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SummaryOne hundred and thirty-seven (Holstein (41), Montbéliarde (42) and Tarentaise (54)) dairy cows in first or second lactation received during winter one of two levels (high, H; low, L) of energy intake, and were later fed identically at pasture. Thrice in winter and twice at pasture, the chemical composition and coagulation properties of individual milks were measured. Milk from Holstein cows had lower casein and calcium contents, and poorer coagulation properties (curd firming time and curd firmness) than that from Montbéliardes and Tarentaises (P < 0·01). These differences practically disappeared when taking into account the distribution of the different κ-casein variants and milk casein content. κ-BB milks had coagulation properties 20–50% superior, according to characteristic, to those of AA milks. In the three breeds, animals from the H group had casein contents higher by 1·4 g/l than those of the L group, which induced a significant improvement in curd firming time, curd firmness and cheese yield. Turning out to pasture induced an increase of 0·02 in milk pH, and improved milk coagulation properties. These changes did not appear to result entirely from the parallel increase in milk casein content.

-Understanding the distribution of grazing activity and its management is valuable to ensure the sustainability and productivity of heterogeneous grasslands. Controlled behavioural studies can provide insight into the cognitive abilities of herbivores and suggest new approaches to improve their grazing distribution. We compared the behaviour and diet selection of sheep and cattle in 1-ha fescue (Festuca arundinacea S.) plots, in which the number and size of preferred ryegrass (Lolium perenne L.) patches were varied. Five different patterns were used to investigate the effects of ryegrass abundance and spatial distribution on the searching success of the herbivores. Both heifers and ewes grazed ryegrass more when its abundance increased from 1.5 to 6% , and also when ryegrass was aggregated into a few 8 × 9-m patches, rather than being dispersed into a larger number of 3 × 3-m ones. Neither the interaction between ryegrass abundance and patch size, nor that between herbivore species and patch size was significant. At 6% abundance, aggregating ryegrass into a single 24 × 24-m patch tended to further increase its use by heifers, but not by ewes. Fescue less than 1 meter from the nearest ryegrass patch was usually selected more than the fescue located further away. Fescue between 1 and 5 m from the nearest patch was never selected more than the fescue located further away. Our results, together with those for the diet selected by sheep, cattle and deer, when offered patches of a preferred food in aggregated or dispersed patterns, indicate that over a wide range of abundance, it is advantageous for herbivores to forage on patchy resources when the preferred vegetation is aggregated. This corroborates the prediction that foraging costs associated with patch distribution are involved in determining diet selection in patchy grasslands, but our results suggest that it is also influenced by some social characteristics of the animals, such as their social attraction and social tolerance. Résumé -Dans une mosaïque de végétation, les choix des ovins et des bovins varient selon l'abondance mais aussi la distribution spatiale des placettes préférées. La compréhension des mécanismes qui gouvernent l'utilisation de pâturages hétérogènes permet de proposer des pistes pour conduire les parcelles et les troupeaux au mieux des attendus de l'élevage et de la préservation de la biodiversité. Les études comportementales en situation contrôlée peuvent aussi nous éclairer sur les capacités cognitives des herbivores. Ici, nous avons comparé le comportement et les choix alimentaires de génisses et d'agnelles dans des parcelles de fétuque (Festuca arundinacea S.) d'un hectare, dans lesquelles ont été implantées des placettes préférées de ray-grass (Lolium perenne L.) de diffé-rentes tailles et en différents nombres. Cinq combinaisons d'abondance et d'agrégation du ray-grass ont été proposées aux animaux. Ceux-ci ont plus sélectionné le ray-grass lorsque son abondance augmentait de 1,5 à 6 % . Ils l'ont également plus sélectionné lo...

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-In a systemic approach, the breeder can be considered as the decisional component of the livestock system, whereas animals are usually depicted to be part of its biotechnical component. The animal itself is a biological system whose ability to survive, grow, reproduce and cope with the environnement and livestock practices play a major role in the ability of the livestock system to sustain. In such a conceptual representation of the system, the reproductive females draw a peculiar attention since they determine in a great part the productivity and the durability of the system through their abilities to maintain their own production level (milk production, numeric productivity) and to save their reproductive efficiency (repeated pregnancies and lactations) over years. Considering the animal level and its lifespan, it is clear that the abilities to adapt rely on behavioural and physiological regulatory processes. The study of the biological mechanisms involved in the adaptation to undernutrition is particularly interesting since regulatory processes implied in energy metabolism may interfere directly or indirectly with the reproductive function, and consequently, with the durability of the livestock system. A biological significance of such relationships between nutrition and reproduction is given that they allow the female to be informed about the associated risk of entering a productive process facing the uncertainty of the nutritional context. Although the general mechanisms implied in the ability to adapt to the underfeeding constraint are conserved among ruminants, the thresholds (or priorities) may largely differ according to the breed within the same species. Hence, in order to evaluate the ability of the ruminant livestock systems to perpetuate in hard environments (maintaining their production levels) or to assess sustainable objectives (opening bushy landscapes by increasing grazing pressure), animals' inherent adaptive potentialities have to be well known. Résumé -Capacités adaptatives des femelles et durabilité des systèmes d'élevage. Synthèse bibliographique. Dans une représentation systémique du système d'élevage, l'éleveur incarne la composante décisionnelle du système, tandis que l'animal constitue, avec la ressource, sa composante biotechnique. L'animal lui-même peut être considéré comme un système biologique soumis à un environnement contraignant, dont les aptitudes à survivre, croître, se reproduire et s'adapter jouent un rôle fondamental dans la pérennité du système d'élevage. Les femelles reproductrices tiennent une place particulière dans cette représentation car non seulement elles déterminent une large part de la productivité du système par leur propre niveau de production (production laitière, productivité numérique), mais elles en assurent également sa reproductibilité au cours du temps (investissement reproductif). A l'échelle de l'individu et de sa durée de vie, les modalités d'adaptation reposent sur des processus de régulations comportementales et physiologiques. De tels pro...