Managing cows for extended lactations has been proposed to balance the genetic milk yield potential of modern dairy cows with the adverse effect of the negative energy balance on reproduction. However, due to the standard course of the lactation curve, extended lactation may decrease daily milk production per cow besides reducing the number of calves, heifers and dry cows per year. In turn, this reduces herd feed use and alters farm feed production requirement. Thus, the effect of an extended lactation strategy may depend on the system level of interest; i.e., cow, herd and farm or even sector. We modeled the effect of different strategies for extended lactation on herd productivity, efficiency and greenhouse gas emission by simulating herd dynamics, production and feed use with a herd model (www.simherd.com). Extended lactation was defined as calving intervals of either 15 or 17 mo and was applied only during first parity, only during second and higher parities or during all parities. The base scenario resembled an average Danish Holstein dairy herd with 10,500 kg energy-corrected milk (ECM) per cow per year and a calving interval of 13 mo. Extended lactation scenarios reduced milk production by 2 to 486 kg ECM and reduced meat production by 5 to 24% per cow per year. Herd feed intake was reduced by 1 to 7% per cow pear year, which led to a 1 to 3% increase in milk production per kg dry matter (DM) at herd level. However, excluding young stock feed intake changed the result to a 0 to 2% loss in kg ECM per kg DM. Including meat production showed that MJ milk and meat produced per kg herd DM intake increased 1 to 2%. Furthermore, extended lactation led to a reduction in feed growing area by 1 to 8%. Thus, MJ milk and meat produced per ha increased 1 to 2% across scenarios. Finally, this reduced the farm level emission from 1.06 to 1.02–1.05 kg CO₂-equivalents per kg ECM across scenarios. Utilizing the freed feed growing area to produce energy crops reduced emissions by 3 to 12%. However, the benefit would be less at sector level because of emissions from beef cattle raised to compensate for the lower meat production.