Agriculture Reference
In-Depth Information
excrete mineral nutrients and nitrogen, altering the effect of dietary min-
eral content. Whitehead [27] notes fi ve homeostatic control mechanisms
that help to maintain ruminant nutrient uptake within a healthy range:
1. Variation in the extent of absorption from the digestive tract.
2. Variation in the extent of excretion via the feces.
3. Variation in the extent of excretion via the urine.
4. Deposition of an excess in the tissues in harmless and/or reserve
forms.
5.
Variation in the extent of secretion into milk.
For dairy cows that excrete signifi cant quantities of some mineral nu-
trients in milk, dietary concentrations may be critical. While there is a met-
abolically active reserve of calcium and magnesium in bone, the reserve
amounts are small relative to intake or excretion. With such a small buffer,
a dietary defi ciency can rapidly have serious negative or even fatal effects
for a cow early in lactation. Therefore the maintenance of a healthy soil
with good soil structure to encourage extensive root systems, good water-
holding capacity to buffer drought or suboptimal irrigation, and a capacity
to provide and retain good soil fertility is fundamental to the sustainability
of grazing-based ruminant livestock systems, particularly in pastures regu-
larly used by dairy cows that excrete high levels of mineral nutrients and
nitrogen on a daily basis.
4.3.1 FORAGE PLANT NUTRITION IN PASTURES
Plant roots, like soils, have characteristic cation exchange capacities
(CEC), influenced by the concentration of carboxylic acids in root cell
walls that provide negative charges, especially if the pH of the soil solu-
tion is a unit or two below neutrality (pH 5-6) [27]. In general, legume
roots have a higher CEC than grasses, and therefore can compete more
successfully for divalent cations, such as Ca
2+
or Mg
2+
, in the soil solu-
tion; among legumes, white clover accumulates more calcium than other
legume species [27]. The balance of cations in the soil can affect their
uptake, because much of the uptake of soil nutrients by roots is passive—
