A Retrospective Analysis of Factors Affecting Postpartum Fertility in Egyptian Dairy Cows: Milk Yield, Season and Parity

Subfertility in postpartum dairy cows represents a major constraint on productivity in intensive systems, particularly in subtropical regions like Egypt, where heat stress compounds metabolic challenges from high milk production. This retrospective cohort study analyzed 8,994 lactation records from 5,415 Holstein cows from a randomly chosen commercial farm in Ismailia Governorate (2021–2024) to quantify the independent and interactive effects of 305-day milk yield (high: >7,000 kg; low: <3,000 kg; medium: 3,000–7,000 kg), season of parturition (spring, summer, autumn, winter), and parity (early: 1–2; late: >5; optimal: 3–5) on reproductive performance. Generalized Linear Models and MANOVA, adjusted for farm, year, body condition score, and postpartum disorders, confirmed significant main effects (P < 0.01). High-yield cows exhibited the longest days open (DO: 185.6 ± 78.4 d) and highest services per conception (S/C: 4.0), versus medium-yield cows (DO: 148.3 d; S/C: 2.8). Summer calving increased DO to 175.3 ± 75.1 d and reduced pregnancy rate by 120 DIM (PREG120) to 20.1%, with 1.7-fold higher metritis/retained placenta. Late-parity cows showed poorest metrics (DO: 197.8 ± 82.3 d; S/C: 4.2). Findings underscore a persistent production–fertility antagonism and heat-stress vulnerability. Targeted interventions, enhanced summer cooling, parity-specific nutrition to mitigate negative energy balance, and rigorous postpartum monitoring, are recommended to shorten DO, improve conception efficiency, and enhance herd profitability and welfare.