Agriculture Reference
In-Depth Information
by applying it to the soil. However, drylot dairies and feedlots import nu-
trients and must compost manure and/or export it to surrounding cropland.
While cropping-based (e.g., hay, silage, and cereal grains) dairy feeding
operations often import nutrients to the farm and distribute the resulting
manure nutrients on nearby cropland two or more times per year, pasture-
based livestock feeding returns nutrients to the pasture during grazing [26].
Pastured ruminants are an important conduit for the return of the nutri-
ents that are not exported from the farm as beef or dairy products to pas-
ture soils. Grazing-based ruminant production allows most of the nutrients
from a pasture to be immediately recycled onto the same pasture, reducing
the loss of volatile nutrients (e.g., ammonia, methane) from manure stor-
age and compost piles and lagoons, and reducing the time and fuel re-
quired to mechanically spread manure solids onto cropland. However, the
distribution of manure on pastures is uneven at best. The nitrogen in urine
patches is suffi ciently concentrated that nitrate may leach from pastures
into groundwater, forage that is soiled by manure will be rejected for a
period of time, and unincorporated manure solids are vulnerable to runoff.
Of the macronutrients, calcium, phosphorus and magnesium are largely
excreted in the feces, while potassium is largely secreted in urine. Sulfur
secretion is variable, but more nitrogen is excreted in urine than in feces,
although this is infl uenced by diet. The form of nitrogen found in urine is
readily converted to a form that can be volatilized by an enzyme present in
feces, while nitrogen is excreted in feces as organic matter, which is min-
eralized much more slowly by soil microbial activity [27]. The manage-
ment of cattle on pastures can infl uence the distribution of manure nutri-
ents; rotational stocking or other types of management-intensive grazing
(MiG) such as strip grazing result in a more even distribution of nutrients
than continuous grazing [28]. Under rotational stocking, reasonably well-
distributed manure nutrients can contribute to a fl ush of soil microbial
activity in pastures, particularly if pastures are irrigated following grazing,
and nutrients can be extracted and utilized by growing forage plants dur-
ing the rest period between grazing events. The grazing pressure exerted
by higher stocking densities or co-grazing with other species can reduce
rejection of forages adjacent to urine or dung patches.
Cederberg and Mattsson [29] modeled Swedish non-organic and or-
ganic dairy nutrient inputs and outputs, and determined that while there
