This study investigated the effects of protein supply on milk production in a herbage-based feeding system. We assessed 138 lactation records from 94 Holstein and Swiss Fleckvieh cows on an organic farm across 3.5 years. During winter, the basal ration consisted of hay (kg dry matter [DM]: 111 g crude protein [CP] and 5.2 MJ net energy lactation [NEL]), and grazing started in late March (kg DM: 178 g CP and 6.1 MJ NEL). On average, the cows consumed 333 kg of concentrate per lactation (as fed). The cows were paired by breed, parity, and calving date and randomly assigned to a low-protein (LOPRO; kg DM: 137 g CP and 7.8 MJ NEL) or a higher-protein (HIPRO; kg DM: 221 g CP and 8.0 MJ NEL) concentrate treatments. Calving occurred from late December to early May. Milk recordings and reproductive parameters were analysed using linear mixed-effects models, and health treatment counts were analysed using Poisson tests. Only in early lactation did LOPRO cows produce less energy-corrected milk (26.8 vs. 28.5 kg/d; p = 0.01) and have lower milk fat (38.6 vs. 40.0 g/kg; p = 0.049) and milk urea (16.1 vs. 20.6 mg/dL; p < 0.001) concentrations than HIPRO cows. Although calving-to-first-insemination intervals were similar (p = 0.18), LOPRO cows showed poorer reproductive performance. No differences in health treatments were observed between the feeding treatments. These findings highlight a trade-off between protein supply for performance and reproduction and increased nitrogen excretion, as reflected in milk urea concentrations.