Male involvement in fertility and factors affecting semen quality in bulls

Fertility varies substantially among bulls. In general, methods to predict fertility are better for identifying bulls with low fertility than for ranking bulls with good to excellent fertility. Compensable sperm abnormalities can be overcome by increasing the dose used for artificial insemination; these are attributed to sperm reaching and penetrating the zona pellucida. In contrast, increasing the insemination dose does not improve fertility for uncompensable defects, implying that the sperm are able to cause fertilization and initiate development, but they do not sustain embryogenesis. Bull testes must be 2 to 6°C cooler than core body temperature for fertile sperm; consequently, increased testicular temperature reduces semen quality. Increased nutrition before 30 weeks of age increased luteinizing hormone pulse frequency, hastened puberty, and increased testicular size at maturity in bulls. However, attempts to correct nutritional deficiencies present during calfhood by supplemental nutrition later in life were unsuccessful.