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Prolonged exposure to high environmental temperatures results in an accumulated heat load that induces a heat stress (HS) response in dairy cattle. Heat stress compromises dairy farm profitability by reducing milk yield, altering milk composition, and hindering reproductive performance. The ability to alternate between carbohydrate and lipid sources for energy production is termed metabolic flexibility (Met Flex). The objective of this study was to evaluate the Met Flex of mammary, muscle, and liver tissue in lactating dairy cows under HS and thermoneutral (TN) conditions. Sixteen Holstein cows were assigned to 1 of 2 treatment groups: pair feeding in a TN environment (PFTN) or HS. All cows experienced a 4 d TN period with ad libitum intake followed by a 4 d treatment period. Heat stress cows were exposed to a temperature humidity index (THI) ranging from 76 to 80 and the PFTN cows were exposed to a THI of 64. Milk production and health data were recorded twice daily. Semitendinosus biopsies were obtained on d 4 of each period and post-mortem mammary and liver samples were obtained on d 4 of period 2. All tissue samples were assayed for Met Flex. Activity of mitochondrial (Mit) enzymes were assessed in skeletal muscle only. Four d of HS decreased milk yield, altered milk composition, and increased respiration rate and rectal temperatures. No differences in Met Flex were observed in mammary or liver tissue during period 2. However, HS, but not PFTN conditions, lowered Met Flex of skeletal muscle by 18.3% when compared with TN ad libitum feed intake conditions of period 1. No treatment differences were observed in skeletal muscle Mit enzyme activity indicating the decrease in Met Flex occurred independently of changes in Mit function. The reduction in Met Flex of skeletal muscle during HS may contribute to reduced milk yield and warrants further investigation.

Aflatoxin M1(AFM1) is a pathogenic metabolite often transferred from feed into milk from aflatoxin (AF) B1, B2, G1, and G2; thus, it poses a human health risk. Therefore, effective mitigation strategies are needed to reduce animal and human exposure to AF. Study objectives were to evaluate a dietary adsorbent (Silicoglycidol, ATX) as a sequestering agent in AF contaminated feed and to broadly examine how AF affects liver function and the immune system. Primiparous Holstein cows (n = 12, 279 ± 88 DIM and 675 ± 19 kg BW) were used in a replicated 3 × 3 Latin square design with 21-d periods in which d 1–14 were considered adaptation and data collected on d 15 to 21 were used for analysis. Treatments were: 1) control (CON) consisting of a basal diet; 2) AF diet consisting of CON + AF challenge (100 µg of AFB1/kg dry matter intake [DMI]); and 3) AF + ATX supplemented at 0.10% of dietary DMI. Feed intake and milk yield were recorded daily, fecal samples were collected on d 20 of each period, blood and urine samples were collected on d 21 of each period, and milk samples were collected on the last 2 d of each period. Data were analyzed using the MIXED procedure of SAS. Milk yield and DMI were unaffected by treatment (26.8 ± 1.3 kg/d and 24.0 ± 0.9 kg/d, respectively). Similarly, neither milk composition nor DMI digestibility were affected by treatment. No AFM1was detected in CON cow's milk or urine. Supplementing ATX reduced AFM1in milk (1.57 vs. 1.14 ± 0.1 µg/L for AF and AF+ATX, respectively) and urine (9.9 vs. 5.6 ± 1.1 µg/L for AF and AF+ATX, respectively). Consuming AF did not affect biomarkers of liver health or immune activation including ALT, AST, GGT, haptoglobin (Hp), and immunoglobulin G (IgG). In summary, feeding ATX reduced the absorption and transfer of dietary AF to milk and urine.

Summary:Hyperketonemia (HYK) is a metabolic disorder in dairy cattle related to difficulties during the transition period and associated with suboptimal reproductive performance. Studies have proposed the monitoring of the transition period based on early-lactation milk yield. Therefore, our objective was to explore the role of milk yield during the first week of lactation on the relationship between HYK and reproductive performance. Cows (N = 2,091) were tested for HYK (blood BHB ≥1.2 mmol/L), between 3 and 10 DIM. First-week milk yield was classified into 3 levels according to parity and herd. Reproductive performance was assessed until 150 DIM. After adjustments, we found no evidence to support an association between HYK and reproduction. However, models stratified by early milk yield showed that HYK was associated with lower risk of pregnancy among low-yielding cows. Results suggests that early milk yield plays a role in the association of hyperketonemia with reproduction.