Reserve Cattle Grazing and Residual Dry Matter (RDM) Monitoring

Summary of the RDM Report:  Residual Dry Matter (RDM) was monitored in 2013, 2014, and 2015 on the Vernal Pools and Grassland Reserve (Reserve), University of California, Merced. RDM monitoring was conducted to assess cattle grazing on the 6,561 acre (2,656 hectares) Reserve; grazing is a requirement of the Reserve Management Plan, and state and federal wetland permits. A herd of about 1,600 dairy cattle grazes the Reserve for about six months each year. Triplicate RDM samples were collected at 55 sites in 2013, 67 sites in 2014, and 71 sites in 2015. Mean overall RDM was 760 lbs./acre (853 kg/ha) in 2013; 430 lbs./acre (482 kg/ha) in 2014; and 956 lbs./acre (1,072kg/ha) 2015.  Lower RDM levels in 2013 and 2014 were attributed to low rainfall rather than to overutilization by cattle. High RDM levels in 2015, more than twice the levels of 2014, were attributed to lower grazing pressure. Mean grass height was about 2.7 in. (6.9 cm) over the three years. For comparative purposes, RDM and grass heights were monitored annually on ungrazed grassland outside the Reserve along La Paloma Road. RDM and grass heights were much higher at these ungrazed locations than in the Reserve. The primary environmental purpose for cattle grazing on the Reserve is to prevent pervasive, non-native, European grasses from growing into the thousands of rare and protected vernal pools found there. Without grazing, excessive grass growth can lead to thatch (phytomass) build up in pools; reduction of rare, endemic wetland plants via competition; reduced hydroperiod; and overall degradation of the aquatic environment. Low RDM levels and overutilization by grazing cattle, therefore, are more desirable from a vernal pool ecosystem perspective than are higher RDM levels and underutilization. RDM monitoring took place during an historic drought (2013-2015) in California in which annual precipitation in eastern Merced County was only 29% to 67% of the long term mean. Low rainfall leads to low RDM levels. RDM levels were 5% below the Management Plan target of 800 lbs./acre in 2013 and about 20% above this target in 2015. We conclude, based on our RDM monitoring, that grazing appears to comply with Management Plan guidelines, although grazing was uneven across the Reserve. Whether the current grazing operations actually result in desirable ecological conditions within the vernal pools has not yet been determined.