|Abstract:||Understanding the role that stress plays on livestock health and production is of utmost importance in today’s agricultural industry. In particular, dairy cattle face many stressful conditions that have an impact on lactation performance and overall animal health. These stress-related factors not only affect cattle wellbeing, but also have a vastly negative impact on the economic state of the dairy industry. For instance, heat stress (HS) can cost the dairy industry between $900 million to $5 billion annually, while mycotoxin contamination issues can cause up to $1.7 billion in annual losses. Therefore, this research aims to provide a further understanding of the biological effects of two stressors in dairy cattle production systems, nutrition-related aflatoxin (AF) challenge and environment-related HS challenge, and help identify potential solutions that would alleviate the negative consequences of these stressors.
Experiments 1 and 2 were conducted to evaluate lactation performance and inflammatory markers in multiparous, lactating Holstein cows during an AF challenge when two separate mitigation strategies were implemented: 1.) trace mineral injection, and 2.) dietary supplementation of aluminosilicate clay. In the first experiment, 58 Holstein cows were assigned to 1 of 3 treatments in a randomized complete block design: saline injection and no AF challenge (NEG), saline injection and AF challenge (POS), and trace mineral injection and AF challenge (MM). Cows in NEG had lower AF excretion in milk and greater 3.5% fat-corrected milk compared to cows in POS. Cows in POS had greater MUN and BUN than cows in MM. Liver concentrations of Se and Fe were greater for cows in MM compared to cows in POS. An upregulation of liver GPX1 was observed for cows in POS compared to cows in MM. In the second experiment, 16 multiparous, lactating, Holstein cows were assigned to 1 of 4 treatments in a replicated 4 × 4 Latin Square design with 21-d periods: no adsorbent and no AF challenge (CON), no adsorbent and an AF challenge (POS), 113 g (4 oz) of aluminosilicate clay (adsorbent) top-dressed on the total mixed ration (TMR) with an AF challenge (F4), or 227 g (8 oz) of adsorbent with an AF challenge (F8). Fat-corrected milk increased as concentration of adsorbent in the diet increased. There was a decrease in milk aflatoxin M1 (AFM1) concentration at d 18 as concentration of adsorbent in the diet increased. Cows in CON had greater concentrations of serum cholesterol and plasma superoxide dismutase compared to cows in POS. The expression of NFKB1 was greater in liver of cows in POS compared to cows in CON. The expression of mTOR was greater in the liver of cows in CON compared to cows in POS. When compared with cows in CON, cows in POS had greater odds ratio for hepatocyte inflammation.
Experiments 3 and 4 were companion studies aimed at evaluating the effects of a commercially-available rumen-protected methionine (RPM) source fed to Holstein cows during a HS challenge on: 1.) lactation performance and physiological responses, and 2.) metabolic and inflammatory biomarkers in blood, as well as immunohistochemical parameters in mammary tissue. Thirty-two multiparous, lactating Holstein cows were randomly assigned to 1 of 2 dietary treatments [TMR with rumen-protected Met (RPM) or TMR without RPM (CON)], and within each dietary treatment group cows were randomly assigned to 1 of 2 environmental treatment groups in a split-plot crossover design. The study was divided into 2 periods with 2 phases per period. In phase 1, all cows were in thermoneutral conditions (TN) and fed ad libitum. In phase 2, group 1 was exposed to a HS challenge (HSC). Group 2 remained in TN but was pair-fed (PFTN) to HSC counterparts. After a 21-d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, while dietary treatments (RPM or CON) remained the same for each cow. Cows in HSC had a greater increase in vaginal temperature and respiration rate compared to cows in PFTN. Cows in PFTN had a greater decrease in DMI and milk yield compared to cows in HSC. Cows in CON had a greater decrease in milk protein concentration for PFTN and HSC compared to cows in RPM for PFTN and HSC. Cows in CON for HSC had greater decrease in milk fat concentration compared to cows in RPM for HSC. At 8 h post-feeding, cows in PFTN had a greater decrease in PUN compared to cows in HSC. At 4 h post-feeding, cows in HSC had a greater increase in plasma glucose compared to cows in PFTN. At 8 h post-feeding, cows in PFTN had a greater increase in plasma NEFA compared to cows in HSC. Cows in PFTN had a greater decrease in plasma insulin compared to cows in HSC at 4 and 8 h. Cows in PFTN had a greater increase in insulin sensitivity compared to cows in HSC at 4 and 8 h. Compared to cows in PFTN, cows in HSC had a greater increase in plasma SAA, serum haptoglobin, plasma LBP, and plasma IL-1β. Compared to cows in CON, cows in RPM had a lower percentage of apoptotic cells and a lower proportion of apoptotic to proliferating cells during HSC.
In conclusion, AF exposure hindered lactation performance, reduced liver function, and increased liver inflammation. Subcutaneous injection of trace minerals helped maintain an adequate antioxidant response in cows during AF challenge. Aluminosilicate clay had a positive impact on milk production and hepatocyte inflammation, while reducing AF transference. Furthermore, HSC altered physiological and production parameters, as well as metabolic and inflammatory biomarkers in blood. Rumen protected methionine feeding maintained milk protein and fat concentration, while aiding mammary cell preservation during a HS challenge.