Agriculture Reference
In-Depth Information
metabolites, nutrients, and hormones throughout the body.
In the gastrointestinal tract, water provides moisture for
rumen fermentation and aides in excretion of waste.
Certain characteristics of water, including its high specifi c
heat, high thermal conductivity, and latent heat of vapor-
ization, assist in body temperature regulation. Water
requires heat to raise its temperature, has a high heat trans-
fer capability, and requires heat to change from liquid
phase to vapor or to solid phase. Vaporization of water is
the main route by which the body will lose heat. Other
functions include lubrication of joints and conduction of
sound through the body.
Body water is gradually displaced by body fat as animals
grow and mature. Milk production increases the water
requirement and body water content. Males tend to have
more body water than females. Turnover rate of water in
the body under normal conditions ranges from 2-8 days
but may increase with potassium and salt intake, tempera-
ture, and humidity.
An animal obtains its water from drinking, from water
in feeds (called bound water or preformed water), and from
metabolic water. Metabolic water is the water released
during oxidation of the carbohydrates, fats, and proteins.
Metabolic water of nutrients relates to their oxidation state.
Fats are the least oxidized compounds and have a high
ratio of carbon and hydrogen to oxygen. One hundred
grams of fat when metabolized will release 109 grams of
water, carbohydrates yield only 60 grams, and proteins
yield only 42 grams. Disposal of end-products of protein
metabolism (urea) requires additional water. Preformed
and metabolic water can meet the water needs of many
grazing animals.
The body loses water through four main channels: (1)
kidneys, (2) skin, (3) lungs, and (4) intestines. Kidney
water excretion through urine is under hormonal adreno-
corticotropic hormone (ACTH) control. Water loss through
skin is either insensible (radiation) or sensible (perspira-
tion) and is affected directly by solar radiation input, tem-
perature, humidity, and wind velocity. Lungs lose water
through vaporization; this loss is affected directly by
temperature and animals' activity. Water loss through the
lungs and skin, accounts for one-third of total water loss.
Intestinal water loss through feces is affected by animal
species, being lower for sheep and goats than cattle. Water
lost by any channel except lungs will also include loss of
electrolytes.
Water requirements are not fi xed. Under normal condi-
tions, voluntary water intake is proportional to feed intake.
However, high ambient temperature will increase water
consumption. An animal may drink only 3 liters of water
per kg of feed at 5-6°C versus about 15 liters at 40°C.
Season of the year also can change the water requirement.
Daily water intake generally is estimated at 5-6% of body
weight but may be 10% of body weight in extreme cases.
Dietary factors as well as environmental factors can infl u-
ence water intake. Dietary factors that increase water
intake include intake of dry matter, protein, fat, and salt.
Feeds with higher water content such as pastures and fresh
cuts may displace intake of imbibed water. However,
water intake often increases with intake of ensiled feeds
because of their higher osmolarity. The primary environ-
mental factors that affect water intake are high temperature
and high humidity; both increase water intake. Design,
accessibility, and cleanness of water also may alter water
consumption by goats.
W
ATER
D
ISTRIBUTION
Total body water (TBW) is distributed into two major
compartments: (1) extracellular fl uid (ECF) that comprises
some 31-38% of TBW, and (2) intracellular fl uid (ICF)
that comprises the remaining 62-69% of TBW. The ECF
includes blood plasma (25%) and intestinal fl uids (75%)
with volume being regulated by maintaining a constant Na
concentration. The ICF includes water plus other regula-
tory electrolytes, potassium, other inorganic ions, and dif-
ferent proteins within cells. The volume of ICF is closely
regulated by volume sensors, hormones, and water trans-
port mechanism supported by the liver, heart, and kidneys.
Fluid exchange between the two pools is important for the
survival of animals under desert conditions and the
Mediterranean environment. Water content of the fore-
stomach will comprise from 10-30% of TBW and is a
major temporary water reservoir whose volume will
change over time with meals and drinking bouts.
Withholding food and water for 20-24 hours is a routine
procedure that will minimize the variation in body weight
measurement or “ shrunk weight. ” Alternatively, body
weight measured without food and water restriction is
called “ unshrunk weight. ”
W
ATER
R
EQUIREMENTS
Requirements for water are not determined directly but
instead the requirement typically is calculated as voluntary
intake of water for animals given free choice access
to water. Water intake usually is calculated from dry
matter intake according to the following equation (NRC,
1985 ):
Total Water Intake liter day
(
) =×
−
386
099
.
Dry Matter Intake
.
