Effects of parity, age at calving and stage of lactation on fatty acid composition of milk in Canadian Holsteins. Can. J. Anim. Sci. 94: 401Á410. The objectives of the present study were to study the effects of parity, age at calving and stage of lactation on individual fatty acids and fatty acid unsaturation indices in milk of Canadian Holsteins. One morning milk sample was collected from each of 3185 dairy cows between February and June 2010 from 52 commercial herds enrolled in the Quebec Dairy Production Centre of Expertise, Valacta. Individual fatty acid proportions (g 100 g (1 of total fatty acids) were determined for each sample by gas chromatography. Editing for registration status, cow, sire and dam identification, breed, country, age at calving and stage of lactation reduced the data set to 2573 cows representing 46 herds. The statistical model included the fixed effects of parity, age at calving and stage of lactation each nested within parity and random effects of herd-year-season of calving and residual. First-parity cows had relatively higher proportions of beneficial unsaturated fatty acids, from a human health standpoint, and lower proportions of potentially undesirable saturated fatty acids in milk fat as compared with later parity cows. Milk fatty acid profile varied with stage of lactation. The proportions of short-and medium-chain fatty acids (C6:0 to C14:0) were low in the beginning of lactation and increased during the early part of lactation. Long-chain fatty acids (especially C18:0 and oleic acid) and unsaturation indices (C16, C18 and total) showed an opposite trend to that of short-and medium-chain fatty acids. The study showed that parity and lactation stage contribute to variation in bovine milk fat composition.

The objective of the present study was to estimate heritabilities of milk fatty acids (FA) and genetic and phenotypic correlations among milk FA and milk production traits in Canadian Holsteins. One morning milk sample was collected from each of 3,185 dairy cows between February and June 2010 from 52 commercial herds enrolled in Valacta (Ste-Anne-de-Bellevue, Quebec, Canada). Individual FA percentages (g/100 g of total FA) were determined for each sample by gas chromatography. After editing the data, 2,573 cows from 46 herds remained. Genetic parameters were estimated using multitrait animal models fitted under REML. The model included fixed effects of age at calving and stage of lactation each nested within parity and random effects of herd-year-season of calving, animal, and residual. The pedigree of animals with data was traced back 5 generations on both the male and female sides to account for relationships among animals. The estimates of heritability for individual FA ranged from 0.01 to 0.39, with standard errors ranging from 0.01 to 0.06. Generally, monounsaturated FA (MUFA) and saturated FA (SFA) showed higher heritability estimates than polyunsaturated FA (PUFA). Overall, SFA were negatively genetically correlated with MUFA and PUFA, whereas genetic correlations between MUFA and PUFA were positive. The SFA showed positive associations with fat yield and fat percentage, whereas unsaturated FA were negatively associated with fat yield and fat percentage. Bovine milk FA composition could be improved through genetic selection, and selection for MUFA or against SFA could alter the bovine milk fat profile in a desirable direction.

The objective of the present study was to estimate genetic parameters of milk yield (MY), intake traits, type traits, body condition score (BCS), and number of breedings (NOB) in first lactation Canadian Holsteins with a focus on the possibility of using type traits as an indicator of feed intake. Data were obtained from the Canadian Dairy Network and Valacta. A mixed linear model was fitted under REML for the statistical analysis. The multivariate (five traits) model included the fixed effects of age at calving, stage of lactation, and herdround-classifier for type traits; age at calving, stage of lactation, and herd-year-season of calving (HYS) for BCS; age at calving and HYS for MY, feed intake, and NOB. Animal and residual effects were fitted as random effects for all traits. Estimates of heritabilities for MY, dry matter intake (DMI), angularity, body depth, stature, dairy strength, final score, BCS, and NOB were 0.41, 0.13, 0.24, 0.30, 0.50, 0.30, 0.22, 0.20, and 0.02, respectively. Genetic correlations between type traits and DMI ranged from 0.16 to 0.60. Results indicate that type traits appear to have the potential to predict DMI as a combination/index of two or more traits.Key words: milk yield, feed intake, type traits, dairy cattle.Résumé : L'objectif de la présente étude était d'estimer les paramètres génétiques de production de lait (MY-« milk yield »), des caractéristiques de prise alimentaire, des caractères de conformation, de cote de condition de chair (BCS-« body condition score ») et le nombre de saillies (NOB-« number of breedings ») chez les vaches holsteins canadiennes à la première lactation avec un accent sur la possibilité d'utiliser les caractères de conformation comme indicateurs de prise alimentaire. Les données ont été obtenues du Réseau laitier canadien (« Canadian Dairy Network ») et de Valacta. Un modèle linéaire mixte a été traité sous REML pour l'analyse statistique. Le modèle multivarié (cinq caractères) comprend les effets fixes de l'âge au premier vêlage, le stade de lactation et le classificateur de troupeau complet pour les caractères de conformation; âge au vêlage, stade de lactation ainsi que troupeauannée-saison (HYS-« herd-year-season ») de vêlage pour la BCS; âge au vêlage et HYS pour la production de lait, prise alimentaire et NOB. L'animal et les effets résiduels ont été réglés comme effets aléatoires pour tous les caractères. Les estimations d'héritabilités pour la production de lait, ingestion des matières sèches (DMI-« dry matter intake »), angularité, profondeur de corps, stature, puissance laitière, cote finale, BCS et NOB étaient 0,41, 0,13, 0,24, 0,30, 0,50, 0,30, 0,22, 0,20 et 0,02, respectivement. Les corrélations génétiques entre les caractères de conformation et la DMI variaient entre 0,16 et 0,60. Les résultats indiquent que les caractères de conformation semblent avoir un potentiel pour la prévision de la DMI en tant que combinaison/indice de deux caractères ou plus. [Traduit par la Rédaction] Mots-clés : production de lait, prise alimentaire...

The objectives of the present study were to estimate genetic parameters of milk fatty acid unsaturation indices in Canadian Holsteins. Data were available on milk fatty acid composition of 2,573 Canadian Holstein cows from 46 commercial herds enrolled in the Québec Dairy Production Centre of Expertise, Valacta (Sainte-Anne-de-Bellevue, Quebec, Canada). Individual fatty acid percentages (g/100 g of total fatty acids) were determined for each milk sample by gas chromatography. The unsaturation indices were calculated as the ratio of an unsaturated fatty acid to the sum of that unsaturated fatty acid and its corresponding substrate fatty acid, multiplied by 100. A mixed linear model was fitted under REML for the statistical analysis of milk fatty acid unsaturation indices. The statistical model included the fixed effects of parity, age at calving, and stage of lactation, each nested within parity, and the random effects of herd-year-season of calving, animal, and residual. Estimates of heritabilities for the C14, C16, C18, conjugated linoleic acid, and total unsaturation indices were 0.48, 0.25, 0.29, 0.14, and 0.19, respectively. Phenotypic and genetic correlation estimates among unsaturation indices were all positive and ranged from 0.20 to 0.65 and 0.23 to 0.81, respectively. The estimates of heritabilities and genetic correlations for milk fatty acid unsaturation indices suggest that genetic variation exists among cows in milk fatty acid unsaturation, and the proportions of desirable unsaturated fatty acids from a human health point of view may be increased in bovine milk through genetic selection.