270 THE INDIAN JOURNAL OF VETERINARY SCIENCE AND ANIMAL HUSBANDRY [ IV, III

highly successful. Such a system of control is based on the nature of the life cycle
of the parasite, which has been worked out in detail by Veglia (1916). There does
not appear to be any complete account of the life cycle of O. circumcincta, but most
workers are of the opinion that it follows the same course as H. contortus in which
the succession of events is as follows :—

The eggs laid by the mature females in the abomasum reach the ground in the
fæces and the larvæ hatch out in about 24 hours. These feed on certain bacteria
in the fæces and must pass the next two stages of their development on the ground
before they are capable of infecting the sheep. The time taken to reach the infec-
tive stage may vary from 3 days to several weeks, depending on the temperature
and degree of moisture present.

It can be readily understood that the time taken for the newly hatched larvæ
to reach the infective stage is the all important factor in regard to the introduction
of any system of control by progressive sectional grazing of the pastures. In this
connection Taylor (1929) states for both H. contortus and O. circumcincta that,
' although in this country development would rarely be delayed because of excessive
dryness, the temperature is usually below that best suited to the larvæ and it can
only be in our warmest weather that they reach the infective stage in less than 10
days'.

Assuming that the larvæ of O. circumcincta require a period of ten days to be-
come infective it follows that any system of progressive sectional grazing whereby
the lambs are shifted on to clean pasture every 10 days should give complete con-
trol. Before advising control measures along these lines, it was decided to carry
out a preliminary experiment with a view to gaining information on the following
points:—

        (1)  The extent to which stomach worm infestation of lambs can be reduced
                    by a system of progressive sectional grazing.

        (2)  Whether 10 days is a safe maximum period to allow between successive
                    shifts in reducing infestation from O. circumcincta.

        (3)  The degree to which lambs can become infected when put to graze with
                    their infested dams on clean pasture.

The experiment was undertaken jointly under the north of Scotland College of
Agriculture and the Rowett Research Institute, the latter providing all facilities for
the investigation.

                                   PLAN OF THE EXPERIMENT.

A three acre field of excellent third year's grass which had never carried sheep
was fenced into equal portions. Both portions were on a gradual slope and compar-
able in every respect as regards grazing and drainage.