Articles 91-100

3. The use of n-alkanes to estimate diet composition, intake and digestibility in sheep fed mixed diets

O. Valiente, A. de Vega, J. A. Guada, C. Castrillo

Departamento de Producción Animal y Ciencia de los Alimentos, Universidad de Zaragoza, Spain

The nutritive value of a forage will depend on its intake (understood as both daily amount and composition of the diet, in terms of proportions of the different botanical fractions actually
consumed by the animals) and digestibility, the estimation of these parameters becoming difficult in grazing conditions. The use of the n-alkanes as internal markers for this purpose has given good results with mixtures of two temperate forages (Dove and Mayes, 1996), but has not yet been tested with other species or mixed diets.
The aim of the present experiment was to compare observed values of intake, digestibility and diet composition obtained in sheep fed different proportions of barley grain and straw to their n-alkane estimates. Sixteen adult sheep were randomly assigned to four diets with different proportions of barley grain and straw (60/40, 45/55, 30/70 y 15/85) and fed at a level which avoided refusals. Intake, digestibility and diet composition (proportion of grains, leaves and stems consumed) were recorded for seven days.
The contribution of leaves and stems to total straw intake was estimated from the comparison of their respective n-alkane profiles. From 14 days before until the end of the balance period the animals were dosed once a day (9 h) with 1.5 g of paper pellet containing equal proportions of tetracosane (C24) and dotriacontane (C32), used as external markers to provide for faecal recovery, and hexatriacontane (C36) used for faecal production calculations. N-alkanes were extracted from samples of grains, leaves, stems and faeces, and their concentrations analysed by means of gas chromatography. Diet composition was estimated according to the procedures proposed by Mayes et al. (1994), using C25 (pentacosane), C28 (octacosane) and C30 (triacontane) as internal markers, and dry matter intake following Mayes et al. (1986). Digestibility was calculated from estimated intake and faecal production. Observed and estimated proportions of grain in the diet were compared by regression, and differences within each treatment assessed by the paired t-test procedure. This last method was also used to compare observed and estimated intake and digestibility values.
Estimates of proportions of grain in the diet were very accurate (estimated = 1.0063 observed – 2.4720; r2 = 0.99; P=0.0001), and mean differences between measured and estimated parameters ranged from –6.9 to 2.7% for dry matter intake, and from –4.6 to 2.1% for dry matter digestibility, depending on treatment (P<0.05 for diets 60/40 and 30/70). There was not a consistent relationship between the magnitude of the deviations and the proportion of grain in the diet.
Due to the scarce differences between observed and estimated values, it is concluded that the n-alkane technique is a powerful tool to estimate intake, digestibility and diet composition of mixed diets and hence may be used in sheep grazing mature whole cereal crops.

References:
Dove H. and Mayes R. W., 1996. J Nutr 126: 13-26.
Mayes R. W. et al., 1986. J Agric Sci 107: 161-170.
Mayes R. W. et al., 1994. Sci Total Env 157: 289-300.