Dietary source and availibility of fatty acids to manipulate ruminal protozoa, metabolism of fat, and milk fatty acid profile in lactating dairy cows.

摘要

With the increasing environmental concern for N excretion in the environment, the nutrition of dairy cows meets new challenges to maintain milk production while decreasing dietary protein. Various strategies to improve efficiency of N utilization in milk production have been developed. Decreasing protozoal abundance and the corresponding improvement of microbial protein efficiency could improve this efficiency of dietary protein utilization. The feeding of fat usually decreases the abundance of protozoa but can have detrimental effect on ruminal fermentation and milk fat production, thus needing further study to ascertain the potential benefits relative to potential detriments.; In a first study, the effects of the availability of dietary fatty acids from cottonseed oil on ruminal metabolism and milk fat production were investigated in a study with 60 lactating dairy cows and a corresponding digestibility trial with five cows. Whole cottonseed is commonly added to dairy diets to provide energy, protein, and a partial forage substitute. However, handling problems limit its use in some feeding situations. Although pelleting and delinting overcome these limitations, rapid release of free oil or lower digestibility, respectively, can decrease their efficacy. In our study, feeding a mix of pelleted and delinted cottonseeds appeared to modify ruminal fatty acid metabolic processes, decreasing the risk of milk fat depression and tending to increase dry matter intake and milk production over time compared with conventional or pelleted cottonseeds. However, pelleting through a larger die provided no comparable benefit. In a second study, the effects of feeding Rumensin (R) in combination with animal vegetable (AV) fat or coconut oil were fed to six rumen-cannulated dairy cows. Using omasal collection, I measured microbial protein efficiency and nutrient digestibility. By feeding R to control the extent of amino acid deamination and combined with fat to control protozoal abundance, the efficiency of microbial protein synthesis might be improved in dairy cows. Animal vegetable fat can be biohydrogenated in the rumen and decrease its effectiveness, but diets supplemented with coconut oil (CO; rich in medium chain FA) are more consistent in inhibiting protozoa. Total protozoal abundance was decreased by CO supplementation for all genera expect for Epidinium, which maintained its numbers but was decreased by AV+R. The low acetate to propionate ratio for CO was associated with a decreased ruminal NDF digestibility. There was no effect of diet on efficiency of microbial protein synthesis. DMI was 5 kg/d lower with CO. Milk production was lower when cows were fed CO than AV and when diets contained R. Milk fat depression (MFD) occurred with AV+R and CO. The decrease in protozoal abundance was not associated with an increase in microbial protein efficiency. The detrimental effect of CO on DMI affected the energy available for milk synthesis.; Omasal flows of FA were characterized by an increased percentage of trans 18:1 for AV and CO diets, a higher percentage of 12:0 and 14:0 for CO, and higher cis 18:1 for AV. Milk FA composition reflected the changes observed for omasal FA digesta flow. The de novo FA synthesis in the mammary gland was inhibited with R and F supplementation. Higher trans 18:1 FA in milk fat was also observed for AV and CO. The feeding of CO did not prevent MFD, and no interactions between R and S were detected. The feeding of CO did compromise ruminal biohydrogenation, with accumulation of trans 18:1 in the rumen and in milk fat.