Mineral Content of Invaded Grassland Forages Is Affected By Precipitation and Management

Abstract Mineral content of grazed forage influences overall health, growth and reproduction of cattle and promotes maximum cattle productivity. However, grassland mineral content may not always meet cattle requirements throughout the grazing season and can be affected by precipitation of the season. Most of the Northern Great Plains grassland is invaded by exotic perennial grass species that have a uniform growth habit and response to precipitation. Growth habitat can be controlled by grazing or mowing, which may alter mineral content. No studies have examined the combined effects of mowing and drought on forage mineral concentration of invaded grassland. Therefore, the objective of this study was to determine mineral concentration of forage in rangeland pastures in seasons of low compared with normal precipitation and mowing status and model recommendations for mitigating low mineral content. This study was conducted at the Northern Great Plains Research Laboratory in Mandan, North Dakota USA. Pastures were sampled across several years and pasture locations, with enclosed plots mowed or not mowed in April, to simulate early season mobgrazing or clipping. Quarter- meter quadrats were taken every two weeks of the grazing seasons from plots in each pasture, one year dry and the other normal. Samples were analyzed for macrominerals and trace minerals via ICP-aa spectroscopy. Forage mineral content was analyzed as a completely randomized design using the MIXED procedure of SAS (SAS 9.4, Cary, NC). Total ash content was different by year (P = 0.0053) but not affected by mowing (P = 0.21). Potassium and phosphorus were not different due to mowing, nor effected by precipitation. Calcium was affected by mowing (P = 0.0065) and tended be affected by precipitation (P = 0.11). Zinc, copper, molybdenum, manganese and iron were all found to be significantly affected by precipitation (P < 0.001). When applied to a beef nutrient requirement model, Ca and P were generally adequate to meet recommended amounts for lactating cows based on a May-calving 1,200-pound cow most months of the season but did not meet needs of a 1400 lb. cow and calf. Forage phosphorus content was significantly less in June of each year, but adequate the rest of the season, suggesting targeting phosphorus supplementation for only the least months would save on mineral costs. Understanding forage mineral fluctuations across the grazing season may provide producers with tools to make the best supplementation strategies and use the most appropriate mineral formulations to maximize performance and reduce input cost.