Reproduction and fertility parameters of dairy cows supplemented with monounsaturated fatty acid-rich canola oil : mRNA gene expression

The main objective of this thesis was to investigate the effects of supplementing pasture-based, primiparous, Holstein-Friesian cows with incremental levels of crude degummed canola oil (CDCO) on milk production, fatty acid composition, plasma metabolites, hormonal profiles and the expression of fat-related genes involved in reproduction and lipogenesis. It was generally hypothesised that CDCO supplementation would influence liveweight, body condition score, milk yield and fatty acid composition, plasma metabolites and reproductive hormonal profiles and mRNA expression of fat-related genes under temperate Australian environmental conditions. A random allocation of cows into treatment groups that consisted of a wheat-based pelleted diet with no supplemental CDCO (control), or with CDCO added at 25 ml/kg (low), 35 ml/kg (medium) and 50 ml/kg (high) was employed in an eight-week feeding trial after two weeks of adjustment. All cows had ad libitum access to pasture. The first experiment investigated the effect of CDCO on body condition score, liveweight, milk yield and composition. It tested the hypothesis that milk yield, fat and protein contents will increase as level and duration of CDCO supplementation increased, while cow body condition score and liveweight will be suppressed. Results indicated that the duration of supplementation significantly (P<0.05) influenced lactation traits and cows receiving CDCO supplements had greater milk yield at the expense of milk fat and protein without any negative impact on body condition score and average daily gain. These initial results provided empirical evidence that in a dairy setting where pasture is the main feed base, energy spared through CDCO-induced milk fat depression is partitioned towards milk yield. This finding raised a further research question on the impact of supplementation on energy-related plasma metabolites circulating in the lactating cow. Therefore, the second experiment investigated the effect of CDCO and ad libitum grazing on plasma metabolite profiles and tested the hypothesis that incremental levels of CDCO supplement will decrease plasma non-esterified fatty acids (NEFA) and β-hydroxybutyrate (BHB), but increase cholesterol and glucose metabolites. It was demonstrated that with the exception of BHB, CDCO did not alter plasma metabolite profiles (P>0.05), indicating that primiparous cows grazing high quality pastures during spring had sufficient energy intakes to prevent negative energy balance at 40 days in milk without the need for added oil supplements. However, duration of supplementation (week of lactation) had a significant effect (P<0.05) on the concentrations of NEFA, BHB and glucose, thus raising a further research question on the potential of CDCO in improving milk quality for beneficial health effects through the manipulation of milk fatty acid composition. Therefore, the third experiment tested the hypothesis that supplementation with CDCO will alter milk fatty acid composition towards increased total monounsaturates (tMUFA) and a decrease in total saturates ((tSFA). Results of significant (P<0.05) increases in 18:1ω9c, 18:1ω7t, tMUFA, tMUFA/tSFA ratio and a decrease in tSFA were observed, thus demonstrating improved milk quality and enhanced beneficial healthy fatty acid profile without any negative impact on the animals or milk taste. The fourth experiment tested the hypothesis that incremental levels of CDCO will alter the profiles of progesterone (P4), luteinizing (LH) and follicle stimulating (FSH) hormones. It was apparent that FSH and P4 profiles were significantly (P<0.05) influenced by duration and levels of supplementation, but not LH. Cows in the high (0.459 ng/ml), medium (0.367 ng/ml) and low (0.251 ng/ml) treatment groups had higher FSH concentrations compared to the control (0.172 ng/ml) cows. The fifth and last experiment investigated the effect of CDCO on the expression of Arylalkylamine N-acetyltransferase (AANAT), B-cell translocation gene-2 (BTG2) and Fatty Acid Synthase (FASN) genes involved in reproduction and lipid synthesis. The hypothesis tested was that post-partum supplementation of primiparous Holstein-Friesian cows with dietary CDCO in a pasture-based system will alter the relative mRNA abundance and expression of AANAT, BTG2 and FASN genes associated with lipid metabolism. Both level and duration of supplementation with CDCO were significant sources of variation (P 0.05). Cows in the high (0.67 fold), medium (0.87 fold) and low (0.56 fold) levels of oil treatments had lower expressions of BTG2 gene compared to the control (1.0 fold) group of cows. It was concluded that the supplementation of grazing cows with lipid-rich feeds could be utilised as a dietary manipulation tool to down-regulate the expression of BTG2 gene and its anti-proliferative attributes. The low expression of BTG2 might be important when the reproductive system of cows is recovering from the effect of gestation and new cell growth is required. The suppression of FASN gene expression can be beneficial in sparing energy from milk fat synthesis and re-directing the surplus to non-mammary tissues in vivo. However, severe milk fat depression may be economically undesirable to dairy farmers because of its contribution to total milk solids upon which milk prices are based. These findings highlight the important role of supplementary nutrition in altering reproductive and lipogenic gene expression in lactating primiparous cows.