Lait 67 (1987) 437-450
DOI: 10.1051/lait:1987426

Les variations individuelles de la lipolyse spontanée du lait de vache : effet du numéro de lactation et répétabilité au cours de deux lactations successives

Marie-Paule CHAZAL^a, Marie-Paule CHAZAL^b and Y. CHILLIARD<sup>a

a</sup>  INRA, Laboratoire de la Lactation, Theix, 63122 Ceyral, France
^b  Institut Technique de l'Elevage Bovin

Abstract - Individual variations in spontaneous lipolysis in bovine milk : effect of lactation number, and repeatability during two consecutive lactations
The effect of lactation number on milk free fatty acid contents (FFA) after milking (initial FFA), and after 22 h storage at 4 °C (FFA-22) has been studied during two successive years (1984, 1985) on 162 to 267 cows, in winter, when all cows were kept indoors, and in spring at pasture.
In winter, milk initial FFA content tended to increase (P < 0.05) with lactation number. In spring, its variation was lower than that in winter and was not related to lactation number. Initial FFA were lower in 1985 than in 1984 (P < 0.05). Milk initial FFA contents of the two successive years at the same season were not well correlated (r = 0.27, n = 111, in winter, and r = 0.11, n = 131, in spring).
Milk FFA-22 content varied non significantly with lactation number in winter during both years. But in spring (or during late lactation) it decreased when number of lactation increased (P < 0.05). However, when the same cows were studied during two successive spring, no decrease was found from one lactation to the next. The " lactation number " effect could have been concealed by a " year " effect.
Repeatability of milk FFA-22 contents between two consecutive lactations was studied on 111 cows in winter and 131 cows in spring, all the cows being in mid-lactation and in early pregnancy or non-pregnant. Milk FFA-22 contents during two successive years were highly correlated (r = 0.55, n = 111, in winter, and r = 0.64, n = 131, in spring). This correlation was not due to differences in milk somatic cell counts between cows.
Twenty-seven cows were studied regularly during two successive lactations, up to at least 28 weeks of pregnancy. Among 16 cows whose milk FFA-22 content increased during late pregnancy in 1984, 10 cows had the same lipolytic pattern in 1985. The 6 other cows produced milk the FFA-22 content of which did not increase during late pregnancy in 1985. In the eleven other cows, milk FFA-22 remained low during both years. Thus, the increase of milk FFA-22 content during late pregnancy, if it occurs, seems to be dependent on both an intrinsic factor repeatable from one lactation to another, and an extrinsic factor probably linked to feeding conditions.

Résumé - L'effet du numéro de lactation sur les teneurs en acides gras libres du lait de vache après la traite (AGL initiaux) et après 22 h de stockage à 4 °C (AGL22) a été étudié en hiver et au printemps, durant 2 années successives, sur 162 à 267 vaches de 3 ou 4 troupeaux expérimentaux. Les teneurs en AGL22 des laits individuels tendent à diminuer avec l'âge des animaux, mais seulement au printemps lorsque les animaux sont à des stades de lactation et de gestation plus avancés qu'en hiver. Toutefois, le suivi des mêmes animaux d'une lactation à la suivante n'a pas confirmé cette tendance.
La répétabilité entre deux lactations successives de la teneur en AGL22 du lait a été étudiée sur 111 vaches en hiver, et 131 vaches au printemps, toutes en milieu de lactation et en début de gestation. Elle explique alors 30 à 40 % de la variance totale des données. Chez 35 vaches d'un troupeau suivi régulièrement au cours de 2 lactations successives, l'augmentation des teneurs en AGL22 en fin de gestation, lorsqu'elle a lieu, semble être à la fois sous la dépendance d'un facteur intrinsèque répétable d'une lactation à la suivante, et d'un facteur extrinsèque, non répétable et lié probablement aux conditions alimentaires.

Key words: Bovine milk / Spontaneous lipolysis / Fat acidity / Number of lactation / Individual variations / Repeatability between consecutive lactations

Mots clés : Lait de vache / Lipolyse spontanée / Acidité de la matière grasse / Numéro de lactation / Variations individuelles / Répétabilité entre lactations