Conjugated linoleic acid (CLA) has the potential to be used as a dietary means of manipulating milk fat production of dairy cows to meet specified short-term targets, as in a supply-managed industry. An experiment was conducted to evaluate the dose-response to calcium salts of CLA fed as a top-dress on a corn- and alfalfa-based TMR. In two 5 × 5 Latin squares of 28-d periods, five primiparous and five multiparous cows were fed 0, 22, 45, 67 or 178 g d-1 CLA (29% trans-10, cis-12) as a calcium salt. Results from one multiparous cow on the 178 g d-1 treatment were removed from statistical analysis because milk production fell to 1.2 kg d-1 by the end of the period. On average, milk, lactose and protein yields increased at the lower doses of CLA and decreased to control levels at the highest dose. Milk fat yield declined progressively from 876 to 770, 689, 676 and 543 g d-1 on the five doses. Net energy balance of cows increased linearly from -1.5 Mcal d-1 on the control to 2.0 Mcal d-1 at 178 g d-1 CLA. The percentage depression in milk fat yield at different doses of trans-10, cis-12 CLA was described by the equation y = 54/(1 + 22.8/dose). Comparison with published literature suggested that feeding calcium salts of CLA was 30% as effective in depressing milk fat production as an abomasal infusion, due to rumen biohydrogenation and intestinal indigestibility.Key words: Milk composition, cattle, diet

. W. 2005. Phosphorus status of lactating dairy cows fed total mixed rations containing 0.24% vs. 0.36% phosphorus. Can. J. Anim. Sci. 85: 409-412. The effect of dietary P content on P balance was evaluated using 14 lactating dairy cows fed a corn silage based total mixed ration (TMR) containing either 0.24% P or 0.36% P. The animals were paired by calving date at dry-off with one cow from each pair randomly assigned to either dietary treatment. All cows were fed a common TMR containing 0.34% P prepartum, and then 0.41% P for 2 wk postpartum. Experimental diets were fed from 3 to 13 wk postpartum. Phosphorus balance was conducted at week 13 postpartum. Cows fed 0.36% P diet consumed more (P < 0.05) P than the 0.24% P diet cows (69.2 vs. 50.9 g d -1 ) and excreted more (P < 0.05) P in the faeces (49.1 vs. 25.7 g d -1 ). Phosphorus digestibility was lower (P < 0.05) for the 0.36% P diet than the 0.24% P diet (29.0 vs. 48.8%). Dry matter intake and milk yield were not (P > 0.05) affected by treatments. These results suggest that short-term P balance in lactating dairy cows can be maintained on a diet supplying 26% below National Research Council 2001 recommendations of P while reducing the amount of P excreted into the environment. Can. J. Anim. Sci. 85: 409-412. Les auteurs ont évalué l'incidence de la concentration de P dans les aliments sur 14 vaches laitières en lactation nourries d'une ration mixte complète (RMC) à base d'ensilage de maïs renfermant 0,24 % ou 0,36 % de P. Les animaux ont été appariés selon la date de vêlage au tarissement et une vache de chaque couple a été affectée au hasard à l'un des deux traitements. Les vaches ont toutes reçu la RMC enrichie de 0,34 % de P avant la mise bas, puis de 0,41 % de P au cours des deux semaines suivant le vêlage. Les rations expérimentales ont été servies de la 3 e à la 13 e semaine après la mise bas. Le bilan de phosphore a été établi la 13 e semaine suivant le vêlage. Les vaches qui recevaient 0,36 % de P ont absorbé plus de P (P < 0,05) que celles en recevant 0,24 % (69,2 c. 50,9 g par jour) et en ont excrété davantage (P < 0,05) dans leurs fèces (49,1 c. 25,7 g par jour). La digestibilité du P était plus faible (P < 0,05) avec la ration en contenant 0,36 % qu'avec celle en renfermant 0,24 % (29,0 c. 48,8 %). L'ingestion de matière sèche et le rendement laitier ne sont pas affectés (P > 0,05) par le traitement. Les résultats laissent croire qu'à court terme, on peut maintenir le bilan du P chez les vaches laitières en lactation avec une ration renfermant 26 % moins de P que la concentration recommandée par le Conseil national de recherches en 2001 et réduire par la même occasion la quantité de P excrétée dans l'environnement. Mots clés:Bilan du phosphore, vaches laitières, lactation, besoins en phosphore Reducing excessive intake of dietary P in dairy cattle can significantly reduce P excretion in the faeces, which can subsequently reduce the impact that application of dairy cow excreta has on soil P content and the potential for P leaching into waterways (Va...