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Effect of dietary protein level on nitrogen excretion in dry cows

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Nitrogen (N) utilisation in dairy cattle is generally low and thereby results in potential downstream ammonia and nitrous oxide emissions as well as nitrate leaching. Despite this, only few studies have investigated N excretion in dry cows. The objective of the present study was to examine the effect of protein level in the diet on N excretion in dry cows and subsequently to develop a simple equation to predict N excretion based on nutrient intake parameters. In addition, the aim was to investigate whether a low diet crude protein (CP) concentration is sufficient to fulfil N requirements of dry cows. Twelve Danish Holstein dry cows were fed two total mixed ration (TMR) diets both containing grass-clover and maize silage, but with different ratios of rapeseed cake and dried sugar beet pulp to vary the concentration of CP, 111 and 135 g/kg of DM, respectively. Cows were randomly allocated to one of the two dietary treatments in a 2 × 2 Latin square design with six replications in early and late dry period, both lasting for two weeks. In both periods, the first eleven days were used as adaptation, while the last three days were used for sampling and recording. Feed and water intake were recorded daily, and blood, faeces and urine samples were collected and analysed for N, nutrients and other compounds. Faecal output of N was not affected by increased dietary CP level, whereas urinary N output increased with increased CP level in the diet, indicating that excess N is mainly excreted in urine. Thus, urinary N output appears to be strongly correlated with CP intake in dry cows. A dietary CP level of 111 g/kg of DM seems sufficient to meet N requirements of dry cows, underpinning the potential of reducing N excretion by dietary manipulation. Using the Lucas principle, equations predicting N excretion in faeces and urine, respectively, were established for dry cows based on dry matter intake (DMI) and N intake, and the equations were considered reliable proxies for N excretion in dairy cattle during the dry period.
TidsskriftLivestock Science
Antal sider9
StatusUdgivet - aug. 2022

Bibliografisk note

Funding Information:
The authors acknowledge Torkild Nyholm Jacobsen, Amin Turki Aljundi and Ester Bjerregaard (AU Foulum, Tjele, Denmark) for their assistance in sample collection. Further, we would like to thank technicians for handling of animals and running the chemical analyses.

Funding Information:
The experiment was conducted as a part of the project “Development of a new model to quantify the excretion of nitrogen in urine and faeces in the cattle production”, and was financially supported by the Danish Milk Levy Fund (Mælkeafgiftsfonden), the Danish Cattle Levy Fund (Kvægafgiftsfonden) and Aarhus University.

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© 2022

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