Demand for organic milk has grown steadily worldwide, thus creating a niche market opportunity for farmers to transition from conventional to organic certified production. However, surplus of organic milk in the last 2–3 years in the US drove prices down and shrank farmer profitability throughout the country. Currently, transition to organic dairy production has slowed down and the only growing segment of the industry is the “grass-fed” milk market. Farmers shipping “grass-fed” milk are not allowed to feed any source of grain or corn silage with molasses as the sole energy supplement permitted by certifiers and milk processors. While this production system brings challenges including losses in milk yield (MY) and the need for increased forage production, it pays additional premiums that may increase farm profitability. The objective of this paper is to provide data on feed efficiency (FE) using the University of New Hampshire Organic Dairy Research Farm as a case study. Surveys from commercial midwestern and northeastern US organic dairies were used for modeling the impact of “grass-fed” diets on greenhouse gas (GHG) output. Results from 14 experiments (7 published and 7 unpublished) using Jersey cows in confinement (n = 8 studies) or grazing management (n = 6 studies) showed (mean ± SD) DMI of 17.5 ± 2.05 kg/d, MY of 18.3 ± 3.7 kg/d, and ECM of 22.6 ± 4.57 kg/d. Feed efficiency expressed as MY/DMI or ECM/DMI averaged 1.05 ± 0.17 and 1.29 ± 0.20 kg/kg, respectively. Lowered production and FE compared with typical conventional dairies in the US may be related to the use of high forage rations, inclusion of pasture in the diets, increased energy expenditure for grazing, and the lack of digestible forage sources such as corn silage. In fact, our modeling work showed that GHG emissions decreased approximately 2-fold in traditional vs. “grass-fed” organic dairies in part due to the use of corn silage in the diets. In conclusion, MY and FE are lower in organic than conventional dairies. Organic dairy farmers, particularly those shipping “grass-fed” milk should improve forage quality and grazing management and optimize the use of molasses.