Efficient exploitation of grass for cattle requires the application of appropriated grassland management strategies to maximize pasture dry matter intake (PDMI) while maintaining high sward quality by allocating cows to an adequate pasture allowance (PA). The aim of this study was to investigate the effect of two PA, low (L) vs. high (H), on sward quality, PDMI and milk performance of Holstein-Friesian cows (n=72) at two lactation stages (LS), early (E) vs. middle (M). Cows were randomly assigned to four groups (LE, LM, HE and HM) in a 2×2 factorial design. The low PA treatments completed 5 rotations, with more grazing days (+13), lower pre- (14.3) and post-grazing (5.3) sward heights (cm) and higher herbage utilization (81.7%). The low PA had higher stocking rate than the high PA treatments (4.8 vs. 3.9 cows ha-1)and lower PDMI (13.4 kg vs. 14.4 DM cow-1 day-1) with higher (p<0.05) (g kg-1 DM) protein (149.1), carbohydrates (166.7) and digestibility (746.8) while lower (p<0.05) DM (18.4%), acid (283.5) and neutral detergent (508.6) fibers (g kg-1 DM). Sward quality is deteriorated (p<0.05) from rotation 1 to 5, lower decrease was found on low PA treatments. Milk yield (MY) was higher (p<0.05) in cows at early LS (24.8 kg cow-1 day-1), with the lowest (p<0.05) protein (28.9) and fat (36.9) content (g kg-1). No differences were found on MY between PA treatments, but higher protein and fat content were observed on low PA. Decreasing PA resulted in higher sward and milk quality without penalizing MY.

The aim of this study was to investigate the effect of different feeding proportions of forage -grazing vs. silageon milk fatty acids (FA) profile and conjugated linoleic acid (CLA) content of autumn calving Holstein-Friesian cows (n = 61) at CIAM (Galicia, NW Spain). Three treatments (S, 100% silage; G/S, 50% grazing + 50% silage; G, 100% grazing) were set and milk FA profile of dairy cows was determined by gas chromatography-mass spectrometry. The G group showed a decrease in short (p < 0.05) and medium chain FA (p < 0.001), with an increase in long chain FA (p < 0.001) in comparison to the G/S and S groups, which showed the lowest levels (p < 0.001) of mono-and polyunsaturated FA. The CLA content in milk fat increased (p < 0.001) linearly in relation to the increased proportion of fresh grass in the diet of dairy cows from 0.49 and 0.82 to 1.14 g/100 g FA for the treatments S, G/S and G, respectively. During spring and summer, the levels of CLA were three times higher (p < 0.001, +0.76 g/100 g FA) in milk from dairy cows at the G group than in cows at the S group and twice higher (p < 0.001, +0.40 g/100 g FA) than in cows at the G/S group. High proportion of grass in the diet of cows increased CLA content, with the highest levels of unsaturated FA and the lowest levels of saturated FA, increasing the added value of milk on grazing systems using available farm resources.Additional key words: dairy cattle; forage proportion; grass feeding value; milk fatty acids. ResumenComunicación corta. Efecto de la fuente de forraje (pasto vs. ensilado) sobre el contenido de ácido linoleico conjugado en la grasa láctea de vacas Holstein-Friesian en Galicia (NO España)El objetivo de este estudio fue investigar el efecto de utilizar diferentes proporciones de forraje, pasto vs. silo, en el perfil de ácidos grasos (AG) de la leche y en el contenido de ácido linoleico conjugado (CLA) en vacas lecheras HolsteinFriesian (n = 61) de partos de otoño. Se establecieron tres tratamientos (S, 100% ensilaje; G/S, 50% pastoreo y 50% ensilaje; G, 100% pastoreo) y se determinó el perfil de AG de la leche por cromatografía de gases-espectrometría de masas. El grupo G mostró una disminución en los AG de cadena corta (p < 0,05) y media (p < 0,001), con un aumento en los AG de cadena larga (p < 0,001) en comparación con los grupos G/S y S, que mostraron unos contenidos menores (p < 0,001) de AG mono-y poli-insaturados. El contenido de CLA en la grasa láctea aumentó linealmente con el incremento en la proporción de pasto fresco en la dieta de las vacas lecheras desde 0,49 y 0,82 a 1,14 g/100 g de AG para los tratamientos S, G/S y G, respectivamente. Durante la primavera y el verano, los niveles de CLA fueron tres veces superiores (p < 0,001, +0,76 g/100 g de AG) en la leche de las vacas del grupo G respecto a la del grupo S y dos veces superiores (p < 0,001, +0,40 g/100 g de AG) a la del grupo G/S. Una alta proporción de pasto en la dieta incrementó el contenido de CLA, con niveles superiores de AG insaturados y menores de saturados, lo...