Abstract:
This study investigated the effects of MUN and climatological factors on the inter calving
period (ICP), reproductive performance (RP%), and reproductive index (RI) in Holstein Friesian
(n = 1177) and Jersey cows (n = 3305) in different seasons in the subtropics. Threshold values for
MUN on the reproduction of dairy cows in the subtropics remain controversial due to complex
environmental interactions, especially with high environmental temperatures. A retrospective
analysis was conducted of data obtained from the National Milk Recording scheme of the Agricultural
Research Council (ARC) in South Africa. The results confirm that MUN influences the reproduction
of dairy cows in the subtropics. MUN concentrations exceeding 18.1 4.28 mg/dL in Holstein
Friesian cows and 13.0 4.70 mg/dL in Jersey cows extended the inter calving period (ICP), and
decreased RP% and RI. Jersey cows have a lower threshold MUN concentration compared to Holstein
Friesian cows, but they are not adversely affected by high humidity or temperatures, while Holstein
Friesian cows are.
