have exhibited accumulation of nitrite to a very high peak value, followed by a fa11 ; with a correspondingly rapid utilisation of nitrate, Such behaviour is unsuitable for the cyclic operations of bacon curing, where the brine is usually fairly stable a t a moderate nitrite concentration. It was hoped that this investigation might throw some light on the reasons for this important difference between the two types of brine. I t is at least probable from the experiments that such differences are not simply a function of salt content since the same general pattern of behaviour was repeated at different salt concentrations and temperatures. Nevertheless, it is possible on the basis of the results obtained in these experiments to put forward a hypothesis to explain the effect of small changes in salt concentration on the nitrite content of bacon brines. While there remains any nitrate unreduced, bacterial destruction of nitrite is negligible-hence disappearance of nitrite is very largely due to absorption by the sides of bacon. The nitrite content of the brine is therefore the resultant of that produced from nitrate less that absorbed and there is a salt concentration at which these two processes are balanced. This concentration will depend on temperature and on the size and nature of the bacterial population. If the salt content falls, more nitrite will be produced than can be absorbed and the nitrite concentration will tend to rise : if the salt content rises, less nitrite will be produced than can be absorbed and the nitrite concentration will tend to fall. This hypothesis is only tentative and develops in one direction the suggestions made by Ingram et aL2 Factory experiments on a pilot scale would be needed to test its validity. Low Temperature Research Station CambridgeKeccivrd 31 >lay, 1960

A rumen‐fistulated steer was used for the manual collection of samples of freshly swallowed herbage, in a grazing‐management experiment on a perennial ryegrass sward. Diurnal and seasonal changes in the in vitro digestibility of the herbage selected by the grazing animal were studied under both strip‐ and continuous‐grazing methods of management. There was no appreciable change in digestibility as the sward was grazed down from upper to lower layers under strip‐grazing management in April and May. In June to October a within‐day fall in digestibility was found, much of which was attributable to an increase in the amount of old dead herbage grazed from the lower regions of the sward. Dead herbage taken in by the grazing steer was considerably lower in digestibility in August than in May. The in vitro digestibility of herbage samples, cut to ground level before and after grazing in a strip‐grazed treatment, fell markedly as the proportion of dead herbage in the sample increased, giving a high negative correlation. In a continuous‐grazing management there was no pattern of diurnal variation, and the seasonal variation in digestibility of the ingested herbage was less than in strip grazing. The implications of these results are discussed in relation to indirect methods of digestibility determination (faecal‐index technique), the measurement of herbage intake, and to some aspects of grazing management.

In vitro digestion of grass samples has shown that the amount of digestible cellulose, hexosan or pentosan is not greatly affected by species or by stage of maturity. A survey of published data on crude fibre and cellulose digestibility has confirmed these findings. Marked differences between grasses and legumes were observed. The rate of digestion has also been shown to be independent of the total amount of structural material present. The relevance of these observations to the selection of herbage varieties with greater intake by ruminants is discussed. INTRODUCTIONRuminant animals are capable of digesting structural carbohydrates, the main constituents of the fibrous cell walls of forage plants. However, the amount of a forage which a ruminant is able to consume appears to be limited in part by the fibre content of the feed, the amount of this fibre which can be digested, and the rate of its digestion (e.g. 3).This paper describes experiments conducted in vitro which were designed to throw light on the relationship between these three factors. EXPERIMENTALThe in vitro digestion procedures have been described already (16,17). Hexosan and pentosan contents were measured on forages, and on the residues from in vitro digestion, by the colorimetric procedures of Deriaz (6); when hexosan only was measured on the residues from digestion, the preliminary stages of extraction with an alcohol/benzene mixture and the first treatment with A' sulphuric acid were omitted, and the 72% sulphuric acid reagent was applied direct to the indigestible residues which had been recovered and oven-dried (1(K)°C) in 50 ml beakers.Cellulose was estimated by the method of Crampton and Maynard (5), on residues in 50 ml beakers; 25 ml portions of the acetic acid/nitric acid reagent were added, the beakers 238 were covered with a glass basin containing about 15 ml of cold water and were then refiuxed gently for 20 min. After cooling, the residues were filtered through asbestos in Gooch crucibles, dried at 100°C, weighed, ashed at 600°C and reweighed. Cellulose content was measured as the loss of weight on ignition. For grasses, the cellulose, hexosan and crude-fibre contents appear to be similar. RESULTSFigure 1 (i) shows data obtained with a wide range of cocksfoot samples (Dactylis ghmerata, cv. S37). As the grass matured, hexosan content rose from 21% to 37% of the herbage DM and the in vitro hexosan digestibility fell from 80% to 45%. However, the content of digestible hexosan (16% to 22%) showed no clear trends, similar values being found for the least and the most mature samples. Corresponding data for perennial ryegrass samples (Lolium perenne, cv. S24) are shown in Fig. 1 (ii). For the range of samples examined, hexosan content was rather less variable at 23 to 29%, hexosan digestibility fell from 85 to 54%, and the content of digestible hexosan varied between 16 and 20%, the lowest values tending to occur in the more mature samples.The rate of digestion of two samples of S37 cocksfoot and two of S24 perennial ryegrass were measured...

The results of estimations of in vitro digestibility of three standard herbage samples, analysed weekly over about 48 weeks per year, have been examined using time-series methods of Box and Jenkins to separate seasonal and non-seasonal trends. The study includes data from March 1971 to February 1981. The dominant feature of variability is the seasonal component with a cycle of 52 weeks; no other significant cycles of longer periods were detected. The model of Box and Jenkins (1976) was found to be the most appropriate in describing the three series. A decrease in the in vitro digestibility ofthe low standard, which is 10 years old, occurred over the last year or so, although the reasons for this apparent storage effect are unknown.introduction