Comparison of three gradients of grazed grass in dairy cows diet in terms of environmental and zootechnical performances

Climatic change induces challenges in grazing management, which could tempt farmers to keep their cows indoors. To assess the environmental and economic impact of diets with different percentages of grazed grass, thirty-three Holstein cows in early lactation were divided into three groups from 27 April to 7 July 2018. These groups were allocated an increasing proportion of grazed grass in their diet. No access to grazed grass was possible for Group 1 (0%), while Group 2 and 3 were granted access to pasture 21w. Group 2’s (100%) diet was composed of 100% grass. Group 3 (50%) received silage in the barn as well as grazed grass. The access to pasture was adapted to achieve a proportion of 50% grass in the diet. Sward height was measured every week with an electronic rising plate meter (EC 20®), and the nutritional composition of grazed grass was evaluated. All the groups’ diet was complemented with concentrates delivered by the automatic concentrate supplier, where the Guardian® was located in order to measure the methane emitted at each visit. Methane emissions were also assessed by predictions based on the mid infra-red (MIR) spectrum of milk samples. Animal performance was recorded and the milk carbon footprint was estimated by the Feedprint®. No difference in milk yield between the groups was recorded. Predictions based on the MIR spectra analysis showed a slight decrease in methane emission per cow and per day in the 100% group, but this decrease was not confirmed by the breath samples measurements. The feeding costs were in favour of the 100% group. The carbon-footprint of the milk produced with 100% or 50% of grazed grass was lower than for the zero-grazing cows.