1. A primary growth crop of perennial ryegrass (Lolium perenneL., cv. S23) was partially wilted and ensiled after the application of eitherformic acid–water (1:3, w/v; 7.1 1/t fresh herbage; control diet C) or formic acid–formaldehyde (1:1, w/w; 8·81/t fresh herbage; formaldehyde treated diet F) which supplied 50gHCHO/kg crude protein (nitrogen (N) × 6·25). The two silages were fed separately and a third diet comprising formaldehyde-treated silage, supplemented with urea (20 g/kg dry matter dm) at the time of feeding was also examined (dietFU).

2. The three diets were fed at a level of 16 g dm/kg live weight to six 3- to six-month-old cattle fitted with rumen and re-entrant duodenal cannulas in two 3 × 3 LatinSquare experiments, and measurements were made of the digestion of energy, carbohydrateand N.

3. The formaldehyde-treated silage had a lower content of fermentation acids and ammoma-N and a higher content of water-soluble carbohydrate and total amino acids. The apparent digestibility of organic matter, energy and N were depressed (P < 0·05, P < 0·05 and P < 0·01 respectively) by treatment with formaldehyde, but cellulose and neutral-detergent fibre digestibility were unaffected.

4. Within the rumen the digestion of organic matter, cellulose and neutral-detergent fibre were unaffected by formaldehyde treatment or supplementation with urea. Microbial protein synthesis in the rumen was similar for the three diets (average 131 g/kg apparently digested organic matter in the rumen).

5. The application of formic acid-formaldehyde increased (P < 0·05) the amount of food protein escaping degradation in the rumen (4·76 diet C, 6·89 diet F; 7·07 diet FU g/kg protein intake). The contribution of amino acidsof dietary origin al the duodenum increased (P < 0·05) from 50 (diet C) to 80 (diet F) and 82 (diet FU) g/kg DM intake, and the flow of total amino acids at the duodenum was 33% higher (P < 0·001) in cattle fed formic acid–form aldehyde silage diets compared withthe control silage due to the reduction in degradation of protein at ensiling and in therumen.

1. The aim of the investigation was to study the effects of lysine, methionine, crude protein, a complete balanced diet, maize starch and potato starch infused continuously into the caecum on the digeslibilly of the nutrients, urinary nitrogen excretion and N balance in the presence of a normal or a depressed (modified) microflora.

2. The effects of the infused nutrients on digestibility differed when diets supplemented with antibiotics were compared with unsupplemented diets.

3. Infused lysine improved the digestibility of crude protein (0·004/g lysine), crude fibre (0·01/g lysine), gross energy (0·003/g lysine) and lysine (0·03g lysine) when the diets were not supplemented with antibiotics.

Infused lysine, methionine and crude protein seemed to be of very little value for protein synthesis in the pigs, as the effects on N balance, although positive, were very small.

5. Despite the fact that a part of the infused lysine passed through the caecum-colon and into the faeces intact, the effect on N balance was negligible, indicating that the ability to absorb amino acids through the gut wall in the caecum-colon was very poor.

6. Crude protein, a complete balanced diet, maize starch and potato starch infused into the caecum were digested almost as well as if they had been given orally, thus demonstrating the large digestive capacity of the caecum-colon.