A quantitative description of the effect of breed, first calving age and feeding strategy on dairy systems enteric methane emission

This work aimed to merge the results of long-term experiment and quantify dairy cows and replacement heifer's enteric methane (CH4) emission and integrate this information at the dairy system. From 2006 to 2015, at INRA dairy research farm, an experiment was conducted to measure, over successive lactations, the effect of breed (Holstein, Ho; Normande, No) and first calving age (two or three years old) on the performance of distinct types of cows in two feeding strategies (FS; High or Low FS). The proportion and composition of feedstuffs used on each FS were measured and it was possible to estimate the heifers and dairy cows total dry matter (DM) intake and enteric CH4. The enteric CH4 was quantified by feeding level (DM intake per% BW), proportion of total intake concentrate and digestible organic matter (OM) in the total tract. All results obtained were integrated and represent eight dairy systems combining two breeds, two first calving age, two feeding strategies. Each dairy system sold the same amount of milk volume (400 t). Daily enteric CH4 was related to DM intake. Dairy cows CH4/head per year varied from 123 to 158 kg. On average, the relationship between enteric CH4 and DM intake was 21.5 g/kg DM intake. The enteric CH4 intensity (CH4 per kg of milk yield) decreased as dairy cow's productivity improved. The number of cows and heifers required for the same milk sold in different systems varied considerably. The heifers replacement rate and first calving age had a strong effect on the number of dairy cows and replacements heifers, and consequently on enteric CH4 emission. On average, the Holstein herd emitted 2.5 t/year of enteric CH4 less than the Normande herd. Reducing first calving age on average decreased dairy systems enteric CH4 by 2.2 t per year. Dairy system annual enteric CH4 emission increased from 13.1 t to 17.3 t in moving from the High to Low FS. Our results at the dairy system level represent an opportunity to develop mitigation strategies. Feeding strategy, breed, first calving age and replacement rate showed potential to reduce enteric CH4 emissions mainly by modifying the number of dairy cows and replacements heifers in the herd.