Les altises des crucifères, la mouche du chou et le mildiou sont les plus à craindre.
Récolte.
Pour le navet de primeur, la récolte est manuelle (confection de bottes si le feuillage est bien vert et non sénescent ou conditionnement en filet après lavage et calibrage). Le rendement peut varier de 15 à 40 t/ha, soit de 3 à 7 kg/m de planche. Pour le navet destiné à la transformation, la récolte mécanique est possible avec l'utilisation d'une récolteuse à bandes. Le rendement est de 40-50 t/ha.
Production.
La France, avec 70 000 t (sur 3 200 ha cultivés), se place au 2e rang européen, après le Royaume-Uni (170 000 t). La production est concentrée dans l'Ouest : Bretagne, Pays de la Loire et Basse-Normandie. Les industries de transformation (surgelés, plats cuisinés) utilisent environ 12 000 t/an.
Péron
navette
Plante voisine du colza cultivée pour son huile alimentaire, comme plante fourragère et éventuellement comme engrais vert (espèce Brassica campestris, famille des crucifères).
La navette (Brassica campestris var. oleifera) se distingue du colza au stade rosette par ses feuilles vert franc, rudes et hérissées de poils, à la floraison par ses fleurs épanouies qui entourent et dominent les boutons floraux (situation inverse du colza), à maturité par ses siliques plus dressées et plus courtes. La fécondation est croisée, et la graine, un peu plus petite que celle du colza, contient de 35 à 42 % d'huile. Plus rustique que le colza, la navette supporte mieux les semis tardifs et la sécheresse, mais sa résistance au froid est plus faible.
La navette est peu cultivée en France pour son huile. En revanche, au Canada et dans quelques pays d'Europe du Nord, elle est utilisée en culture de printemps à la même échelle que le colza oléagineux.
Roger-Estrade (A.)
NDF
Abréviation pour Neutral detergent fiber.
La NDF est la 1re étape du dosage des constituants pariétaux des végétaux selon la méthode de Van Soest. Ce résidu insoluble, obtenu par hydrolyse au moyen d'un détergent neutre après d'éventuels prétraitements (délipidation, hydrolyse enzymatique à la protéase ou à l'amylase), est assimilé à l'ensemble des constituants pariétaux (hémicellulose + cellulose + lignines) des végétaux.
Chapoutot/Schmidely
nécrose
Mort d'une cellule ou d'un tissu à l'intérieur d'un organisme vivant, animal ou végétal.
En phytopathologie, la nécrose est un symptôme très fréquent sur les différents organes d'un végétal (feuilles, rameaux, tiges, branches, racines, etc.). Les agents responsables sont très nombreux (champignons, bactéries, virus, accidents physiologiques). Lorsqu'ils déterminent une nécrose de forme et de couleur particulière, ou lorsque cette manifestation est le caractère principal de l'extériorisation de la maladie ou de l'accident, le terme de nécrose sert à dénommer la maladie.
Raynal
nectaire
Organe végétal sécrétant le nectar, habituellement placé à l'intérieur d'une fleur.
Les nectaires ont des positions diverses en fonction des espèces : sur le réceptacle ou à la base des pétales, dans un éperon de la corolle, sur les étamines ou dans la paroi de l'ovaire. Il existe aussi des nectaires extrafloraux à fonction mal définie, chez quelques plantes (par ex. sur les stipules de la vesce).
Henry
nectar
Liquide sucré, sécrété par les nectaires ou glandes nectarifères de certains végétaux, que les abeilles récoltent pour en faire du miel.
Mazoyer
nèfle
Fruit du néflier.
Mauget
néflier
Arbre originaire d'Europe méridionale et d'Asie occidentale, cultivé pour son fruit, la nèfle (espèce Mespilus germanica, famille des rosacées).
Le néflier peut atteindre jusqu'à 5 m de haut. Il fleurit en mai-juin. Son feuillage, vert mat, prend des tons jaune-orangé à l'automne.
Les nèfles sont récoltées en automne, après les premières gelées. Acides au moment de la cueillette, elles ne sont consommées que plusieurs semaines plus tard.
Mauget
négril
Insecte coléoptère dont les larves attaquent les plants de luzerne.
Les larves de négril apparaissent en mai-juin dans les luzernières du sud de la France. D'abord jaunâtres, puis d'un noir brillant, les larves consomment les feuilles et ne laissent, en cas de forte infestation, que les tiges de la luzerne.
Pour éviter ces dégâts, il est nécessaire de faucher précocement afin d'éliminer les larves. Les traitements insecticides éventuels ne sont envisageables qu'en production grainière.
Strebler/Raynal
neige
Précipitation de cristaux de glace agglomérés en flocons, dont la plupart sont ramifiés, parfois en étoile.
Les flocons, formés par coalescence, sont plus ou moins gros et en état de fonte plus ou moins totale selon les conditions des basses couches d'air. La neige représente un volume important avec beaucoup d'air (35 % d'eau). Selon la zone, elle s'accumule, et, malgré une fonte partielle, peut représenter une accumulation importante d'eau et de frigories. L'accumulation de neige permet une restitution progressive de l'eau au cours du printemps, très utile pour son infiltration et pour l'alimentation des écoulements.
Les frigories stockées permettent de ralentir le refroidissement en période hivernale et limitent le réchauffement de printemps (réflexion forte de l'énergie solaire et fonte de la neige, qui consomme beaucoup d'énergie).
Perrier
nématicide
Substance qui a la propriété de tuer les parasites nématodes phytopathogènes (une substance capable de tuer les nématodes parasites des animaux est appelée anthelminthique).
Guillot
nématode
Ver cylindrique et effilé, à tube digestif complet, généralement de très petite taille, pullulant dans le sol, dont de nombreuses espèces mènent une vie libre, tandis que d'autres sont des parasites de plantes, d'invertébrés et de vertébrés, dont l'homme.
Les nématodes sont des vers allongés, dont le corps est couvert d'une cuticule. Ils ont une biologie extrêmement variée. Les sexes sont séparés.
De nombreuses espèces sont libres, vivant dans la terre, les eaux douces ou marines ; certaines sont phytophages, d'autres parasitent les animaux.
Chez les animaux.
Les nématodes parasites des animaux se rencontrent dans de nombreux organes (tube digestif, muscles, appareil respiratoire, tissu sous-cutané, vaisseaux sanguins, etc.). Ils déterminent chez l'hôte des troubles divers (obstruction des vaisseaux, perforation de la muqueuse, consommation de sang, de tissu, perturbation des métabolismes, inoculation de germes pathogènes, sécrétion de toxines, etc.). Ce sont des antigènes qui provoquent la formation d'anticorps.
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