The Problem of Measuring the Nutritive Value of Herbage

Abstract

As emphasis in modem grassland thought is placed increasingly on herbage quality (7), our inability to measure the feeding value of a sample of herbage, without recourse to a digestibility experiment, becomes more evident. This has been emphasized by Orampton (5); Mitchell (22) has suggested that the most promising approach to this problem is in the use of equations which relate the digestibility of a feed (in this case herbage) to its chemical composition.Watson (36) in 1936 suggested that for a limited range of swards the digestibility (from which they estimated the Starch Equivalent) of the herbage dry-matter could be estimated from its crude protein content. Similar regressions on crude protein have been suggested by Boyd (4) and Forbes (10). The correlation of digestibility with crude fibre has been proposed by Axellson (3) and Hallsworth (15) and with lignin by Forbes (11, 12). More general relations have been suggested by Mitchell (22) using protein and by Lancaster (20) using crude fibre, cellulose and lignin. Schneider (28) has shown that the use of such regressions can give a prediction error variance 25-40 per cent lower than when ' average ' digestion coefficients are used.The present paper considers the methods by which such relationships are established, and the likely limitations of the present approach to the problem, in view of the errors that are possible in these methods. In developing a suitable equation, the digestibilities and chemical compositions of a wide range of herbage samples must be measured. The validity of such an equation will depend on the reliability of these data, and we must thus consider how they are obtained, and how accurate they are likely to be.

Studies in Herbage Digestibility

“…The effect of advancing maturity of grasses on the feeding value of herbage has been shown by several workers, e.g. Jari and Helleday (II) and Raymond (17). At the time when the grasses are rapidly declining in digestibility white clover remains high and thus has an important function in maintaining the overall digestibility of the herbage at this time (Fig.…”

“…Many endeavours have been made to relate the nutritive value and digestibility of herbage to some fraction within the herbage. Recently Raymond (17) has questioned the value of the herbage nitrogen content as an indicator of herbage digestibility, since his data have illustrated a large spread in coefficients of digestibility even at similar levels of nitrogen in the herbage. Data obtained from experiments at Nottingham also throw doubt on nitrogen or crude-protein contents as indicators of nutritive value.…”

Studies on the Digestibility of Herbage

“…More recent work has placed particular emphasis on the dc\'clopment of indirect laboratory methods of assessing tbe quantity and feeding value of herbage grazed. [Garrigus (11), Woodman (39), Sears (28), Forbes (8), Raymond (24), Lancaster (19), Reid (26), Kane (18) ].…”

Studies in the Digestibility of Herbage

“…The productions of faecal dry matter by grazing sheep and by caged sheep feeding at a HIGH level of intake on similar cut herbage were found to be as 100 to 73, 82 and 95 on three occasions. If we assume the same digestibility of herbage by the two groups (this cannot be strictly true because of the effects of level of intake and selective grazing (8)), this represents levels of herbage intake of 5 : 4, i.e. similar to the ratio of HIGH to LOW levels in the present experiments.…”