Four experiments were carried out on individually penned Finn Dorset and Suffolk × Finn Dorset ewes receiving complete diets of milled hay and concentrates and rationed at various levels below voluntary intake. In the first experiment, 20 non-pregnant non-lactating ewes were each given daily 1·6 kg food and injected intramuscularly with either isotonic saline (control), progesterone plus oestrogen (20 mg + 80 ng per 24 h) to mimic hormone status in pregnancy, bromocriptine (1 mg per 12 h) to suppress plasma prolactin, or thyrotrophin releasing factor (TRF) to elevate plasma concentrations of prolactin and the iodothyronines. Fractional outflow rates of chromium-mordanted fish meal from the rumen were estimated from the rate of excretion of chromium in faeces and were proportionately 0·25 higher for ewes given TRF than for controls (P < 0·025). None of the other treatments had a significant effect on outflow rate.
In experiment 2, fractional outflow rates from the rumen were determined during July (long daylength) and again during December (short daylength) in 12 ewes that had their seasonal pattern of plasma prolactin modified by removal of their anterior cervical sympathetic ganglia. Another 12 sham-operated ewes were used as controls. Neither the seasonal changes in prolactin nor their modification by cervical ganglionectomy significantly affected outflow rate.
Twelve non-pregnant non-lactating ewes were used in the third experiment to test the effects of mid-winter shearing (mean ambient temperature −4°C) on fractional outflow rates from the rumen and on the plasma concentrations of prolactin, thyroxine (T4) and cortisol. Shearing caused a drop in plasma prolactin, gave proportional increases (P < 0·05) of 0·47 and 0·16 in T4 and cortisol respectively and gave a 0·12 increase (P < 0·01) in the outflow rate of chromium-mordanted fish meal from the rumen. The increases in T4, and outflow rate were sustained for at least 2 weeks.
For experiment 4, 22 ewes each suckling two lambs were used in three trials to see if variation between ewes in milk production and in lamb growth rate was correlated with variation in outflow rate from the rumen. The only significant correlation (r = 0·68, P < 0·01) was between outflow rate and lamb growth rate for the eight ewes in trial 1. The results of all four experiments are discussed in relation to the mechanisms by which non-dietary factors, via their effect on the rate of outflow of digesta from the rumen, may alter the quantity of protein reaching the abomasum and the likely significance of this alteration on animal performance.