Discipline: Reproduction Keywords: MUN, urea, reproduction, season, subtropics

Conception and re-conception rates of high-producing dairy cows remain of concern in dairy production systems, especially in the subtropics. Apart from the major effects of nutrition and specific nutrients on reproduction, other risk factors such as negative energy balance, inflammation and impairment of the immune response have been shown to affect re-conception rates. Dietary energy levels interact with blood urea nitrogen (BUN) and may influence the pregnancy rates of dairy cows. Milk urea nitrogen (MUN) (as proxy for BUN) provides an estimation of the level of N loss after the absorption of ammonia from the rumen. In terms of reproduction, conception rates of first service dairy cows are negatively associated with milk production and MUN, whereas high concentrations of MUN at the time of insemination is a contributing factor to higher risk pregnancy failures. Also, seasonal variations in MUN influence cow reproduction, whereas the negative interaction between heat stress and high MUN concentrations will reduce the reproduction potential of cows. However, since the threshold values for MUN and the related effects on the reproduction of dairy cows in the subtropics are not well documented, and extreme temperature and humidity indices do occur in South Africa, the objective of the study by the authors cited below was to investigate the influence of MUN and season on the reproduction of Holstein Friesian and Jersey cows; the Gauteng area being the study site.

The effects of MUN and climatological factors on inter-calving period, reproduction performance, and reproductive index in TMR system Holstein Friesian (n = 1177) and Jersey cows (n = 3305) in different seasons were investigated. A retrospective analysis was conducted of data obtained from the National Milk Recording scheme of the ARC.

The results confirm that MUN influences the reproduction of dairy cows. MUN concentrations exceeding about 18.1 mg/dL in the Holstein Friesian cows and 13.0 mg/dL in the Jersey cows extended the inter-calving period, and decreased reproduction performance and the reproductive index. The Holstein Friesian and Jersey cows took longer to conceive as the concentration of MUN increased above these threshold values. The reproduction performance of the Holstein Friesian cows was compromised by high relative humidity (>54.3%) and high maximum temperatures (>24.8oC), whereas the Jersey cows were adversely affected by minimum daily temperatures below 9.4oC, but not adversely by high humidity or temperature.

It is suggested that sustainable strategies to reduce heat stress and manage MUN concentrations effectively, should be considered important to improve reproduction efficiency in South Africa and other sub-tropical countries.


Webb, E.C. & de Bruyn, E., 2021. Effects of Milk Urea Nitrogen (MUN) and Climatological Factors on Reproduction Efficiency of Holstein Friesian and Jersey Cows in the Subtropics. Animals 2021, 11, 3068. ani11113068