Milk urea concentration and nitrogen use efficiency of dairy cows grazing different levels of forage plantain with ryegrass-white clover

Abstract

Context: Milk urea (MU) is a by-product of crude protein metabolism in dairy cattle whereby diets with lower nitrogen (N) utilisation are typically associated with greater MU concentrations. Thus, MU is recognised as a useful indicator of dietary N use efficiency (milk N divided by N intake; NUE). Forage plantain has been shown to decrease MU, potentially due to its lower content of ruminal degradable protein compared with ryegrass-white clover (RGWC). However, there are limited data on how including plantain in a farm system affects MU concentration and NUE in dairy cattle throughout an entire lactation period. Aims: In this study, we investigated the effects of different levels of forage plantain in the farm system on MU and NUE. Methods: Data were collected over two dairy production years (2021/22 and 2022/23) as part of a farm system study established in late July 2021 at Lincoln University Research Dairy Farm. Nine herds (12 cows each) were randomly allocated into one of three replicated pasture treatments (n=3) sown with an increasing plantain (cv. Ecotain) seed rate: (i) RGWC with nil plantain (PL0); (ii) RGWC+3 kg/ha plantain (PL3); or (iii) RGWC+6 kg/ha plantain (PL6). Individual cow milk volume was measured at each milking (DelPro, DeLaval), with milk composition determined fortnightly (CRV, Hamilton, New Zealand) using samples collected at consecutive afternoon and morning milkings. Fortnightly pre-grazing pasture samples, coinciding with milk samples, were collected to determine botanical and chemical composition. Dry matter (DM) intake was back-calculated from the energy requirements for cows in each herd. Statistical analysis was conducted using General Analysis of Variance with pasture treatment, season of the year and production year included as factors, and farmlet as the experimental unit. Key results: There was an interaction between treatment × season for plantain % in diet DM, N intake, diet N:ME ratio and NUE, and between treatment × season × year for N intake and diet N:ME ratio. Regardless of season and year effects, average dietary content (% DM) of plantain leaves was 0.1% in PL0, 14.7% in PL3 and 17.7% in PL6. Average estimated daily intake of pasture (14.4 kg DM/cow), supplement (baleage; 1.2 kg DM/cow) and N (476 g/cow) were similar across treatments. Average daily milksolids production per cow was similar between PL0 (1.65 kg) and PL3, and PL0 and PL6, but higher (P=0.015) in PL6 than PL3 (1.66 vs 1.61 kg). Average MU concentration was higher in PL0 than PL3 and PL6 (P<0.001), which did not significantly differ from each other (Table 1). The two-way interaction effect showed that NUE was lower in PL3 than PL0 and PL6 in late-winter, similar between treatments in spring, higher in PL6 than PL3 in summer and higher in PL6 than PL0 in autumn. Conclusions: While the effect of plantain incorporation into RGWC- based dairy systems at either 14.7 or 17.7% of diet DM resulted in a seasonally dependent response in cows' NUE, it consistently reduced their MU concentration. Implications: The presence of forage plantain may need to be considered if MU concentrations are used as indicators to help manage herd nutritional status and environmental risk.