CHAPTER I Effects of Inorganic and Organic Mineral Supplementation on Blood Antioxidant Status and Rumen Microbiota Composition of Holstein Calves under Short-term Heat Stress and Recovery Condition
The study was carried out to investigate the effects of inorganic and organic minerals on the physiological responses, reduction of oxidative stress, and rumen microbiota in Holstein calves during short-term heat stress (HS) and recovery period. Eight Holstein calves were randomly assigned to four treatment groups: mineral-free (Con), inorganic minerals (IM), organic minerals (OM), and high-concentration organic minerals (HOM) in a 4 x 4 Latin square design with 4 periods of 35 days and kept in a temperature-controlled barn. The experimental periods consisted of 14 d for HS, 14 d for recovery, and 7 d for diet adaptation for the next period. Results showed that ADG and FE were high in HOM-supplemented calves in HS condition (p>0.05). Meanwhile, body temperature and respiration rate were higher in HS than in recovery condition (p<0.05). Selenium concentration in serum was high in the HS period (90.38 μg/dL) and recovery period (102.00 μg/dL) of HOM-supplemented calves (p<0.05). During HS period, the concentration of serum cortisol was 20.26 ng/ml in HOM group, which was 5.60 ng/ml lower than that of the control group (p<0.05). The total antioxidant status was the highest in the OM group (2.71 mmol Trolox Equiv./L) during HS and in the HOM group (2.58 mmol Trolox Equiv./L) during the recovery period (p<0.05). Plasma Malondialdehyde and HSP70 were decreased by HOM supplementation during HS and the recovery period, while SOD was increased (p>0.05). At the phylum level, Firmicutes and Actinobacteria decreased, and Fibrobacteres, Spirochaetes, and Tenericutes increased (p<0.05) in HS condition. Treponema increased in HS condition (p<0.05), while Christensenella was high (p<0.05) in HOM and OM during HS and recovery conditions, respectively. Therefore, supplementation with HOM during HS reduced concentrations of cortisol and increase total antioxidant status in Holstein calves, suggesting that high organic mineral supplementation in calves may alleviate the adverse effects of HS.
CHAPTER II Higher Concentration of Dietary Selenium, Zinc, And Copper Complex Reduces Heat Stress-Associated Oxidative Stress and Metabolic Alteration in the Blood of Holstein and Jersey Steers
This study investigated the influence of high concentrations of dietary minerals on reducing heat stress (HS)-associated oxidative stress and metabolic alterations in the blood of Holstein and Jersey steers. Holstein steers and Jersey steers were separately maintained under a 3 x 3 Latin square design during the summer conditions. For each trial, the treatments included Control (Con; fed basal TMR without additional mineral supplementation), NM (NRC recommended mineral supplementation group; [basal TMR + (Se 0.1 ppm + Zn 30 ppm + Cu 10 ppm) as DM ba-sis]), and HM (higher than NRC recommended mineral supplementation group; [basal TMR + (Se 3.5 ppm + Zn 350 ppm + Cu 28 ppm) as DM basis]). Blood samples were collected at the end of each 20-day feeding trial. In both breeds, a higher superoxide dismutase concentration (U/mL) along with lower HSP27 (μg/L) and HSP70 (μg/L) concentrations were observed in both miner-al-supplemented groups compared to the Con group (p < 0.05). The HM group had significantly higher lactic acid levels in Jersey steers (p < 0.05), and tended to have higher alanine levels in Holstein steers (p = 0.051). Based on star pattern recognition analysis, the levels of succinic acid, malic acid, γ-linolenic acid, 13-methyltetradecanoic acid, and tyrosine decreased, whereas pal-mitoleic acid increased with increasing mineral concentrations in both breeds. Different treat-ment groups of both breeds were separated according to the VIP scores of the top 15 metabolites through PLS-DA analysis; however, their metabolic trend was mostly associated with the glu-cose homeostasis. Overall, the results suggested that supplementation with a high-er-than-recommended concentration of dietary minerals rich in organic Se, as was the case in the HM group, would help to prevent HS-associated oxidative stress and metabolic alterations in Holstein and Jersey steers.
CHAPTER III Influence of Dietary Organic Trace Minerals on Enteric Methane Emissions and Rumen Microbiota of Heat-Stressed Dairy Steers
Ruminants are the main contributors to methane (CH₄), a greenhouse gas emitted by livestock, which leads to global warming. In addition, animals experience heat stress (HS) when exposed to high ambient temperatures. Organic trace minerals are commonly used to prevent the adverse effects of HS in ruminants; however, little is known about the role of these minerals in reducing enteric methane emissions. Hence, this study aimed to investigate the influence of dietary organic trace minerals on rumen fermentation characteristics, enteric methane emissions, and the composition of rumen bacteria and methanogens in heat-stressed dairy steers. Holstein (n=3) and Jersey (n=3) steers were kept separately within a 3x3 Latin square design, and the animals were exposed to HS conditions [Temperature-Humidity Index (THI), 82.79 ± 1.10]. For each experiment, the treatments included a Control (Con) consisting of only basal total mixed rations (TMR), National Research Council (NRC) recommended mineral supplementation group (NM) [TMR + (Se 0.1 ppm + Zn 30 ppm + Cu 10 ppm)/kg dry matter)], and higher concentration of mineral supplementation group (HM) [basal TMR + (Se 3.5 ppm + Zn 350 ppm + Cu 28 ppm)/kg dry matter]. Higher concentrations of trace mineral supplementation had no influence on methane (CH₄) emissions and rumen bacterial and methanogen communities regardless of breed (p>0.05). Holstein steers had higher ruminal pH and lower total volatile fatty acid (VFA concentrations than Jersey steers (P <0.05). Methane production (g/d) and yield (g/kg dry matter intake) were higher in Jersey steers than in Holstein steers (P <0.05). The relative abundances of Methanosarcina and Methanobrevibacter olleyae were significantly higher in Holstein steers than in Jersey steers (p<0.05). Overall, dietary organic trace minerals have no influence on enteric methane emissions in heat-stressed dairy steers; however, breed can influence it through selective alteration of the rumen methanogen community.