131Cs PASSAGE FROM PRECIPITATION TO MILK
709
The age of cows, the stage of lactation, or the stage of pregnancy
did not appear to have significant effects on the percentage of BIOs
secreted
into
milk. On the other hand, the type of feed did have a
significant effect. This effect was noted when comparisons were made
between rations containing large amounts and those containing small
amounts of grain with proportionate decreases or increases in the
amount of hay. The transfer coefficient was always lower for those
animals fed small amounts of grain and large amounts of hay. One
explanation for the difference is that *"Cs is bound by fibrous residues
in the feed as suggested by the experiments of Mraz and Patrick.°
They found increased '°"Cs excretion by rats fedfibrous feeds. Compari-
sons were made between the crude-fiber content of the diet and the
transfer coefficient. These comparisons showed that the coefficient
decreased rather consistently as the fiber contentof the ration increased.
In the Colorado State University study, thirty-three observations
were available from individual Holstein cows whose diet included from
about 10 to 40% crude fiber. The regression coefficient calculated from
these data was
Y = 0.785 — 0.11 x crude-fiber intake (kg/day)
where
Y
is
the transfer coefficient (ics secreted per liter of milk
daily as a percentage of the total daily intake of '8’Cs in the feed).
Our results indicate that it is very important to consider the
crude-fiber content of the diet if comparisons are to be made between
herds which vary in their intake of this constituent.
There are, of course, numerous differences between high-grain
and high-hay rations besides the difference in crude-fiber content.
One difference is that hay contains four to five times the concentration
of potassium found in grain. An increased potassium intake in high-hay
diets, according to the Observed Ratio concept, would decrease the
amount of '8’Cs found in milk. However, in one feeding trial rice hulls
were added to the ration. Thus this feed had a fiber content similar
to that of hay but a potassium content similar to that of grain. The
transfer coefficient was 0.0030 as compared to 0.0063 for a diet where
mixed grain replaced rice hulls. The experiences of other investigators
have indicated only a minor influence on '°"Cs retention by the addition
of extra potassium to the diet. Additional experiments are contemplated
for the study of the quantitative relation between the fiber content of
the ration and the levels of '"Cs found in milk and in urine as well as
the effect of supplemental potassium.
The mean transfer coefficient of 0.0038 (Table 3) is similar to the
average amount secreted in 66 hr reported by Cragle® but lower than
the 0.01 suggested by Garner.®