A factorial experiment is described in which thirty-six aged and thirty-six maiden spayed crossbred ewes were used in four successive tests at intervals of 15 days in a study of factors affecting the detection of oestrus. Variates introduced into each test were: (a) freedom of rams (free or tethered); (b) size of mating paddock (17 acres or ⅕ acre); (c) age of ewe (maiden or aged); and (d) dose of oestrogen (oestradiol benzoate, ODB: 10·0, 15·6, 24·4 μg.).
All ewes were primed with progesterone for 12 days prior to ODB injection and joining. The number of ewes detected in oestrus and the number and identity of rams which served each ewe were recorded.
In the four successive tests 29, 33, 36 and 30 ewes were served.
Significant main effects were: (a) freedom of rams—free 77, tethered, 51 (P < 0·001); (b) size of mating paddock—17 acres 49, ⅕ acre 79 (P < 0·001); (c) age of ewes—aged 57, maiden 71 (P < 0·05); and (d) dose of ODB—10·0 μg., 20; 15·6 μg., 41; 24·4 μg., 67 (P < 0·001).
Significant interactions were: (a) test number × age (P = 0·01), and (b) test number × dose of ODB (P < 0·05). The number of maiden ewes served increased and that of aged ewes decreased withsuccessive tests. There was a barely significant ohange in the slope of the ODB D/R line.
Ewes showed no obvious preference for tethered rams of any particular breed. There was a positive correlation between the performance of rams when free and tethered.