Amending soil in organic dairy pastures

by Tamara Scully
Getting optimal milk production from cows in an organic pasture-based dairy system requires excellent soil health. Amending poor quality soils will result in economic rewards in terms of added milk production. Dr. Cindy Daley, of California State University, recently shared her experience with amending the dairy’s very poor soils as they transitioned to an organic system. The university’s dairy farm has 90 milking cows, primarily with New Zealand genetics, and 60 acres of irrigated pasture, along with 30 acres of alfalfa and 10 acres in winter forage.
The soil matrix is composed of the physical, chemical and biological soil components. Soil is about 45 percent mineral particles, five percent organic matter — which includes living bacteria, viruses, insects, worms and more, plus dead (plant residues) and very dead (well-decomposed, humus layers) — and 50 percent air space and water, Daley said.
Physical soil traits impact oxygen and water movement. Chemical traits include the ph and available nutrients and the biological traits include nutrient cycling, biodiversity, and pest and disease resistance. A balanced mineral profile and optimal pH, plus a healthy soil matrix, need to be achieved in order to support a healthy soil biology.
“We want a really rich biology,” Daley said. “So, biodiversity beneath the soil is really an important component that we really need to strive for in our soils.”
In order to know what the soil’s characteristic are, “you really have to measure,” Daley said. Soil nutrient testing, soil organic matter (SOM) assessments, soil penetrometer, pH, and CEC (Cation Exchange Capacity) are important tests, which were done on the university’s dairy pastures before beginning the remediation program.
The CEC “is a measure of its (the soil) ability to hold and release nutrients,” Daley explained. More time is needed to amend soils with high CEC than with a low CEC. Clay soils have higher CECs.
Researching Soil Amending
Fifty acres of pasture were divided into 10, five-acre paddocks. The blocks were randomly selected to be amended or not. All of the paddocks were intensively grazed in a rotational system with the cows on pasture for more than 12 hours/day during the March-October grazing season. Each paddock had a rest period of 18-21 days in the spring, and 30-35 days in the summer.
Soil samples were taken in April, June and August for three years, from all paddocks. A composite sample from each paddock was sent to the same lab throughout the experiment. The unamended soil on the farm had a very poor mineral profile, as well as micronutrient issue, particularly boron and sulfur. The calcium/magnesium balance was off, with excessive magnesium and low calcium levels, resulting in decreased plant growth. Because calcium is a facilitator for all nutrients, the non-amended soil was not able to provide the nutrients needed for optimal forage health. The soil was extremely compacted, so aeration was not able to occur properly. The non-amended soils were used as controls, and only the natural inputs from the grazing cows were added to the system.
During the three year period, the amended soils averaged 14,000 pounds more forage production than the non-amended. The ADF (Acid Detergent Fiber) of the amended soils was less than that of the non-amended, which is significant because the overall digestibilty of the forage decreases as the ADF increases. In addition, as the NDF (Neutral Detergent Fiber) increases the amount the DMI (Dry Matter Intake) decreases.
“There’s just not enough room in the rumen,” Daley said, adding that dry matter intake is crucial. “The more DMI, the higher the milk production.”
The net energy available in a forage for milk production and body maintenance was greater in amended soils than in the non-amended. In vitro tests of true digestibility confirmed that the amended soils had increased levels of fiber digestibility.
“It is important just how digestible the fiber component is, because there is unrealized energy and nutrients there,” Daley said. Measures of the RFQ (Relative Forage Quality) of the amended soil were at 189.85, while the non-amended were much lower, with an average RFQ of 158.59.
“By amending our paddocks, we increased the overall quality of that feed.”
Pasture Economics
The cost of amending the soil was spread over a three year period. The amended paddocks were treated with a spring and a fall application of gypsum each year, as well as lime calcium. Boron, zinc sulfate, maganese sulfate, and compost were also applied over the three year period, based on the results of soil testing. The total cost of amendments was under $300/year for each of the three years.
The amended soils not only yielded increased forage production, but increased quality of forage. Daley measured the milk pounds per DM, which shows the additional milk values of the amended forages based on available digestibility and energy content. The amended soils produced 3,298 milk pounds with this measure, while the non-amended produced 2,967 milk pounds.
The economic impact can also be measured in increased milk production per cow. Based on 50 percent pasture DMI, and a 200 day grazing season, plus a dry matter demand of 40 pounds per day, per cow, Daley calculated that a cow on the amended forage will produce 663 more pounds of milk per year than on the non-amended soils. At $30/cwt cost of milk, each cow generates an additional $199 of milk each grazing season.
Daley also calculated that the amended pastures, totaling 25 acres, saved $6,336 in replacement feed costs by generating more forage. The higher quality forage would also mean less of a need for supplemental grain, which would mean additional savings as well, and increase net profits per cow.
Improved soil organic matter would mean better water-retaining abilities, and the soils would most likely require less irrigation, also reducing costs. Improved SOM would also impact nutrient retention, meaning higher quality forage could be produced as SOM improves, likely increasing forage yields and nutritional value, again bringing economic value to the farm.
“Balanced soils do make better forages,” Daley said. “Amending these soils, trying to balance that profile…really does improve the overall forage quality.”
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