ratio means the material is low in nitrogen. Alfalfa hay, which is high in nitrogen, has a C:N ratio of 18:1, whereas maralfalfa, which is low in nitrogen, has a C:N ratio of 19:1. Almost the same difference in C:N ratio was observed in the mixed diets A and M. It has previously been observed that animals prefer the higher nitrogen C3 plants to the C4 plants, as they have a lower C:N ratio (Lauder, 2000).
The main differences between diets were the greater fibre (aNDF and ADF) and lower NFC and ADL contents with diet M. The higher digestibility of M diet than A diet could also be related to the lower lignin content, as well as the lower DM intake, which could have slowed the ingesta transit along the digestive tract, thus improving digestibility. The average rumen pH never fell below 6.2, so the values obtained can be considered sufficiently high to maintain normal rumen fer- mentation (Ørskov and Fraser, 1975). Despite a higher aNDF content of diet M, rumen propionic acid was greater in this diet (17.5 mol/100 mol vs. 15.6 mol/100 mol, for M and A respectively) due to the lower forage/concentrate ratio of the ingested M diet. Other VFAs related with lipid metabolism such as isovaleric, n-valeric and n-caproic were greater in A diet compared to M. In the study of Sosa et al. (2006), maralfalfa was the control diet and the treatments were partial replacement of maralfalfa with maize grain or molasses. Therefore, as almost the whole diet was maralfalfa a lower DM digestibility was to be expected, compared to the results from our study, where maralfalfa accounted for around 40% of the dry matter intake.