forage (table 3). The resulting error in forage 
intake increases with an increase in true mois- 
ture content of forage. When the moisture con- 
tent exceeds 65 percent, the magnitude of error 
(associated with a high probability of occur- 
rence) becomes unbearably large. This is a lim- 
iting factor because we cannot attain infinite 
precision and accuracy in sampling this char- 
acteristic. 
TABLE 3.—Percentage of error in forage in- 
take estimates resulting from an 
error of +2 percent in the estimate 
of mean moisture content of forage 
True moisture Mean air temperature (°F.), 
content of percentage of error in forage 
forage, intake estimates 
percentage 
40 50 60 70 80 
20 1.4 nh 0.8 0.5 0.6 
30 1.9 ar 1.2 1.0 11 
40 2.3 2:2 1.9 1.8 1.1 
50 4.3 3.6 3.0 2.5 2.0 
55 6.3 5.3 4.3 3.5 2.9 
60 9.0 7.9 6.1 4.6 4.0 
65 16.4 13.3 9.6 7.2 5.2 
70 44.4 30.5 18.8 12.3 8.6 
Sources of Bias 
Factors affecting vapor output.—Increasing 
temperatures increase the requirement for 
water in the control of body temperature. 
Thus, ambient temperature was evaluated by 
Winchester and Morris (1956) as the primary 
factor in the water-intake rates of cattle. They 
reviewed literature showing that the effect of 
humidity was negligible below 75 F. Wind, 
also, is most likely to become important only at 
high temperatures, when respiratory and cuta- 
neous excretion of water is great (Brody et al. 
1954). The literature apparently does not in- 
clude data on the effects of radiant heat inde- 
pendent of air temperature. However, it has 
been shown that shades of insulating materials 
are beneficial to cattle in hot weather (Ittner 
et al. 1954). Several environmental factors can 
affect the water-intake rate, especially when 
temperatures exceed 90 F. The high variability 
in water-intake rates at high temperatures 
probably is due to the effects of variations in 
wind, humidity, and radiant heat as well as to 
a magnification of inherent animal variability. 
Although these environmental factors seem 
unlikely causes of bias in estimating forage in- 
take under our field conditions, consideration 
of them leads to the assumption that: if one 
could determine the amounts of water excreted 
through the skin and lungs, and express the 
water to dry matter relation in terms of the 
water requirement for urine and feces, then 
the usefulness of the water-intake method 
could be improved and extended to a wider 
range of conditions. 
Factors affecting liquid output.—According 
to the literature reviewed by Winchester and 
Morris (1956), the intake of large amounts of 
nitrogen and salt increases the water require- 
ment. Water intake by sheep increased 3.4 li- 
ters per 100 grams of increase in salt intake 
(Wilson 1966a). Forage containing high con- 
centrations of salt increased the water intake 
and decreased the dry matter intake (Wilson 
1966b). Most of the salt (89 to 98 percent) is 
excreted in urine (Riggs et al. 1953; Wilson 
1966b). Excess waste products from nitrogen 
metabolism, as well as salt excess, must in- 
crease water output in urine, and thereby in- 
crease the water-intake rate. With crude-pro- 
tein contents of forage varying from about 18 
percent in the spring to about 5 percent in the 
fall, we must expect some seasonal pattern of 
bias in estimating forage intake. Generally, 
high crude-protein contents of forage are asso- 
ciated with high moisture contents, which we 
have suspected had a positive bias in the esti- 
mate of forage intake. In all probability, we 
need an adjustment based on the nitrogen con- 
tent of forage. 
Variations in the water content of feces also 
must be considered. Russian thistle appears to 
have a laxative effect, which might result 
from its high oxalate content (Cave et al. 
1936; Christensen et al. 1948). We found an 
increase in the moisture content of feces of 
as much as 4 percent when yearling steers 
were eating a large proportion of Russian this- 
tle. This factor alone increased water output in 
feces about 1 gallon per day, and surely must 
have caused a positive bias of 3 to 5 pounds in 
the estimate of forage intake. Consequently, 
we anticipate the need for an adjustment based 
on the water content of feces. 
Synopsis.—Our greatest difficulties are asso- 
ciated with high moisture contents of forage. 
Inherent limitations, inadequate sampling pre- 
cision, and potential sources of bias have been 
encountered simultaneously with high mois- 
ture contents. Adjustment procedures that will 
permit an extension of usefulness for the wa- 
ter-intake method surely can be developed 
without losing the advantages of low cost, ease 
of application, and lack of requirements for 
restricting animals and land area. Meanwhile, 
opportunities to improve the efficiency of graz- 
ing practices on semiarid grasslands are most 
likely to coincide with low moisture contents of 
forage (Hyder 1967). Needless to say, we like 
the water-intake method, even though we must 
express a very strong desire to improve it and 
extend its usefulness (for estimating forage in- 
take and quality, pasture productivity, and 
herbage growth rates as discussed by Be- 
ment) 2 to a wider range of pasturing condi- 
tions. 
2 Bement, R. E. 1968. Herbage growth rate and for- 
age quality on shortgrass range. Ph.D. Thesis on file at 
Colo. State Univ., Fort Collins, Colo., June 1968, 53 pp. 
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