Using n-alkanes to estimate herbage intake and diet composition of cattle fed with natural forages in Madagascar

Accurate estimates of feed intake are important in order to develop accurate ration formulations, for selecting livestock according to production efficiency, and for reducing environmental impact of livestock. The present study assessed the accuracy of n-alkanes to estimate individual herbage intake and diet botanical composition of cattle fed natural forages, typical to Madagascar. The effect of two different forage allowances and seasons on the herbage intake was also tested. Eight Norwegian red pie sires (232 +/- 20 kg BW) were orally dosed twice daily with paper pellets containing 456.61 +/- 21.64 mg of C32 alkane as external marker. Animals were housed in individual pens and fed with mixtures of five forages species typically used by farmers in the rainy season (Aristida multicaulis, Hyparrhenia rufa, Imperata cylindrical, Urochloa brizantha and Stylosanthes guyanensis) and the dry season (Chrysopogon serrulatus, Cynodon dactylon, Imperata cylindrica, Lolium multiflorum and Leersia hexandra Sw). The sires were randomly assigned to two different forage allowances: (i) ad libitum with a refusal of 5 % and (ii) 1.1 % DM of body weight (BW). The n-alkane pairs C31:C32 and C32:C33 and the ratio C32/Acid Insoluble Ash (AIA) in plants and faeces were used to estimate the herbage intake. The botanical composition of the diet was estimated using the n-alkane profile from C27 to C35 in individual plant species and faeces, by least squares optimization. The n-alkane pairs underestimated (P < 0.0001) the herbage intake by 25 % in the dry season, for both forage allowances and 26 % for animals receiving ad libitum diet, regardless of the season. In contrast, the intake estimates based on both n-alkane pairs did not differ from the measured intake for animals receiving low forage allowance during the rainy season. The C32/AIA underestimated the actual herbage intake, by 50 %, for animal consuming ad libitum diet, for both seasons (P < 0.0001). The n-alkane faecal recovery (AFR) corrections factors set affected (P < 0.01) the estimated proportions of each plant species, that comprised the diet in both seasons. The application of appropriate AFR permitted to have a better accuracy of diet botanical composition estimates. It is concluded that plant wax n-alkanes are advantageous for estimating both diet botanical composition and herbage intake in cattle. However, for improving the prediction, it is important to measure the actual AFR before the calculation.