Studies in the Digestibility of Herbage

Abstract

INTRODUCTIONIn the previous paper (Part III) the use of techniques which assume certain constituents in forage to be indigestible was discussed. These, together with the feed : faecal ratio techniques suffer from the main disadvantage that they require the sampling of a sward for 'herbage as grazed', which is a subjective operation. They also assume 100 per cent recovery in the faeces of herbage constituents such as lignin or ehromogen, while particular care must be taken in obtaining representative samples of faeces in the field.However the main error in these techniques lies in herbage sampling, and alternative methods which do not require this sampling are being investigated.Using data from indoor digestion experiments, RAYMOND (12) suggested that the composition of faeces produced by grazing stock might be used as an indirect measure of the composition of the herbage grazed. This showed a relationship between the nitrogen contents of herbage and the resulting faeces, % N in ash-free herbage = 0-28 i 0-85 x % N in ash-free faeces [1] and suggested that information on the nutritive value of herbage might be obtained from such relationships. Using similar data, LANCASTER (8, 9) proposed a general relationship between the digestibility of the organic matter in herbage and the nitrogen content of faeces produced:% digestibility of organic matter in herbage = 100(1 \ [2] \ % N in ash-free faeces/ Based on all the data, C ^ 0-76, but more precise estimates were obtained by using C ^ 0-67 for herbage feeds below 15 per cent crude protein and C = 0-80 for feeds above 15 per cent crude protein. HOMB and BRIEREM (4), commenting on this technique, have implied that LANCASTER assumes nitrogen excretion per 100 grm. dry matter intake to be constant, whereas these authors show this to be positively correlated with per cent crude protein in the forage, the correlation depending on the type of forage. However, as noted above, LANCASTER (8) made some allowance for the nitrogen content of the forage. FORBES (2) has proposed an equation taking full account of forage composition: ", , , , ,. , "" /% protein in herbagex % herbage dry matter digested =100 -I \ x \% protein in faeces/ (^100 -42-64 X (% protein in herbage -5%) \ [3] In this however the main advantage of the faecal index method has been lost, in that an estimate of the composition of 'herbage grazed' is required.In connection with their study ofthe use of chromogen as an 'indigestible tracer', REID et al. (16) have noted that an estimate of herbage digestibility can be made without measuring, directly 69 or indirectly, the chromogeti contetit of 'herbage grazed'. They proposed a relationship between herbage digestibility and the chromogen content ofthe resulting faeces: % digestibility of herbage D.M. = O'O168 X + 8-47 log X + 32-74 [4] where X = units chromogen/grm, dry faeces. (Standard error of estimate of digestibility = 0-44%).In a discussion ofthe use of faecal chemical analysis, RAYMOND (13) suggested a study ofthe correlation between herbage digestibility...

The growth of hill lambs on herbage diets

“…There are limitations to the use of faecal techniques for the prediction of the intake and digestibility of grazed herbage. The error associated with the prediction of the intake of grazed herbage is likely to be relatively greater than that for predie- tion of OMD (Raymond et al 19546). Though the opportunity for a degree of selection was available to the animals on all grazing treatments the predicted values for the lambs and adults were in close agreement (Table 7).…”

Estimation de la digestibilité de l'herbe ingérée au pâturage à partir de l'azote fécal et de quelques autres paramètres fécaux

“…La digestibilité de l'herbe pâturée par les animaux peut également être estimée à partir de la composition chimique des fèces (teneur en azote, en chromogènes, en groupement méthoxyl, en silice, etc.). Parmi ces caractéristiques, c'est la teneur en azote fécal qui, depuis les travaux de L ANCASTER (1949) et R AYMOND , K EMP & H ARRIS (1954), a été le plus utilisée.…”

IN VITRO AND FAECAL NITROGEN TECHNIQUES FOR PREDICTING THE VOLUNTARY INTAKE OF CHLORIS GAYANA

“…Indirect methods based on faecal nitrogen have been suggested for measuring the digestibility (16), voluntary intake of DM (II) and intake of digestible organic matter (8) of pasture selected by grazing animals. When applied to a wide range of pasture species these techniques have large errors (I I, 13) which severely limit their usefulness.…”