Whole-Farm Nutrient Balance
A "Whole Farm Nutrient Balance" evaluation is a tool that can be used to evaluate the potential for generation of excess nutrients on the farm and can form the basis for developing plans to deal with nutrient buildups. Nutrients are transported along multiple pathways and in a variety of forms in a livestock operation.
A picture of the flow of nutrients is presented in the above figure. Within the farm boundaries, nutrients are ideally "recycled" between the livestock and crop components. Manure nutrients are used for crop production and feed crop nutrients are in turn recycled as animal feed. Nutrients can enter a livestock operation as purchased products (fertilizer, animal feed, and purchased animals), nitrogen (N) fixed by legume crops, and nitrates in rain and irrigation water.
Most nutrients exit a livestock operation in the form of animals and crops sold and possibly other products moved off farm (e.g., manure sold or given to a neighboring crop producer). Ideally nutrients that exit the farm as losses to the environment (nitrates in groundwater, ammonia volatilized into the atmosphere, and N and phosphorus into surface water) should be minimal. Some nutrients (especially phosphorus) can accumulate in large quantities in the soil if over-applied over a period of several years. Although not a direct loss to the environment, an accumulation of nutrients in the soil can result in environmental losses in the future.
An "imbalance" is the difference between the Inputs and the Managed Outputs and represents either a direct environmental loss or the buildup of nutrients in the soil. Farm operations with a significant nutrient imbalance are at a greater risk to water quality. In contrast, operations that achieve a closer balance represent more sustainable production systems.
Size is a poor indicator of nutrient imbalance. A review of the whole farm nutrient balance for 33 Nebraska swine confinements and beef feedlots did not observe any connection between size and the extent of an imbalance. Some of the worst imbalances were observed for livestock operations with less than 1,000 animal units (1000 beef animals, 700 dairy cows, or 2500 hogs).
A P balance provides a good indicator of the risk to water quality. An imbalance in N does not distinguish between the relatively benign losses (e.g., denitrification of nitrate to N2 gas) and the relatively harmful environmental losses (e.g., nitrate loss to water). In contrast, P losses impact water quality through increased soil P levels and greater concentration of P moving with surface runoff water.
Purchased animal feeds are often the most significant source of the N and P inputs on livestock operations, with most of the rest coming from commercial fertilizer. In the Nebraska study, N inputs as feed varied from 33% to 77% of total N inputs for farms with less than 250 animal units and more than 2,500 animal units, respectively. Phosphorus inputs as feed was the largest nutrient source for most livestock and poultry farms. With the growing concentration of livestock and poultry, purchased animal feed is often the most significant source of nutrients even in regions that grow most animal feeds locally. (National Curriculum, MWPS)
Evaluating nutrient balance from a whole farm perspective provides a more complete picture of the driving forces behind nutrient-related environmental issues. The following four management strategies should help reduce nutrient imbalances:
- Efficient use of nutrients in crop production can offset fertilizer nutrient inputs.
- Alternative feed rations and efficient utilization of on-farm feeds can offset nutrient inputs as purchased feeds and forages.
- Exporting of manure nutrients to off-farm users can increase managed nutrient outputs.
- Manure treatments allow disposal of manure nutrients. Some treatment options enhance the value of manure nutrients and complement manure marketing efforts.