Improving Dairy Farm Sustainability II: Environmental Losses and Nutrient Flows

This paper continues the analysis of nutrient management on a case study dairy farm in New York State. In Part I, it was found that 60 to 70% of the imported N and P were not accounted for in the exported milk, crops, and animals. The purpose of this paper is to present a process for accounting for the fate of the excess nutrients and to determine the extent to which they were contributing to air and water pollution from the farm. Environmental losses of N and net excess of P in different subsections of the farm were estimated. Losses of N from volatilization on the barn floor and in storage were estimated to be 16% of excreted N. As a partial check on these results, manure nutrient composition for lactating cows was analyzed at excretion, entering storage, and leaving storage. Soil leaching losses from the farm were calculated using the LEACHN model, and were 9% of total N inflows to the farm. Predicted nitrate N concentrations in the leachate were 10.6 ppm. Results from monitoring a stream originating from the farm gave an annual average of 14.4 ppm of nitrate N. About 80% of the total N inflows were accounted for as milk sold (25%), animals sold (2%), leaching losses (9%), and volatilization/denitrification losses (46%). Environmental losses accounted for 75% of the excess N. Projected scenarios for increased use of farm-produced forages, reduction in fertilizers, and increased feed conversion to milk resulted in only minor improvements in the nutrient imbalance on this farm. Research QuestionAssessment of the nutrient imports and exports for the case study dairy farm in Part I of this study indicated that 72% of the N and 57% of the P were not accounted for in the export of animal and plant products. In this paper we present a process to account for the fate of excess nutrients and to determine the extent to which they are contributing to the pollution of air and water. Literature SummaryMass nutrient balances on other dairy farms indicate a similar excess in imported nutrients. Nutrient flows on a Pennsylvania dairy farm showed N and P excesses of 50 to 60%, while excess N for a model farm was approximately 55%. Reports of excess N and P were 64 to 79% for four New York farms ranging in size from 45 to 1300 cows. Environmental losses of nutrients from dairy farms are difficult to measure, and estimates of N loss from manure vary from 0 to 75%. Mathematical models have been developed to predict volatilization losses of N from manure. The LEACHN model was developed to predict water flow in soils, volatilization and denitrification losses of N, and leaching of nitrate to the groundwater. Study DescriptionFlows of N and P into and out of the barns, manure storage, fields, and the whole farm were determined for the case study farm. The goal was to measure as many flows as possible and to calculate only those flows that could not be measured. Inflows of purchased feeds, fertilizers, and N fixation, and outflows of milk and animals were taken from Part I of this study. Volatilization losses from manure were estimated for lactating cows from the change in manure N concentrations at excretion, entering storage, and leaving storage. For all cow groups, losses of N from manure were calculated using a model of urea conversion and N volatilization. The LEACHN model was used to simulate the movement of water and the losses of N from the soil as volatilization/denitrification and leaching to the groundwater. As a partial check on these calculations, concentrations of nitrate N, total P, and sediment were measured in a stream that originated in the center of the farm. To determine the extent to which all nutrient sources and losses were accounted for, mass nutrient balances were performed on subsections of the farm comprising the barns, manure storage, and fields. Applied QuestionsWhat was the fate of excess N and P on the case study farm? About 80% of the N inflows to the farm were accounted for as milk sold (25%), animals sold (2%), leaching losses (9%), and volatilization/denitrification losses (46%). Volatilization losses of N from manure on the barn floor and in storage were 16% of excreted N. Atmospheric losses of N through volatilization and denitrification from the fields amounted to 27% of the fertilizer and manure N applied to the fields. About 70% of the leaching occurred on 25% of the soils, and was associated with well drained areas. Over 65% of the excess P coming onto the farm was associated with an excess of P applied to the fields in fertilizer and manure. What are the implications for water and air quality? Average nitrate N concentration in leachate entering the groundwater was predicted to be 10.6 ppm, which is close to current water quality standards. The stream monitored on the farm had an annual average of 14.4 ppm. Average P concentration in the stream was 0.4 ppm, which is much higher than the value of 0.03 ppm reported for uncontaminated streams. Soil test results confirmed high P levels in the fields closest to the barns. Atmospheric losses of N accounted for 65% of the excess N on the farm. It would appear that excess nutrients were directly related to water and air pollution on this farm. Can excess nutrients be reduced by increasing nutrient efficiencies? Increasing the use of farm-produced feeds in the ration from 50 to 75% of N intake was projected to result in a slight reduction of N excess, from 72 to 68%. Eliminating the use of N fertilizers, assuming no loss in crop yield, was projected to result in an excess of N of 65%. Increasing the animal conversion of feed N to milk N from 19 to 25% still gave a nutrient excess of 64%. What are the implications for dairy farm sustainability? The farm in this study was highly profitable and productive, had a sufficient land base with acceptable animal density, and a progressive management toward environmental protection. Thus, the significant environmental impacts from this farm are cause for wider concern about the sustainability of dairy farming as currently practiced. Projected effects of increasing nutrient efficiencies on this farm caused slight improvements in the whole-farm nutrient balance for N. What changes would improve the situation? Major improvements in nutrient balances will probably be achieved not by a single change, but by a combination of changes in a number of areas. These include increased use of precise ration balancing systems, collection of better quality information on feeds and animals, optimum use of forages and other farm-produced feeds, excellent silage preservation management, conservation of nutrients in manure prior to land application, crop nutrient management planning to minimize fertilizer use, and the best possible crop, harvest, and pest management.