Influence of growth of kikuyu (Pennisetum clandestinum) and farmers pratices on dairy and beef production in La Réunion island

In La Réunion island, kikuyu (Pennisetum clandestinum) has been introduced in the years 70ies from East Africa. It forms nowadays highly productive pastures in high-altitude swards, from 600 to 1 400 m, settled down mainly for cattle milk and meat production, where it is managed under rotational grazing. Cattle producers have to cope with a higher productivity, and a subsequent lower nutritive value, of kikuyu during the hot season from mid- November to mid-May. The present work studies the interaction between growth of the kikuyu, farmers¿ practices and animal production on kikuyu pastures. In an farm located at 880 m of altitude, 24 samples of kikuyu from 3 to 10 weeks of age of regrowth were collected on pastures, from 3 squares of 1 m-, in both hot season (HS) and cold season (CS). Dry matter (DM, 48 h-60°C) percentage and productivity, and in vitro organic matter digestibility (IVOMD, Aufrère and Demarquilly, 1989) were determined, and compared with DM percentage and productivity of 785 samples collected in the fields (302 and 483, respectively in HS and CS). Experimental results pointed out a higher productivity during the hot season (4.42 t DM / ha, vs 2.71 in CS, at 10 weeks of regrowth), where IVOMD decreases faster than during the cold season. On the investigated private farms, UAFP and Cirad recommend to enter a paddock when kikuyu is about 25 cm high, and to leave it when its height is equal to 10-12 cm, with a rotational grazing rhythm depending on the season: many paddocks have to be untapped during the hot season, and rates of nitrogen fertilisation must depend on the season. This policy results in an equal productivity of the grass in the grazing paddocks between seasons (2.35 and 2.46 t / ha, respectively in HS and CS, in despite of an higher DM percentage in HS 26.0 % vs. 18.8 % in CS). Although kikuyu pastures yields can be high, stocking rate is not too intensive (less than 1.5 head / ha). In La Réunion island, the high protein content of the complementary feed leads to nitrogen concentrations far in excess of animal requirements. In beef farms, calves are complemented from 4 months old all the year with 1 kg / d of a commercial feed, whereas cows receive a complementary feed only during the cold season. Daily weight gains from the birth to the age of 4 months are close to 700 g / d during the hot season, showing that kikuyu grass as only dietary resource meets the requirements of suckling cows. This can be explained by the ability of the animal to select herbage according to its protein concentration (Dugmore et al., 1991). (Texte intégral)