The Evaluation Of Dietary Betaine, Pre And Probiotics, Transitional Substrates, And B-Mercaptoacetate On Physiological, Metabolic, Hormonal And Production Responses In Lactating Holstein Cows Subjected To Thermal Stress

This dissertation evaluated nutritional approaches such as the addition of betaine, prebiotics, probiotics, transitional metabolic substrates, and β-mercaptoacetate (MAA; a compound which inhibits β-oxidation) to the diet of lactating dairy cows to determine their impact on physiological, metabolic, hormonal and production responses during thermal stress. The first objective was to evaluate the use of an organic osmolyte, betaine to reduce the impact of heat stress (HS). Cows were fed either 0 (control; CON), 57 mg/kg BW (mid) or 114 mg/kg (high; HI) body weight (BW) betaine and subjected to thermoneutral (TN) and HS conditions. There was an increase in milk yield during TN with HI betaine over controls (P< 0.01), but the advantage was lost during HS. Plasma glucose increased during HS in HI dose cows compared to control (P < 0.01) as did plasma insulin (P = 0.01). Betaine increased milk production during TN and plasma glucose in HS, but did not improve the HS response. Objective two evaluated the use of a probiotic or direct fed microbial (DFM), Calsporin (Bacillus subtilus C-3102) to decrease the effects of HS in dairy cows. We hypothesized that feeding Calsporin prior to and during HS would reduce pathogenic strains of bacteria, maintain commensal microbes, and improve ruminal anaerobic fermentation resulting in improved milk yield (MY). Milk yield was numerically increased (1.26 kg, P = 0.11) in cows fed Calsporin during TN but was reduced under HS (-2.67 kg, P < 0.01) and milk protein content was decreased (P = 0.05). The DFM tended to decrease somatic cell count (SCC) across periods (P = 0.07). Calsporin addition to the diet did not affect respiration rates and was associated with higher rectal temperature at 1800 in HS (P = 0.02). The expression of heat shock protein 27 (HSP27) was decreased with Calsporin treatment (P = 0.03) and in both HS and TN. The fecal microbial count did not change with the exception of the Calsporin strain in treated animals (P < 0.01). The third objective was to feed OmniGen-AF (OG) to dairy cows before and during thermal stress. We hypothesized that feeding OG to HS dairy cows will improve the immune response, and decrease production losses associated with HS. Cows fed OG maintained lower SCC compared to control (P < 0.01) during the recovery period. We did not detect differences between groups in serum calcium while serum non-esterified fatty acid (NEFA) concentrations (P = 0.10) tended to be greater in OG fed cows across the Agricultural Research Center (ARC) portion including HS. Serum Adrenocorticotropic hormone (ACTH) levels were greater in OG cows (P<0.0001) across all sample days. Feeding OG reduced the HS response including serum Cortisol. The final study measured the effects of the metabolic substrate β-hydroxybutyrate (BHB) during HS on feed intake and metabolites. Under TN conditions the cows received a bolus dose of BHB and dry matter intake (DMI) and metabolites were measured. The second part of this study used a bolus of MAA to limit the up-stream production of acetyl-CoA available for ketogenesis by inhibiting ß-oxidation. We proposed that dosing lactating dairy cows with BHB would decrease DMI, increase plasma insulin, decrease NEFAs and increase skin temperature by vasodilatation. The same cows were then subjected to HS and dosed with saline and MAA on different test days. The infusion of BHB increased skin temperature (time 0.5, 1, 2, 3 and 4°C r² =0.98 with serum BHB) and decreased serum NEFA levels (P < 0.01). There was no change in mean DMI, glucose or insulin. The bolus of MAA decreased feed intake, vaginal temperature, and insulin. There was an increase in serum BHB with the initial dose of MAA and an initial decrease in serum glucose (P < 0.0001) with MAA. Serum glucose increased as insulin decreased with MAA. The infusion of BHB did not alter feed intake in this study despite high plasma levels of BHB