Abstract
Background:
Heat stress (HS) is a major environmental challenge that significantly affects the reproductive health of livestock.

Aim:
This study aimed to explore the efficacy of N-acetyl-L-cysteine (NAC) or Origanum vulgare oil (OV) on the physiological attributes, semen traits, and blood biochemistry of heat-stressed goat bucks.

Methods:
A total of 24 goat bucks aged about 2-2.5 years and weighing 56.3±1.34 kg were divided into 3 groups. The bucks were fed a basal diet or supplemented with NAC (0.3 g/animal/day) or OV (1 mL/animal/day) for 3 months during the summer season. The Temperature-Humidity Index (THI) was 88.44±2.18, indicating severe heat stress.

Results:
The treatments did not have statistically significant effects on rectal temperature, respiratory rate, skin temperature, and pulse rate (P > 0.05). Both OV and NAC treatments significantly improved total protein concentrations relative to controls (P < 0.05). Albumin levels were enhanced only in the NAC group compared with the other groups (P<0.05). Additionally, oral administration of OV or NAC significantly reduced cholesterol and triglyceride concentrations, blood urea levels, and liver enzymes (ALT, AST) in stressed goats (P<0.05). Stressed bucks treated with NAC had lower cortisol levels compared to the control group (P<0.01) and higher testosterone levels (P<0.05). NAC supplementation also had superior effects on ejaculate volume (mL), wave motion (Score), sperm concentration (10⁹/mL), sperm motility (%), and total sperm output (106/ mL) compared with OV and other groups (P<0.01). Dead spermatozoa (%) and abnormal spermatozoa (%) were significantly reduced in all treated groups, with the lowest values observed in the NAC group (P<0.01).

Conclusion:
N-acetylcysteine improved the metabolic profiles and testosterone levels of heat-stressed bucks, resulting in superior semen quality and demonstrating its potential as an antistress modulator.

Key words: Blood biochemistry; Goat; N-Acetyl Cysteine; Origanum vulgare oil; Semen.