Agriculture Reference
In-Depth Information
ions move with water in the soil solution, and move into roots as part of
the transpiration stream. Uptake of nutrients that are passively acquired
from the soil solution may be higher in mid-summer, when evapotranspi-
ration is high. In contrast, nitrate, phosphate and sulfate are all negatively
charged, and therefore are repelled rather than attracted to the negative
charges on soil particles and roots.
An excess of potassium (K
+
) in the soil solution, which is more likely
in heavily manured soils, can result in ―luxury uptakeǁ by forages in ex-
cess of plant requirements, and lead to an imbalance of high potassium
and therefore a relative defi ciency of calcium and magnesium [37]. High
K
+
concentrations in forages can result in metabolic issues in grazing ru-
minants due more to antagonisms with other elements such as calcium
(Ca) and magnesium (Mg), than to simple defi ciencies. Grass tetany (hy-
pomagnesemia) is characterized by low blood Mg and usually occurs in
beef cows and ewes grazing lush grass pastures during periods of cool
and cloudy weather [38-40]. Grass tetany tends to occur when the dietary
intake of total Mg is not particularly low, but instead when factors exist
that increase the animal's Mg requirement (early lactation) or reduce the
availability of Mg to the animal. Potassium inhibits the animals' ability
to absorb Mg resulting in a relative defi ciency, and therefore the ratio of
milliequivalents of K to (Ca+Mg) in forage has been used to predict the
tetany hazard of forages [39]. Furthermore, milk fever (hypocalcemia) is
characterized by low levels of blood calcium and most commonly occurs
in high producing dairy cows 12 to 24 h after calving when the sudden de-
mand for Ca required by the onset of lactation tests the Ca homeostatic ca-
pabilities of the animal [41]. Excessive K concentrations in the prepartum
diet of high-producing dairy cows decreases Ca resorption from the bone
resulting in an imbalance in Ca homeostasis and a possible defi ciency re-
gardless of dietary concentrations of Ca [41]. Soder and Stout [37] noted
that the high K concentrations observed in orchardgrass pastures fertilized
with a dairy manure slurry could predispose lactating dairy cows to milk
fever. Hardeng and Edge [42] examined the incidence of disease between
31 organic and 93 non-organic Norwegian dairy herds and unexpectedly
found no differences between the production systems for cases of milk
fever. The authors noted that for each kg increase in peak milk production,
the risk of milk fever increased by 5%; however, the mean maximum milk
