Effect of season on nutritive value of an alpine pasture

Pasture is the cheapest source of nutrients for dairy cows, but supplementation with energetic concentrate can balance a diet with low energy/nitrogen ratio at the beginning of the grazing season or a low energy content at the end of the grazing season. Aim of this study was to evaluate the effect of seasonal variation and botanical composition on nutritive value of an alpine pasture over 3 grazing seasons. In 22 ha alpine pasture (Prà Maslino, SO, 1650 m sl) most important botanical species (20) were sampled every 15 days through the 3 seasons (194 samples) and were analysed for chemical composition and Gas Production. Two types of botanical association included the most important part of pasture sward: Festuca rubra (47%) and Trifolium repens (44%), with 91 different species. Gramineae species had more DM content (38.6  12.1 % as fed), NDF (67.9  8.6 % on DM) and ADF (36.0  6.4 % on DM) but less CP (10.4  3.0 % on DM) than every other species (DM 27.7  10.5 % as fed; NDF 52.5  11.2 % on DM; ADF 33.8  7.2 % on DM; CP 15.0  5.4 % on DM). From June to September plant growth determined a decrease of quality, although there was a great variability for species and year of trial (average daily decrease of 0.5 g/kg DM for CP, average daily increase of 1.2 g/kg DM for NDF, while for gramineae species OM digestibility decreased 0.25 %/d and 0.18 %/d for other species). Multiple regressions of dOM on several analytical parameters were performed and the best significant equation for all samples was: dOM = 98.13 - 0.5305 DM (%) - 0.6071 ADF (% DM) (n=144; R2=0.63; RSD=7.69) According to the Cornell system, sward samples showed a high content of slow-degradation carbohydrate fraction “b2” (46.0  6.8 % CHO) and a high content of medium-degradation protein fraction “b2” (75.1  2.3 % CP), while the immediately-available carbohydrate fraction “a” was 22.3  1.2 % CHO, and the immediately-available protein fraction “a” was 19.1  2.3 % CP. These data suggest that grazing dairy cows need concentrates which takes into account the high protein content and fermentability of pasture at the beginning of the grazing season, and the low energy and protein content of pasture at the end of the grazing season.