31 mars 2005
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Dans les régions à climat méditerranéen sub-humide d'Afrique du Nord, le bersim (Trifolium alexandrinum L.) est très utilisé pour l'affouragement en vert de fin d'hiver et de printemps des élevages bovins laitiers. L'irrigation permet-elle d'allonger sa courte période de production (90 jours) en culture pluviale ?
Dans deux essais (semis du 18/10 et du 20/11 en Mitidja, Algérie), avec comme témoin le régime pluvial (T0), trois règles de décision d'irrigation ont été comparées : T1 avec déclenchement de tensiomètres à 0,05 Mpa, et T2 et T3 selon l'évaporation du bac classe A et irrigation tous les 30 ou 50 mm. Quel que soit le régime hydrique, la période d'efficience d'utilisation de l'eau optimale se situe de mi-février à mi-mai (4 à 5 kg MS/m3 d'eau). L'avancement du semis d'un mois et l'irrigation ont permis d'allonger la durée d'offre fourragère exploitable de 15 jours environ. Les règles de décision d'irrigation se sont traduites par différents degrés de satisfaction des besoins en eau, auxquels la production de matière sèche est très corrélée, mais l'effet est inverse, défavorable, sur la qualité du fourrages (valeur énergétique et protéique).
Berseem (Trifolium alexandrinum L.) is an annual legume sown in autumn which is much used for the green-feeding of dairy cattle at the end of winter and in spring in the regions of Northern Africa with a sub-humid Mediterranean climate. The short production season of the rainfed crop may be extended by the use of irrigation, sowing ahead of time, and improving growth at the end of spring.
Two trials comparing three decision rules for irrigating a berseem crop were set up in Mitidja in Northern Algeria, with the rainfed crop as control (T0). The two trials differed by their sowing date (respectively 18th October and 20th November). The rules for the decision to irrigate were : T1, irrigation when tensiometres at depths of 40 and 60 cm showed a value of 0.05 Mpa (maintained near soil field capacity) ; T2, irrigation whenever the evaporation of a class A pan cumulated to 30 mm ; T3, the same but when the cumulation reached 50 mm.
The effects on soil water content and reserve and on actual evapotranspiration were measured with neutron probe and tensiometer techniques. The duration of production and the water use efficiency were observed; the DM yield and the quality of the forage were measured at each cut.
The various treatments resulted in different values for the ratio of actual to potential evapotranspiration in spring, highly correlated to the DM production (T1>T2>T3>T0). In all treatments the water use efficiency was optimal between mid-February and mid-May, with an average value 4.7 kg DM/m3 H20 ; later on, around mid-May, when growth gave way to flowering, it decreased dramatically. The advancement of the sowing date to 18th October resulted in a duration of production of 110 days. A further extension of this duration would require a much earlier sowing date, with irrigation. The positive effects of the irrigation on DM yield were offset by concomitant adverse effects on the forage quality (water and crude fibre contents, energy and protein values).
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