Agriculture Reference
In-Depth Information
production for organic herds was 4.6 kg lower compared to non-organic
herds. Therefore, they suggested that the lower milk production found in
the organic herds resulted in reduced Ca depletion from milking compared
to non-organic herds [42].
Nitrate toxicity (methaemoglobinaemia) in ruminants grazing forages
grown under stressful conditions or on soils that have received high appli-
cations of either inorganic (synthetic) or organic (manure) fertilizers and
can be an issue in both organic and non-organic grazing systems. Plants
take up nitrogen from the soil largely in the form of nitrate (NO
3
−
), which
under normal growing conditions is rapidly converted to nitrite (NO
2
−
),
then to ammonia (NH
3
) and fi nally to plant proteins. However, when plant
growth is slowed by stressful environmental conditions (frost, drought,
shade), nitrogen in the form of NO
3
−
accumulates in the plant more rapidly
than it can be converted to protein. Nitrate itself is not toxic; however, mi-
crobial action on NO
3
−
in the rumen results in the conversion of hemoglo-
bin to methemoglobin, which greatly reduces the oxygen carrying capac-
ity of the blood [39]. Nitrate levels remain stable after forage is harvested
and cured for hay, but potential for accumulation is variable across forage
species, with annual cereals (sorghum-sudan, sudangrass, oats, etc.) and
some weeds (pigweed, lambsquarter, smartweed, etc.) documented as ni-
trate accumulators [38].
Forage micronutrient content is affected by soil content and plant up-
take. The availability of the micronutrients iron, zinc, cobalt, manganese
and copper is greater in acidic soils, while the availability of molybdenum
and selenium is higher in alkaline soils. Iron is primarily used by rumi-
nants to enable hemoglobin to carry oxygen in the blood for respiration;
a defi ciency can lead to anemia. Manganese, molybdenum, selenium and
copper are needed for enzyme activity, and cobalt is a component of vi-
tamin B-12. Zinc is required for enzyme activity, but is also needed to
stabilize RNA and DNA, and for membrane function [27]. Defi ciencies
of manganese are uncommon, but the primary symptom is lameness. De-
fi ciency symptoms of zinc and copper are not specifi c, but include poor
growth in young stock and increased susceptibility to disease. Copper de-
fi ciencies are relatively common, and reduced copper absorption can be
caused by high dietary molybdenum, sulfur, or zinc. Cobalt defi ciencies
result in loss of appetite, and are common either where soils are naturally
