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National audience; Connectedness between countries is a critical point for a reliable international evaluation. Weak genetic links between countries can affect predicted genetic values and ranking of the animals. A new method is presented here to measure connectedness between countries: a systematic difference between true genetic levels of countries is introduced by simulation, a BLUP is performed based on true pedigree, then the percentage of the initial difference that can be found between the estimated genetic levels of the corresponding countries is measured. The more 2 countries exchange genes, the closer are their true genetic levels. The better 2 countries are connected, the higher is the percentage of true re-estimated genetic difference. It was then shown that among countries involved in the Interbull Simmental and Montbéliard breed evaluation, Italy is one of the best connected countries and Slovenia probably the worst connected one. Ireland and the Netherlands use only Montbéliard French bulls. This method was very easy and rapid to implement and could be used in routine.; La connexion génétique entre pays est un élément essentiel de la fiabilité d’une évaluation génétique internationale. Un faible niveau de connexion peut biaiser la comparaison d’individus de pays différents et fausser le classement général des reproducteurs. Cet article présente une nouvelle méthode pour mesurer la connexion : des différences de niveau génétique entre pays sont simulées, puis un BLUP est réalisé utilisant les pedigree réels afin de réestimer les différences introduites initialement. Plus deux pays échangent de gènes et plus leurs niveaux génétiques seront proches. D’autre part, plus ces deux pays sont connectés, mieux on réestime les différences initiales. Cette méthode a été testée sur les populations qui participent aux évaluations conjointes Interbull en race Simmental et Montbéliarde : l’Italie est un des pays le mieux connecté, tandis que la Slovénie apparaît comme le pays le moins bien connecté. L’Irlande et les Pays-Bas utilisent principalement des taureaux Montbéliards français. Cette méthode est très simple et rapide à mettre en application en routine.

International audience

Background Heat stress negatively influences cattle welfare, health and productivity. To cope with the forecasted increases in temperature and heat waves frequency, identifying high-producing animals that are tolerant to heat is of capital importance to maintain milk production. This study, based on the joint analysis of on-farm performance and weather data, had two objectives: (1) to determine the response in production performances (milk, fat and protein yields, fat and protein contents) and udder health (somatic cell score) to temperature-humidity index (THI) variations in Montbeliarde cows, and (2) to estimate the interactions between genotype and THI, to enable the identification of the most adapted animals for facing the expected increases in temperature. Results Test-day records from first and second lactations from 2016 to 2020 were associated with the average THI during the three days before the test-day record. In total, 446,717 test-day records from 55,650 cows in first lactation and 457,516 test-day records from 58,229 cows in second lactation were analysed. The optimal THI was below 55 (i.e. ~ 12–13 °C) for all traits. Individual responses to THI were estimated by random regression models, which also included individual responses to days in milk. Regardless of the stage of lactation, genetic correlations along the THI gradient were above 0.80, which suggests that genotype-by-THI interactions were weak for production and udder health traits. Nevertheless, a variability in the individual slope of decay could be highlighted at high THI. The genetic correlation between production level at moderate THI and the slope at high THI was negative, while for somatic cell score, it was positive, indicating that heat stress amplifies the susceptibility to mastitis. Conclusions The optimal THI for French Montbeliarde cows is below 55 for production and udder health traits. Genetic-by-THI interactions are weak in French Montbeliarde cows for production and udder health traits, but not all animals react in the same way to high temperatures. Even if there is little room for improvement, using a heat tolerance index in cattle selection would be relevant to anticipate the expected increases in temperature. Further investigations are needed to interpret this variability on production traits. However, the current selection for mastitis resistance seems appropriate to adapt cattle to rising temperatures.

International audience