Effects of source of supplementary trace minerals in pre- and postpartum diets on reproductive biology and performance in dairy cows.

The transition period in dairy cows imposes metabolic challenges that often reduce milk production and reproductive efficiencies. Energy, nutrients and oxidative balances are often compromised during this period, resulting in impaired immunity, increased susceptibility to disease, thereby further compromising nutrient intake and partition. Health problems and extensive mobilization of body reserves after calving have enduring effects on reproductive performance. Supplementation of trace minerals (TM) is required as it can benefit the oxidative balance and health of the cow in transition, but whether the TM should be in organic form (more expensive) or whether inorganic minerals will suffice, is not clear. Therefore, the objective of the authors cited was to evaluate the effects of complete replacement of inorganic salts of trace minerals (STM) with organic trace minerals (OTM) in both pre- and post-calving diets on ovarian dynamics, oestrus behaviour, pre-implantation conceptus development, and reproductive performance of dairy cows.

Pregnant cows and heifers were blocked for parity and BCS and randomly assigned to either STM or OTM diets at about 45 days before expected calving. Pre- and post-calving diets were formulated to meet 100% of recommended levels of each trace mineral in both treatments, being Co, Cu, Mn, Se and Zn. The STM group was supplemented with Co, Cu, Mn and ZN sulphates and sodium selenite, while the OTM group was supplemented with Co, Cu, Mn and Zn proteinates and selenized yeast. The treatments continued until 156 days in milk (DIM) and were assigned to individual cows using automatic feeding gates. Starting at 21 DIM, ultrasonography examinations of the ovaries were performed weekly to determine the presence of a corpus luteum and post-calving resumption of ovarian cyclicity. The cows were pre-synchronized with two injections of PGF_2α at 42 and 56 DIM. Oestrus behaviour was monitored using electronic activity tags that indirectly measured walking activity. Cows detected in oestrus after the second PGF_2α were inseminated, and those not detected in oestrus by 67 DIM were enrolled in a synchronization program. Cows that returned to oestrus after artificial insemination (AI) were re-inseminated. Pregnancy was established 33 days after AI and non-pregnant cows were resynchronized. Transcript expression of interferon-stimulated genes in blood leukocytes was performed in a subgroup of both STM and OTM cows, while a different group received uterine flushing 15 days after AI for recovery of conceptuses and uterine fluid for analyses of transcriptomics and metabolomics, respectively. In addition, dominant follicle diameter, luteal size and blood flow, and concentration of progesterone in plasma were measured on day 0, 7 and 15 in relation to AI. After flushing, PGF_2α was given and the dominant follicle was aspirated two days later to measure the concentration of trace minerals.                                                                                      

The results showed that oestrus behaviour, size of the dominant follicle and corpus luteum, concentration of progesterone, time to pregnancy and proportion of cows pregnant by 100 days of the breeding period did not differ between treatments. A greater proportion of cows supplemented with OTM had a corpus luteum detected before pre-synchronization, and multi-lactation cows supplemented with OTM tended to resume cyclicity earlier than STM cows. OTM cows had a greater concentration of Cu in the follicular fluid than STM cows. In pregnant multi-lactation cows, expression of receptor transporter protein 4 in blood leukocytes was greater in the OTM group. Conceptuses of the two treatments had 589 differentially expressed transcripts, with many indicating advanced elongation and greater transcript expression of selenoproteins in the OTM group. In pregnant cows, 24 metabolites were more abundant in the uterine fluid of OTM, including spermidine, sucrose and cholesterol.

It was concluded that OTM compared to STM caused moderate improvements to resumption of ovarian cyclicity and important changes in pre-implantation conceptus development, but it did not alter conception risk and pregnancy rate. Replacement may therefore not be economically justifiable.