Development of nitrogen and methane losses in the first eight weeks of lactation in Holstein cows subjected to deficiency of utilisable crude protein under restrictive feeding conditions

Low-protein diets are increasingly being used in dairy cow nutrition to minimise noxious nitrogen (N) emissions. However, at parturition, the lower milk yield at that time may mask deficiency in dietary utilisable crude protein (uCP; equivalent to metabolisable protein). Under restrictive feeding conditions, farmers would limit the feed allowance to match the lower measured milk yield, thereby exacerbating the deficiency. The consequences for N emission intensity per kg milk yield and methane emissions are unknown. In this study, two diets were fed to nine Holstein cows each from parturition onwards. One diet was complete and the other was calculated as 20% deficient in uCP. Feed allowance was always oriented towards the measured milk yield. In each of the first eight lactation weeks, intake and excretion were measured for 5 d. On the last 2 d of this period, methane emission was measured in respiration chambers. The statistical model included treatment, week and interaction as effects. The real levels of uCP and energy supply across the 8 weeks were 33% and 15% below requirements, respectively, in the Deficient cows. In addition, the Deficient cows consumed 18% less dry matter (caused by substantial refusals in week 1, where energy supply was according to requirements) and produced 25% less milk (26 vs. 34 kg/d). Cows in both groups used dietary N with similar efficiency for milk protein synthesis and excreted similar proportions of the N ingested via urine and faeces. This resulted in both treatments having similar N emission intensities per kg milk N and similar urinary N as a proportion of total excreta N, suggesting a similar potential for gaseous N emissions from the manure per kg of milk. The Deficient cows emitted 22% less methane overall but had similar methane yield and emission intensity to the Controls. In conclusion, a reduction in crude protein intake immediately after parturition does not reduce N emission per unit of milk when associated with uCP deficiency.