[en] To maximize milk output and milk solids per hectare from grazing dairy cows it is necessary to identify the level of grazing swards in order to achieve high pasture dry matter intake (PDMI). The aim of this study was to investigate the effect of pre-grazing herbage mass (HM) and daily herbage allowance (DHA) on perennial ryegrass (Lolium perenne L.) swards structure, PDMI and milk performance. Sixty-four spring-calving Holstein-Friesian dairy cows were balanced and randomly assigned to one of four (n = 16) grazing groups (LL, LH, HL and HH) in a 2 × 2 factorial design. The treatments consisted on two pre-grazing HM, low (L- 1,600 kg DM ha–1) or high (H- 2,400 kg DM ha–1) and two DHA, low (L- 15 kg DM cow–1day–1) or high (H- 20 kg DM cow–1 day–1). Swards structure, PDMI and milk performance were examined across the grazing season. The low HM groups completed 9.5 rotations of average 22 days length compared to 6.5 rotations of 32 days for the high HM groups. Herbage utilization was greater (p &lt; 0.001) when animals offered the low DHA (98.0%) compared to the high DHA (89.9%). Stem and dead DM yield (&gt; 4.0 cm) were lower (p &lt; 0.001) for the low HM (221 and 170 kg ha–1, respectively) than for the high HM (388 and 303 kg ha–1, respectively). The high DHA had higher (p &lt; 0.001) milk output (19.5 kg day–1) and milk solids (1.46 kg cow–1). The highest milk output per ha and per cow was achieved by LH, with low post-grazing residuals and high sward quality.

[es] Para maximizar la producci&oacute;n de leche en vacuno y el contenido de s&oacute;lidos en suspensi&oacute;n por hect&aacute;rea es necesario identificar el nivel de pastoreo para lograr alta ingesti&oacute;n de pasto (PDMI). El objetivo de este estudio fue investigar el efecto de la oferta (HM) y disponibilidad diaria de hierba (DHA) sobre la estructura morfol&oacute;gica del pasto de raigr&aacute;s ingl&eacute;s (Lolium perenne L.), la ingesti&oacute;n y la producci&oacute;n de leche. Sesenta y cuatro vacas Holstein-Friesian de partos de primavera fueron distribuidas y asignadas al azar a cuatro grupos (n = 16) en pastoreo (LL, LH, HL y HH) utilizando un dise&ntilde;o factorial 2 &times; 2. Los tratamientos consistieron en dos HM pre-pastoreo, baja (L- 1.600 kg MS ha&ndash;1) o alta (H- 2.400 kg MS ha&ndash;1) y dos DHA, baja (L- 15 kg MS vaca&ndash;1 d&iacute;a&ndash;1) o alta (H- 20 kg MS vaca&ndash;1 d&iacute;a&ndash;1). Se estudi&oacute; la estructura e ingesti&oacute;n de pasto y la producci&oacute;n de leche durante el pastoreo. Los grupos con baja HM completaron 9,5 rotaciones de 22 d&iacute;as en comparaci&oacute;n con 6,5 rotaciones de 32 d&iacute;as en los grupos con alta HM. La utilizaci&oacute;n del pasto fue superior (p &lt; 0,001) con baja DHA (98,0%) que con alta DHA (89,9%). La producci&oacute;n en MS de tallos y material senescente (&gt; 4,0 cm) fue menor (p &lt; 0,001) con baja HM (221 y 170 kg ha&ndash;1, respectivamente) que con alta HM (388 y 303 kg ha&ndash;1, respectivamente). La alta DHA logr&oacute; mayor (p &lt; 0,001) producci&oacute;n de leche (19,5 kg d&iacute;a&ndash;1) y s&oacute;lidos en suspensi&oacute;n (1,46 kg vaca&ndash;1). Los valores m&aacute;s altos de producci&oacute;n de leche por ha y por vaca se alcanzaron en LH, con menos rechazos y alta calidad del pasto.