Milk production, milk composition, and reproductive function of dairy cowsfed different fats

Thirty-five non-gestating multiparous Holstein cows averaging 571 kg of BW (SE = 8) were allotted at 9 wk postpartum to one of two dietary fat supplem ents based on either Megalac (R) (Volac Ltd., Roston, Hertfordshire, UK) an d solvent extracted flaxseed meal (MEGA) or whole flaxseed treated with for maldehyde (FLAX) to determine the effects on milk production and compositio n, follicular development, gestation rate, and fatty acid (FA) composition of blood. Cows were fed a total mixed diet based on ryegrass silage and fat supplements for ad libitum intake. The experiment was carried out between weeks 9 and 19 of lactation. Dry matter (DM) intake and change in body weig ht were similar for cows fed MEGA and FLAX. Milk production was higher for cows fed MEGA than for those fed FLAX (19.8 vs. 18.6 kg d(-1)) as was 4% fa t-corrected milk yield (22.9 vs. 20.2 kg d(-1)). Increased fat mobilization could have contributed to increased milk yield when cows were fed MEGA com pared with when they were fed FLAX as plasma concentrations of non-esterifi ed FA and cholesterol increased more from weeks 9 to 19 of lactation for co ws fed MEGA. Milk fat percentage tended (P = 0.06) to be greater for cows f ed MEGA (4.62%) compared with those fed FLAX (4.37%). Milk protein percenta ge was higher for cows fed FLAX (3.09%) than for those fed MEGA (2.95%), in dicating that formaldehyde protection of flaxseed was adequate to partly pr event ruminal degradability of protein in the seed. Milk fatty acid concent rations of C8:0, C10:0, C12:0, C14:0, C14: 1, Cl 8:0, C18:3, and C20:5 were higher for cows fed FLAX than for those fed MEGA while the inverse was obs erved for C16:0, C16: 1, C18:1, and C18:2. Cows fed FLAX had lower blood co ncentrations of C16:0 than those fed MEGA. There was a significant interact ion (P < 0.05) between week and diet for C18:0 and C18:2 with a decrease in C18:0 blood concentration for cows fed MEGA and an increase for those fed FLAX between weeks 9 and 19, while the inverse was observed for C18:2. Bloo d concentrations of C18:1 were similar for both treatments. Conception rate was significantly lower for cows fed MEGA (50.0%) compared to those fed FL AX (87.5%). Diet had no effect on the size of the largest and second larges t follicles, or on the difference between the diameter of the largest and s econd largest follicles. The number of class 1 (1.09 vs. 0.86), 2 (1.33 vs. 0.86), and 3 (1.28 vs. 0.98) follicles was similar for MEGA and FLAX altho ugh the total number (3.70 vs. 2.70) of follicles tended (P = 0.09) to be g reater for cows fed MEGA than for those fed FLAX. These data suggest that d ietary FA have an effect on gestation rate, but this could not be explained by differences in follicle dynamics or number. However, additional trials with greater numbers of animals are needed to confirm the reproductive resu lts.