Effects of prepartum micro-nutrient supplementation of pasture-based dairy cows on the composition of colostrum and transition milks

The prepartum period, also known as the dry period, is not only a resting phase prior to parturition but is also a metabolically stressful period for the dairy cow. Nutritional management during this period is critical to prevent metabolic disorders postpartum and to aid a smooth transition into the subsequent lactation. Depending on accessibility of essential nutrients and silage quality, minerals may be supplemented into the diet to ensure a strong mineral reserve is accumulated for the demanding calving period and subsequent early lactation period. Selenium, which can be supplemented in an inorganic or organic form, is a mineral routinely supplemented to dairy cows during the dry period as dairy cows have a nutritional requirement (0.3 mg/kg dry matter (DM)/d) for selenium. Another nutrient of interest is choline, which is not routinely supplemented to dairy cows, has gained interest in recent years as research has reported that choline supplementation increases milk yield and improves colostrum quality. Previous studies supplementing selenium or choline to dairy cows was carried out on total-mixed ration systems, with limited studies supplementing these micro-nutrients within a pasture-based system. In this thesis, pasture-based dairy cows were supplemented prepartum with selenium at the maximal dose of 0.5 mg/kg DM/d or rumen-protected choline at a moderate dose of 20 g/d. Therefore, the objective of this thesis was to examine the impact of these micro-nutrients on colostrum and transition milk composition in a pasture-based system. An impact of supplementation on lactation performance in the subsequent lactation was also examined.
A variety of analytical techniques such as of HPLC, GC-FID, UHPLC-DAD, ICP-MS, 1H-NMR and LC/MS-MS, were used to examine the impact of prepartum supplementation on colostrum and transition milk composition. Supplementing organic selenium or rumen-protected choline prepartum produced colostrum with higher concentrations of αs2-casein and β-casein compared to cows supplemented with inorganic selenium. This thesis provides an in-depth analysis on the composition of colostrum and transition milk derived from pasture-based dairy cows. This research addressed a gap in the literature regarding the impact of supplementing selenium at the maximum supplemental rate on colostrum composition in pasture-based dairy cows. This research is also the first to examine the impact of supplementing rumen-protected choline on colostrum composition in pasture-based dairy cows. The results presented herein are important for dairy cow nutritionists and farmers alike to understand that these micro-nutrients do not have a deleterious effect on colostrum quality, in particular immunoglobulin concentrations.