - 25.2 g/kg DM), as expected, since soil moisture directly influences the availability and uptake of N by grasses. Generally, P concentration ranged from 2.63 - 4.26 g/kg DM and was significantly affected by season on all farms, with the exception of one dairy farm (P=0.034). The high level of P found in the present study could be as a result of P accumulation in the soil because of the immobile nature of P making it resistant to leaching. The range obtained indicated that P levels were adequate to satisfy the nutritional requirements of both beef and dairy cattle. The potassium concentration ranged from 4.16 - 12.3 g/kg DM and was influenced by season only in areas when the dry season was severe combined with locations where soil water retention capacity was low. Calcium ranged from 2.05 - 4.19 g/kg DM and was generally highest in the dry season and lowest in the wet season, suggesting the accumulation of Ca under drier conditions in response to drought and heat tolerance mechanisms in grasses. Only on the dairy farms did season significantly affect Mg concentration. Season had no influence on the concentrations of Na, Zn, Fe and Mn. Calcium/phosphorus ratio was significantly affected by season on all farms (p < 0.05) and was higher in the dry season. It was concluded that the macro-mineral profile of Jamaican pastures supporting beef and dairy cattle is generally superior in the wet season while micro-minerals seem to be less sensitive to seasonal variations. The concentrations of Ca, Na and Cu in these pastures were below the dietary requirement for lactating dairy cattle and would therefore require supplementation.