Water-soluble carbohydrate (WSC)-rich pasture or alfalfa hay, with high organic matter digestibility (OMd), are known to increase the voluntary dry matter intake (DMI) of dairy cows and diet digestibility, often resulting in higher dairy performance. However, little is known about the effect of highly digestible hays from botanically diversified natural grasslands, rich in WSC and their interactions with concentrate type on dairy performance. The present trial tested the effect on dairy cow performance of dietary inclusion of hay of greater digestibility and WSC content (high OMd, WSC-rich or low OMd, WSC-poor) from botanically diversified natural grasslands and its possible interactions with concentrate type with low or high starch degradation rate in cow breeds with different production potential. Twenty-eight Holstein and 28 Montbéliarde cows after the lactation peak were randomly allocated to four equivalent groups of 14 cows each. Cows were fed for 6 weeks with 5 kg DM/day of concentrate, 3 kg DM/day of the same 2nd cut hay and two different types of 1st cut hay (ad libitum). Two groups received the 1st cut hay characterised by a high (A) WSC content and high OMd and the two other groups received the 1st cut hay (B) with low WSC content and low OMd. One group per hay type received a wheat-based concentrate, with a high starch degradation rate and the other group received a maize-based concentrate, with a low starch degradation rate. Cows fed A hay diet ingested more WSC (+551 g/day) and had a higher WSC/CP ratio (+0.24) than B hay diet-fed cows. The resulting higher OM total tract apparent digestibility (+2.1%) of ingested diet improved milk yield (+1.4 kg/day), milk fat yield (+58 g/day) and feed conversion efficiency (+0.05 g milk /kg DMI) of cows fed A hay compared to B hay diets. The A hay diet also induced a lower milk urea (-91 mg/kg) and higher milk protein content (+1.1 g/kg) and yield (+59 g/day). The A hay diet milk was richer in de-novo synthesised fatty acids (FA) and poorer in FA derived from ruminal biohydrogenation of dietary polyunsaturated FA. The effect of concentrate type on dairy performance was not significant for either the A hay or B hay diets. Regardless of cow breed and concentrate type, including botanically diversified natural grasslands highly digestible and WSC-rich hay in cow diet improved milk yield, milk fat, and protein synthesis, and feed conversion efficiency.