Supplementation of urea to a basal pasture diet fed to dairy cows to model N-partitioning relationships

S J Hendriks; N Lopez-Villalobos; A J Sheahan; D J Donaghy; J R Roche

doi:10.3168/jdsc.2020-0016

Supplementation of urea to a basal pasture diet fed to dairy cows to model N-partitioning relationships

JDS Commun. 2020 Dec 11;2(1):21-26. doi: 10.3168/jdsc.2020-0016. eCollection 2021 Jan.

Authors

S J Hendriks¹, N Lopez-Villalobos¹, A J Sheahan², D J Donaghy¹, J R Roche^{2

3}

Affiliations

¹ School of Agriculture and Environment, Massey University, Palmerston North 4442, New Zealand.
² DairyNZ Ltd., Hamilton 3240, New Zealand.
³ School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand.

Abstract

The main objective of this study was to investigate whether altering dietary crude protein (CP) through the supplementation of urea to a basal pasture diet fed to dairy cows accurately modeled N-partitioning relationships. To test this, we first needed to establish safe tolerance levels for urea in this setting. Fifteen multiparous, rumen-fistulated, mid-lactation Holstein-Friesian dairy cows were offered spring pasture (~20 kg of dry matter/cow per day) and allocated to 1 of 3 urea supplementation treatments: low N [0 g/d urea; 21% total dietary CP of dry matter (DM)], medium N (350 g/d urea; 26% total dietary CP of DM), or high N (690 g/d urea; 31% total dietary CP of DM), in a completely randomized design. The amount of urea provided daily increased gradually for all cows over a 21-d period, with target urea supplementation reached by d 21. Milk yield decreased linearly at a rate of 2.35 kg/100 g of urea intake when urea supplementation exceeded 350 g/d for 4 d (~2% of DM intake). Cows from the low- and medium-N treatments subsequently entered metabolism stalls from d 25 to 31 to collect urine, feces, and milk for total N collection. Estimated urinary N output (g/d) increased linearly with N intake (g/d), and the slope of the relationship (slope = 0.86; R² = 0.82) was consistent with international published results. Because of the consistency of our results with previously documented relationships, our findings indicate that supplementation of urea to a basal pasture diet is a suitable technique for modeling different N intakes from pasture diets to evaluate urinary N mitigation strategies. Urea supplementation, however, should not exceed ~2% of DM intake.