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Nitrogen excretion and ammonia emissions in dairy cows fed low-N fresh grass and maize silage

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Abstract

Dairy cattle farms must limit their negative impacts and nitrogen (N) losses to the environment. In particular, they must decrease greenhouse gases and ammonia (NH3) emissions. Cow’s N excretion in faeces and urine and the related NH3 emissions are affected by diet intake and composition, especially by N intake. In many farms, particularly in spring and summer, fresh grass and conserved forage are fed together. The influence of these mixed diets on N excretion at the cow level is poorly known. This study aimed at quantifying the effect of 4 maize silage proportions in a fresh grass diet (0:100, 17:83, 34:66, 51:49 of maize silage:fresh grass ratio, DM basis) on the N excretion in faeces and urine and on the total ammonia N (TAN) excretion in slurry, at the cow level. For this, 7 Holstein cows were offered the 4 dietary treatments according to a Latin square design, with individual measurements of intake, urine and faeces amounts, and their N concentration. The TAN concentration was analysed on slurry reconstituted from a urine and faeces mixture, in proportion to their production. Potential NH3 emissions at barn level were estimated from the TAN excretion multiplied by the 0.24 emission factor for cattle housing (EMP/EEA national inventory guidelines). The unusual low grass crude protein (CP) concentration involved very low diet CP concentrations ranging between 107 and 86 g/kg DM. Dry matter intake decreased with increasing maize proportion, from 15.7 to 12.4 kg DM/d, along with the milk production, from 16.4 to 13.2 kg/d, and the N exported in milk, from 90 to 68 g/d. The N excretion decreased linearly from 172 to 119 g/d, with increasing maize proportion, with similar urinary N proportion in N excreted between diets (39%). The TAN excretion in slurry decreased with increasing maize proportion from 34.7 to 21.0 g/d, as the related NH3 estimated emissions. The TAN excretion in reconstituted slurry corresponded to 19% of the N excreted in faeces and urine, which is widely outlying from the 60% commonly used in the EMEP/EEA methodology. This study suggests that the emission estimations from national inventory guidelines should better consider the variability in TAN excretion, which is the primary factor that influences NH3 emissions, especially for poor N-diets.
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Dates and versions

hal-03938337 , version 1 (13-01-2023)

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  • HAL Id : hal-03938337 , version 1

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Manon Ferreira, Remy Delagarde, Nadège Edouard. Nitrogen excretion and ammonia emissions in dairy cows fed low-N fresh grass and maize silage. 73. Annual meeting of the european federation of animal science (EAAP), EAAP, Sep 2022, Porto, Portugal. pp.315. ⟨hal-03938337⟩

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