Fertility in dairy cows and ways to improve it
The fertility of dairy cows affects the genetic improvement and financial sustainability of dairy herds. Fertility is a complex trait that is affected by several factors. Genetically, fertility is difficult to improve because of low heritability. Cows that do not become pregnant are usually culled from the herd. This paper reviews results from studies conducted in South Africa that are aimed at improving the reproductive performance of dairy cows. Records from 9 046 cows in 14 Holstein herds showed that, while lactation number, calving year and calving season affected fertility traits significantly, herd management had the largest effect. Mean ± sd for calving to first service (CFS) and from calving to conception (DO) intervals were 77 ± 30 and 134 ± 74 days, respectively. The number of services per conception (SPC) was 2.55 ± 1.79. The proportion of first services within 80 days post partum and cows confirmed pregnant within 100 and 200 days post partum were 0.64 ± 0.48, 0.36 ± 0.48, and 0.71 ± 0.45, respectively. Heritability (h²) estimates were 0.06, 0.08 and 0.07 for CFS, DO, and SPC, respectively. Albeit low h² estimates were consistent with literature results, the genetic correlation between CFS and DO was positive (0.56), and negative (-0.29) between CFS and pregnancy success. Crossbreeding, using a dual-purpose breed, improved fertility, similar to studies conducted overseas. Increasing the energy content of the total diet of Holstein cows on kikuyu-ryegrass pasture by feeding 7 kg versus 12 kg concentrates/cow/day, improved fertility as a higher proportion of cows were pregnant by 150 days in milk, being 0.52 versus 0.84 and 0.56 versus 0.76 for primi- and multiparous cows, respectively. Estimating breeding values for fertility traits for breeding sires would assist in improving fertility in dairy cows.
Keywords: Concentrate level, crossbreeding, days open, energy source, interval traits, genetic parameters, management-induced infertility