Last year, I visited a large, progressive dairy in Vermont with the “Agriculture and the Environment” class taught by Steve Kramer. Our host was very gracious and gave us a guided tour of the farm where we learned a lot about their operation and nutrient management.
At one point I asked about manure application, which elicited not only an answer, but a manure management “jab” at New York to boot.
It went something like “… and by the way, when are you New Yorkers going to stop spreading manure in the winter?” Being an agronomist, the can of worms was immediately opened wide.
I didn’t have much time to respond, but I did say that the existing water quality numbers didn’t necessarily support the hypothesis in that question.
When it comes to P loading to the lake, New York contributes less (nearly fourfold less according to a study) nonpoint source P (which is presumed to be mostly from agriculture) than Vermont; much of this is due to Vermont’s larger drainage area, but the fact remains.
So, how about P loading from streams? According to a recent USGS study, rivers of major agricultural watersheds in Vermont had greater average annual P concentrations and loads during 1990-2008 compared to rivers in New York.
The take-home message is that drawing conclusions about water quality at a large scale is very complex, and it’s difficult to attribute water quality outcomes to specific agricultural practices.
In reality, the potential for nutrient loss from manure application depends on many factors including application rate, incorporation method/timing, soil type/drainage, slope and, of course, rainfall/weather. Many CAFO-sized farms don’t spread much, if any, manure in the winter.
If manure is applied in the winter, fields must be carefully selected to minimize the potential for surface runoff. While not spreading in the winter may result in the loss of fewer nutrients in some situations, its impact on P loading in the basin isn’t apparent.
Landscape complexity and rainfall are two of the biggest factors that drive runoff generation and P transport. Agricultural practices do impact P loading from the landscape; however, their impact is often masked by other factors, particularly at the watershed scale.
Rainfall variation alone explains much of the year-to-year variation in P loading among watersheds. In addition, not all nonpoint P in the basin is from agriculture. For example, streambank erosion can contribute significant amounts of sediment and P, although its contribution to the lake’s total P load is poorly quantified.
It’s also worth mentioning that the methods used to allocate P loads by land use in the basin are not flawless: Many assumptions go into these methods and they are based on simple export coefficient models, which drastically oversimplify runoff processes and P transport.
The bottom line is that the dairy industry in New York and Vermont has been very proactive in addressing nutrient management and water quality concerns. CAFO-size farms in the basin are required to meet strict nutrient management guidelines for barnyards and on every field they manage.
CAFO farms and farms that receive public money for best management practices must implement a nutrient management plan and every field must be evaluated for P runoff risk with the P site index to determine how much manure can safely be applied.
Of course there’s room for improvement with respect to nutrient management on both sides, but let’s give credit where credit is due and not finger-point. The dairy industry in the basin has stepped up to the water quality plate and will continue to do so in the face of challenging regulations and uncertain economic times. PD
References omitted due to space but are available upon request to email@example.com.
—Excerpts from William H. Miner Agricultural Research Institute Farm Report, December 2010