No abstract

Three comparisons were made, with non-lactating fistulated cows, of the voluntary intake of silage and hay prepared from similar herbage. On average 28% more dry matter was eaten as hay than as silage. Silage and hay had similar digestibilities, but silage residues tended to remain in the gut longer than those of hay. The amount of digesta in the reticulo-rumen immediately after a meal ad lib. was greater with hay than with silage. The cows spent longer eating and ruminating per kg dry matter of silage than of hay. The results are discussed in relation to possible factors determining the voluntary intake of silage.

It has often been supposed that the amount of roughage cows will voluntarily consume at any one meal is largely determined by the effect of that meal in filling the reticulo-rumen and giving rise to a feeling of satiety (cf. Kruger & Miiller, 1955; Makela, 1956). Earlier experiments (Campling & Balch, 1961) provided direct evidence of a relationship between the amount of the contents of the reticulo-rumen and the voluntary intake of hay. There have, however, been few measurements of the amount of digesta in the reticulo-rumen of cattle receiving roughages ad lib.It is clearly important to know whether, in individual cows, the voluntary intake of roughages offered ad lib. ceases when the amount of digesta in the reticulo-rumen reaches some critical level. We interpreted our earlier experiments as evidence for the existence of such a critical level. We have now attempted, therefore, to find whether this level is determined by the capacity of the reticulo-rumen or whether it is fixed in relation to the amount of digesta in the reticule-rumen at some time of the day other than during a meal. In the latter event it would be expected that the critical level would differ with different roughages. We have also considered factors influencing the rate of breakdown of roughages in the reticulo-rumen and the time of retention of residues in the reticulo-rumen, since they influence the amount of the reticulo-ruminal contents during a meal and determine the decrease between meals. Blaxter, Graham & Wainman (1956) and Crampton (1957) stressed the probable importance of the rate of disappearance of digesta from the reticulo-rumen in determining the voluntary intake of roughages.

STTMMARY. Seven changeover experiments were conducted to examine the effect of giving restricted amounts of concentrates on the voluntary intake of different roughages by non-lactating dairy cows. The addition of up to 6 kg concentrates daily to the diet of cows receiving roughage ad lib. caused little change in the intake of hay and a small increase in the intake of barley straw. Larger amounts of concentrates of 6 and 8 kg daily reduced hay intake by between 0-2 and 0-4 kg dry matter/kg concentrate dry matter given. When concentrates were given to the cows the rate of decline in intake of hay tended to be greatest with the hays of highest digestibility. The daily addition of up to 8 kg concentrates to the diet of cows offered silage ad lib. caused only small depressions in the intake of silage. The results are discussed in relation to recent work on this subject with cattle and sheep.

SUMMARY1. In the first experiment, the digestibility of diets containing ratios of 1:1 and 1:4 hay to concentrates was determined at five levels of feeding in castrated male sheep. With diet 1.1, increasing the daily dry matter intake from 600 to 1400 g resulted in a linear decline in organic matter digestibility from 74·4 to 68·6%. The same increase in level of feeding led to a curvilinear decline in the organic matter digestibility of diet 1.4 from 83·0 to 75·9%.2. In a similar way the crude fibre digestibility of the diets declined as food intake increased. Also the increase in level of feeding caused a decrease in the mean retention time of stained hay in the alimentary tract.3. In a second experiment the digestibility of long dried grass offered ad libitum to eight dry and eight lactating cows was measured and compared with the digestibility of similar dried grass offered at maintenance level and ad libitum to eight castrated male sheep.4. On a metabolic live-weight basis (kg W0·73), the lactating cows ate 34% more organic matter than the dry cows and the mean voluntary intake of sheep was only 54% of the intake of dry cows.5. The organic matter digestibility for the dry cows was 1·9 units higher than that of the lactating cows and at ad libitum intake the organic matter digestibility for the sheep was 5·6 and 3·7 units lower than that of the dry and lactating cows respectively. The results are discussed in relation to the validity of extrapolating results obtained with sheep to cattle, and with non·lactating to lactating animals.