Fifteen tests carried out at an average interval of 4 weeks for 14 months showed an annual rhythm in the proportion of spayed crossbred ewes which exhibited oestrus after an injection of 15.6 µg oestradiol benzoate (ODB) following a 12 day period of progesterone treatment. Reactivity was highest in late summer and early autumn and lowest in winter and early spring. The periods of minimum reactivity coincided with the periods of low environmental temperatures and low body weights. The significance of the annual rhythm is discussed with particular relation to the phenomenon of "silent oestrus". No similar rhythm was detected in the 7-aginal response to oestrogen.

Digestible organic matter (D.O.M.) intakes of sheep grazing various pastures were measured by faecal index methods in three 4- to 6-week experiments. No correlation could be found between the weight of pasture available (over the range 150–2900 Ib dry weight per acre of green herbage) and the D.O.M. intake of sheep. During two experiments the sheep were shorn. Shortly after, voluntary D.O.M. intakes rose significantly by 42–62%. Concurrently the hay intake of other sheep yarded nearby rose by 20–51% following shearing. Cold stress is probably responsible for these increases.

Small groups of Merino wethers were maintained at virtually constant liveweight in four "environments" and their maintenance feed requirements estimated from the relationship between their fluctuations in liveweight over 14-day periods and their feed intakes, measured directly or estimated by chromium oxide dilution and faecal nitrogen concentration. When adult wethers of equal size but a varying degree of fatness were fed on fresh pasture herbage in metabolism pens, liveweights of 26, 32, and 46 kg could be maintained on a daily intake of approximately 200, 300, and 420 g digestible organic matter (D.O.M.) respectively. When the metabolism pens were placed outdoors in winter the maintenance requirements rose, the rise appearing to be greatest for the thin sheep. When wethers were allowed to graze for about 1½ , 2½, and 4 hr in abundant pasture, liveweights of 26, 33, and 46 kg were maintained on intakes of approximately 420, 480, and 490 g D.O.M. respectively. When the same wethers were allowed to graze freely and weight was controlled by the scarcity or abundance of the pasturage, the same liveweights were maintained on intakes of about 750, 780, and 560 g D.O.M. respectively. The rise in maintenance requirements on short pasture seems too great to be explained by the increased energy cost of locomotion and grazing. The possibility of an endocrine stimulation of metabolic rate is pointed out.

Twenty-four sheep, 12 fitted with rumen cannulae, were divided into three similar groups and kept at pasture. The grazing intake of group A was not restricted, but groups B and C were undernourished for a period of about 4 months and were then maintained in thin condition for a further 9 months, when all sheep were killed. The mean fat content was then 27, 9, and 5% of mean body weight in groups A, B, and C respectively. Undernourished sheep had a significantly higher (P < 0.01) water content in the fat-free empty body than well-nourished sheep. Multiple regression equations relating the fat, protein, and estimated energy contents of the body to tritiated water space and body weight differed between well-nourished and undernourished sheep. This was due to the larger amount of water in the rumell-reticulum of thin sheep and to the high water content of their fat-free body. __________________ *Part II, Aust. J. Agric. Res., 23: 499 (1972)

Statistical analysis of the results of digestion trials on a wide range of fresh pasture herbages shows that their digestibility might be estimated as the intake factor or feed faeces ratio (Y) from the equation: YO.M. = (2.04 – 0.24XN ± 0.186X2N) ± 0.53 where YO.M. is the intake factor for organic matter, and XN is the percentage of nitrogen in faecal organic matter. The results were subdivided arbitrarily into "summer" (September–April) and "winter" (May–August) periods, and these proved to yield significantly different linear equations. The summer regression yielded higher intake factors (corresponding to 2–3% higher digestibility) for a given value of faecal nitrogen percentage. This subdivision reduced the standard deviation from regression only slightly, to about 0.50, which amounts to ± 17% for pasture of 75% digestibility. These equations give considerably lower values of digestibility for a given nitrogen concentration than regressions hitherto published. The present pooled equation, based on short leafy herbage, probably gives sounder estimates for grazing sheep than do the existing equations derived from trials with more mature herbages. When sheep with a wide range in body weight were all fed a maintenance ration, it was found that feed digestibility was not detectably reduced at high levels of feeding. The undoubtedly higher feed intake of grazing than of pen-fed animals, due in large measure to their higher maintenance requirements, therefore may not cause the reduction in digestive efficiency, and thus the bias in estimates of feed intake, that has been supposed. On the basis of the pooled regression, which is felt to be preferable to a subjectively selected "seasonal" equation, estimates of the intake of digestible organic matter (D.O.M.) by sheep in metabolism pens fed on fresh pasture herbage averaged 97 ± 22% of the true figures, or ± 80 g D.O.M.