Substituting a pasture-based diet with plantain (Plantago lanceolata) reduces nitrogen excreted in urine from dairy cows in late lactation

•Urine N concentration decreases non-linearly as percentage of plantain in the diet of dairy cows increases.•Dietary N is partitioned away from urine to into milk and faeces when plantain is fed to dairy cows.•30% plantain in the diet increases cumulative urine volume through more frequent urination events.•Increased urine volume is associated with water intake in feed. Recent research has demonstrated that grazing cows fed diets including plantain (Plantago lanceolata) express reduced urinary nitrogen (UN) concentration relative to cows fed perennial ryegrass, while milk production is similar or improved. This study explored the effect of increased substitution of ryegrass-based pasture with plantain for cows fed ad libitum in late lactation on intake, nitrogen (N) excretion and partitioning. Sixteen mixed age (4.1 ± 0.5 y) multiparous Holstein-Friesian cows were randomly allocated to four dietary treatments, which comprised ryegrass only (RG), or ryegrass substituted with 15 (15PL), 30 (30PL) and 45% (45PL) plantain. In an incomplete cross over design cows were adapted to a dietary treatment over 14 days prior to a six-day measurement period, then reallocated to another dietary treatment for a second six-day measurement period following 14 days adaptation. Cows offered 45PL consumed 2.6 kg DM/day more than cows fed RG (P = 0.021). Milk and milk solids yield were greater (P < 0.01) when plantain comprised 30% of the diet or more. Daily mean urinary N (UN) concentration followed a quadratic response to increasing plantain content (R2 = 0.62; P < 0.001). Cows fed 30PL and 45PL excreted urine with 19 and 41% less N (g/100 g) respectively, relative to cows fed RG or 15PL. The mechanisms for reduced UN concentration of cows fed diets including plantain included dilution of urine through greater total daily urine volume and alteration of dietary N partitioning. Cows fed 45PL produced 9 L (~25%) more urine per day than cows on other diets; but between 0800 and 1600 h there was a progressive increase in the cumulative volume of urine as the percentage of plantain in the diet increased, due to increased urination frequency rather than individual urination volume which was similar between diets. Regression analyses indicated that cumulative urine volume was correlated with water intake (r = 0.61; P < 0.001), largely from water in the feed of the plantain diets. Nitrogen intake was similar across diets (545 ± 23 g N/cow/day) but N output in urine was reduced by 39 a