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Surplus wastewater accumulates in winter and requires a discharging solution. In our study we examined the effects of using treated wastewater (TWW) as supplemental pasture irrigation in winter. To test the effect of TWW irrigation on the vegetation and soil, 10 grazing prevention exclosures were established at Ramat‐Hanadiv Park, Israel, with half of them irrigated with TWW. Soil and vegetation were sampled during 2 years of irrigation and 1 year after cessation of irrigation. Vegetation composition, species richness, and productivity were sampled at the peak of the growing season and various indices of herbage quality were measured. Our results show that irrigation with wastewater increased herbage production by an average of 38% and total pasture protein production by 9% relative to the control subplots. Conversely, TWW irrigation decreased plant protein content while increasing herbage fibre content and lowering its digestibility. Over a period of 2 years, species richness decreased in the irrigated subplots. Most of the soil indices examined were not affected by irrigation, but in the irrigation subplots, sodium values and the sodium adsorption‐ratio were higher than in areas without irrigation. Additional resources in winter enabled the plants to grow rapidly and increased interplant competition. Changes in vegetation composition were revealed through functional group reshuffling and species richness decline. The results of our study show that wastewater irrigation causes an increase in productivity but a decrease in forage quality. At the pastoral system level, the significant increase in productivity leads to increased availability of protein and herbage for animals. [ABSTRACT FROM AUTHOR]