When it comes to monitoring manure components and calculating on-farm manure use, there’s no room for guessing. Dr. Quirine Ketterings and graduate student Agustin Olivo of Cornell University have conducted extensive research to develop assessment tools for managing nitrogen and phosphorus on both a whole farm and field basis.
“New York is the fifth-ranked milk production state,” said Ketterings. “We have a lot of dairy farms and a lot of cropland that supports the dairy industry here. Quite a few dairy farms are under the Concentrated Animal Feeding Operation (CAFO) header and are spread throughout the state.”
Ketterings explained that New York has abundant water, which makes it important to address what is done on the landscape to ensure clean water stays clean. New York is surrounded by, and is part of, major watersheds including the Chesapeake Bay Watershed. One measure to keep water clean is the CAFO designation.
“CAFO regulations came to New York in 1999 and 2000,” said Ketterings. “They were focused on trying to address water quality challenges. There was a big focus on phosphorus, and it was mostly planning-based. Farms had to put plans together with application rates that did not exceed the nitrogen needs of particular crops.”
Once CAFO regulations were in place, limits for phosphorus (P) were then regulated by the P index (PI). “All of this was plant-based,” she said. “How would we do evaluations and outcome-based assessments to see if we were reaching our targets? That’s when we started working with two evaluation tools: the Whole Farm Nutrient Mass Balance and Field-Based Nutrient Balances.”
The concept for Whole-Farm Nutrient Mass Balance (NMB) involves drawing a boundary around a farm and looking at nutrients coming across the farm boundary – specifically nitrogen, phosphorus and potassium.
“Primary [nutrient] drivers are feed and fertilizer – anything imported onto the farm in terms of N, P and K,” said Ketterings. “Dairy farms’ major export is milk, but there are farms that export crops and manure.”
She explained that an NMB involves subtracting the nutrients exported from the farm from those imported onto the farm. The difference between the two is the balance.
“We can express the balance in two ways,” she said. “One is based on per tillable acre of cropland basis, and the other is on a per hundredweight of milk basis.”
The balance per tillable acre provides information on whether the nutrients present can be recycled on the land base. The balance per cwt. provides a sense of how efficiently nutrients are being returned to the product (milk).
Using on-farm data for about 100 New York dairy farms, Ketterings set feasible balances for N, P and K. “A farm that needs nutrient balance per acre for nitrogen between 0 and 105 is meeting the feasible balance,” she said. “For phosphorus, it’s between 0 and 12; and for potassium, between 0 and 37. Once we have those ranges, we can report back to farms and they can start monitoring their progress over time. We can also do that in terms of balance per cwt. and look at efficiency of production.”
She added that it’s important to consider all feasible balances to hit target goals.
Participating farms can compare their progress to prior years and compare their farm to all other farms in the database. Because of the number of participating farms, an “opportunity table” was created. This is a list of specific farm key performance indicators (KPIs) associated with a high risk of exceeding feasible balances.
Managing manure is a key aspect of keeping nutrient surpluses at feasible levels. But managing manure nutrients is challenging because of the fixed ratio between nitrogen and phosphorus. Photo by Courtney Llewellyn
“What we’ve learned so far from the database is that feed and fertilizer are by far the biggest drivers for balances,” said Ketterings. “Animal density is also a key driver.”
She noted that it’s more difficult for farms with higher animal density to achieve desired goals.
As of 2022, 21% of the milk produced in New York has undergone the Whole Farm NMB assessment, and the database is growing. Phosphorus levels on New York farms are good for both high- and low-density farms.
For N, lower density farms are leading both feasible balances per acre and per cwt. but struggling to meet the feasible balance per acre when animal density exceeds one. Ketterings said this highlights the importance of being aware of animal density.
Agustin Olivo, a Ph.D. candidate at the time of research, explained another tool for managing nutrient balance. “For this project, we collected nitrogen and phosphorus balance information for 994 field year observations across five years on eight New York dairy farms,” he said. “For each, we estimated both the nitrogen balances and phosphorus balances.”
Olivo explained that he estimated N by subtracting N uptake from N supply. Nitrogen supply was from fertilizer N, organic and inorganic N in manure, previous manure applications, rotation credits from previous crops and endogenous soil N.
“We compared that to the uptake, which was mostly driven by yield of individual fields, to obtain a weighted balance,” said Olivo. “Phosphorus was simpler – for the supply side, we considered P supplied by fertilizer or manure applications and compared it to P uptake, which was mostly driven by yield.”
In general, like the Whole Farm NMB, nutrient input is driving the outcome. “Manure is one of our largest drivers of balances,” Olivo said. “Managing manure is a key aspect of keeping surpluses at feasible levels. But managing manure nutrients is challenging because of the fixed ratio between nitrogen and phosphorus.”
In averaging data from the eight farms, the total N:P ratio was higher than crop uptake, indicating that manure could supply all of a crop’s N requirements. However, not all of the N is plant-available. This is the reason N-based applications led to overapplication of P.
“The overapplication of P with N-based manure application ranged from 82% to 290%,” Olivo said. “However, not all farms needed to conduct N-based applications because they had land base to spread manure.”
Farms with higher animal density or higher manure P showed a larger nutrient supply from manure, resulting in the overapplication of P.
Olivo said farms with lower animal density may have an opportunity to mix manure with fertilizer to maintain lower balances. Farms with higher animal density can consider exporting manure to avoid overapplication of P.
“Whole farm mass balances give good overall farm-level pictures,” he said. “Field-based balances can help assess how we’re doing at the field level and see where farms may need to reallocate nutrients.”
Whole farm assessment tools are valuable for farms that produce manure. “Whole farm assessment tools can help identify opportunities for improvements across farm components – cows, crops, storage,” said Olivo. “We need assessment tools to evaluate outcomes, document progress and share achievements with stakeholders. It has become important to put a number to environmental performance and share that.”
by Sally Colby
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