Effect of pre-grazing herbage mass and daily herbage allowance on perennial ryegrass swards structure, pasture dry matter intake and milk performance of Holstein-Friesian dairy cows

Effect of pre-grazing herbage mass and daily herbage allowance on perennial ryegrass swards structure, pasture dry matter intake and milk performance of Holstein-Friesian dairy cows A. I. Roca-Fernandez1*, M. A. O’Donovan2, J. Curran2and A. Gonzalez-Rodriguez1 1 Departamento de Producción Animal. Centro de Investigaciones Agrarias de Mabegondo (CIAM). INGACAL. Apdo. 10. 15080 La Coruña. Spain 2 Dairy Production Department. Moorepark Dairy Production Research Centre. TEAGASC. Fermoy, Co., Cork. Ireland Abstract 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 (L1,600 kg DM ha–1) or high (H2,400 kg DM ha–1) and two DHA, low (L15 kg DM cow–1day–1) or high (H20 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<0.001) when animals offered the low DHA (98.0%) compared to the high DHA (89.9%). Stem and dead DM yield (> 4.0 cm) were lower (p<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< 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. Additional key words: grass intake; grazing management; herbage utilization; milk output per hectare; pasture allowance; spring-calving cows; sward quality. Resumen Efecto de la oferta y disponibilidad diaria de hierba sobre la estructura morfológica del pasto de raigrás inglés, la ingestión de materia seca y la producción de leche de vacas Holstein-Friesian Para maximizar la producción de leche en vacuno y el contenido de sólidos en suspensión por hectárea es necesario identificar el nivel de pastoreo para lograr alta ingestión 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 morfológica del pasto de raigrás inglés (Lolium perenne L.), la ingestión y la producción 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 diseño factorial 2 ×2. Los tratamientos consistieron en dos HM pre-pastoreo, baja (L1.600 kg MS ha–1) o alta (H2.400 kg MS ha–1) y dos DHA, baja (L15 kg MS vaca–1 día–1) o alta (H20 kg MS vaca–1 día–1). Se estudió la estructura e ingestión de pasto y la producción de leche durante el pastoreo. Los grupos con baja HM completaron 9,5 rotaciones de 22 días en comparación con 6,5 rotaciones de 32 días en los grupos con alta HM. La utilización del pasto fue superior (p< 0,001) con baja DHA (98,0%) que con alta DHA (89,9%). La producción en MS de tallos y material senescente (> 4,0 cm) fue menor (p< 0,001) con baja HM (221 y 170 kg ha–1, respectivamente) que con alta HM (388 y 303 kg ha–1, respectivamente). La alta DHA logró mayor (p< 0,001) producción de leche (19,5 kg día–1) y sólidos en suspensión (1,46 kg vaca–1). Los valores más altos de producción de leche por ha y por vaca se alcanzaron en LH, con menos rechazos y alta calidad del pasto. Palabras clave adicionales: calidad del pasto; disponibilidad de hierba; ingestión de hierba; manejo en pastoreo; producción de leche por hectárea; utilización del pasto; vacas de partos de primavera. * Corresponding author: [email protected] Received: 14-04-10; Accepted: 15-11-10. Instituto Nacional de Investigación y Tecnología Agraria y Alimentaria (INIA) Spanish Journal of Agricultural Research 2011 9(1), 86-99 Available online at www.inia.es/sjar ISSN: 1695-971-X eISSN: 2171-9292 Introduction Irish milk production systems are mainly pasturebased spring-calving dairy cows in order to maximize milk performance per hectare (ha) and per cow by increasing the organic matter digestibility (OMD) of the swards and the pasture dry matter intake (PDMI) of the cows (Stakelum and Dillon, 2007). With the abolition of European milk quotas in 2015 and the feeding costs projected to rise, the major purpose for the Irish dairy industry is to increase the efficiency use of grazed grass in the diet of the lactating dairy cows (McEvoy et al., 2009). Dillon et al. (2005) have shown that a 10% increase in the proportion of grazed grass managed for Irish dairy farms may help to reduce the cost of milk produced by €0.025 L–1 and to reduce ever-increasing costs of milk production (Keneddy et al., 2007). Using efficiently perennial ryegrass swards grazed rotationally as the primary diet source to feed dairy cows affords Irish farmers the opportunity to increase milk output per cow (Dillon et al., 2002), milk protein content (Kennedy et al., 2005) and herbage utilization (O’Donovan et al., 2004) affecting sward quality for subsequent grazing rotations (Keneddy et al., 2006). Understanding the important role that pre-grazing herbage mass (HM) and daily herbage allowance (DHA) plays on swards morphological composition, PDMI and milk performance of dairy cows at pasture is a key tool for a better knowledge of the interaction between swards and animals to improve future grassland management practices which can be implemented at farm level with the aim of reducing feeding cost of production and increasing both swards and milk quality and quantity. Maintaining a proper balance between optimizing dairy cows’ milk performance and improving swards feeding value is a dual purpose for using more efficiently farm resources and to increase profitability of sustainable dairy systems worldwide (Kennedy et al., 2007; González et al., 2008; McEvoy et al., 2009; Roca et al., 2009). In rotational grazing systems, rapid changes in swards structure can appear which affect subsequently dairy cows’milk performance (Combellas and Hodgson, 1979). Maintaining high swards quality as the grazing season progresses is the major challenge for appropriate grassland management systems at farm level nowadays. At maximum grass growth, the herbage DM offered exceeds the demand of animals and quality tends to deteriorate due to a higher proportion of senescent material in the swards. Reducing HM at swards can increase the grass feeding value with higher digestibility, more leaf proportion and, less senescent material across the grazing season, increasing also the PDMI of dairy cows (Holmes et al., 1992; Peyraud et al., 1996; O’Donovan and Delaby, 2008). Hoogendoorn et al. (1992) reported an increase in milk yield, milk fat and milk protein content when cows grazed low HM as opposed to medium HM at a similar DHA. Stakelum and Dillon (2007) also found an increased selection of higher digestibility herbage and green leaf content, with higher milk output per cow, on severely grazed swards in comparison to laxly grazed swards. As the quantity of DHA increases, the level of refused pasture normally increases and leading to a decrease in swards quality in subsequent grazing rotations (Lee et al., 2008), but sometimes the levels of milk production can show an increase (Stakelum, 1986). It seems that the combination of low HM and high DHA by using stocking rates (O’Donovan and Delaby, 2008) or just setting the treatments directly as happened in a previous work realized by the same research team, studying the effect of two HM (1,700 or 2,200 kg DM ha–1) and two DHA (16 or 20 kg DM cow–1 day–1), obtained the highest milk output per ha and per cow, maintaining low post grazing residuals and high sward quality (McEvoy et al., 2009). The objective of this new research was to investigate how similar treatments to those established previously, two levels of pre-grazing HM (1,600 or 2,400 kg DM ha–1) and two levels of DHA (15 or 20 kg DM cow–1 day–1), affect the sward structure, the PDMI and the milk performance of spring-calving Holstein-Friesian dairy cows across the grazing season. Material and methods The study was conducted at Moorepark Research Centre, Fermoy, Co. Cork, Ireland (50° 7’N; 8° 16’W) from April 4 to October 31 in 2008. The soil type was a free draining, acid brown earth with a sandy loamto-loam texture. The experimental area was a perennial ryegrass sward (Lolium perenne L.) from an initial mixture of three late-heading diploid cultivars (Twystar, Effect of herbage mass and allowance on perennial ryegrass swards and milk performance 87 Abbreviations used: BCS (body condition score), BW (body weight), DHA (daily herbage allowance), DIM (days in milk at the start of the experiment), HM (herbage mass), OMD (organic matter digestibility), PDMI (pasture dry matter intake), SR (stocking rate). Cornwall and Gilford) with no white clover (Trifolium repens L.), which after six years old, under a good grazing management, showed a very low proportion of other species. Weather climatic conditions Main climatic weather variables (Tª and mean rainfall) during the experimental period were atypical compared to the last 10-years average (1998-2008). Total monthly rainfall was greater during the grazing season in 2008 than the 10-years average for the months June (94 mm, +32%), July (135 mm, +125%) and August (118 mm, +45%). However, in April (38 mm, –45%) and May (51%, –28%), total rainfall was lower than the 10-years average. The total rainfall during the experimental grazing trial in 2008 (639 mm) was 75 mm higher than the 10-years average (564 mm). Mean daily temperatures in 2008 were 0.5-1°C lower than the 10-years average. April and October in 2008 (7.9 and 9.2°C) were colder months compared to the other five months and were 0.8 and 1.4°C colder than the 10-years average, respectively. Experimental design and treatments The experiment investigated the effect of offering two target levels of pre-grazing HM (> 4cm), low (L1,600 kg DM ha–1) or high (H2,400 kg DM ha–1) and two target levels of DHA, low (L15 kg DM cow–1 day–1) or high (H20 kg DM cow–1 day–1) on perennial ryegrass swards structure, PDMI and milk performance of spring-calving Holstein-Friesian dairy cows. A randomized block design with a 2 ×2 factorial arrangement of treatments was applied and results in this paper are expressed as means of thirty weeks at grazing pastures. Two levels of pre-grazing HM were created experimentally with a differential dry matter (DM) yield maintained at 800 kg DM ha–1 across the grazing season. Two levels of DHA were offered across the two HM treatments, these had a differential of 5 kg DM cow–1 day–1 maintained in all the rotational grazing paddocks. The following four grazing treatments were imposed during seven months in four separately experimental farmlets: LL (low HM, 1,600 kg DM ha–1 and low DHA, 15 kg DM cow–1 day–1), LH (low HM, 1,600 kg DM ha–1 and high DHA, 20 kg DM cow–1 day–1), HL (high HM, 2,400 kg DM ha–1 and low DHA, 15 kg DM cow–1 day–1) and HH (high HM, 2,400 kg DM ha–1 and high DHA, 20 kg DM cow–1 day–1). Animals and grazing groups Sixty-four spring-calving Holstein-Friesian dairy cows, primiparous (n= 24) and multiparous (n= 40), were selected from the experimental Moorepark dairy herd. Cows grazed in one large herd from calving to the commencement of the experiment and they were offered 4 kg of concentrate per cow and per day. In the week before the starting week of the study, concentrate supplementation was reduced progressively and animals were removed from the diet on the day prior to the experimental grazing period. Cows were balanced on calving date (February 11 ± 23.9 days), lactation number (2.6 ± 1.74), preexperimental milk yield (29.3±4.8 kg cow–1), milk fat content (43.9 ± 0.88 g kg–1), milk protein content (33.4± 0.29 g kg–1), body weight (BW) (513 ±74.4 kg) and body condition score (BCS) (2.96± 0.55). Animals were then blocked into the following four grazing groups (LL, LH, HL and HH) and each group was randomly assigned to one of the four different grazing treatments (n= 16). Cows were on average 53 days in milk (DIM) when the experiment commenced. No concentrate was offered at all during the experimental period. Grazing management The entire experimental area was rotationally grazed once during the pre-experimental period (February 11 to April 3) to a similar post-grazing height of 4.0± 0.07 cm and differences in pre-grazing HM were created by varying the time period between initial grazing and applying the experimental grazing treatments. The pastures designated to the high HM treatments were grazed initially followed by the low HM pastures thus creating a longer regrowth interval. The experimental grazing area comprised of 21.3 ha divided into ten paddocks. Five paddocks were randomly assigned to the low HM treatments and the remaining five paddocks were assigned to the high HM treatments. These paddocks were subsequently divided into 2 sub-paddocks and permanently fenced. Subpaddocks were randomly assigned to one of the two DHA treatments, with five sub-paddocks per treatment. 88 A. I. Roca-Fernández et al. / Span J Agric Res (2011) 9(1), 86-99 A similar grazing area (farmlet) was available for each treatment and each treatment was managed independently of all the other treatments. The first three subpaddocks grazed per treatment were assigned as grazing only paddocks. The remaining two sub-paddocks per treatment were conserved as silage when the farm cover reached surplus grass supply. The first sub-paddock grazed by each treatment was referred to as the base paddock according to the methodology described by McEvoy et al. (2009). If animals were in a grazing only paddock when the base paddock reached target HM, they finished the rotation in that paddock and returned to the base paddock to commence a new rotation. If animals were in a silage paddock when the base paddock reached target HM, they immediately returned to the base paddock. Silage paddocks were cut immediately to ensure that regrowth interval was similar for grazed and ungrazed paddocks. The area for silage was excluded from the calculation of mean stocking rate (SR). Fresh herbage (> 4.0 cm) was allocated to each individual herd on a daily basis after the morning milking using temporary fencing. No access to the previous days grazing area was allowed and was restricted by back fencing with temporary wire. Paddocks were dusted during April and May with calcined magnesite on a daily basis in order to ensure adequate intake of magnesium in the early lactation period and to prevent hypomagnesaemia. Pastures were not topped at all during the experimental period. In late autumn, pregrazing HM was allowed to increase in order to ensure that adequate pasture was available to the animals to continue grazing until November, with a mean difference reduced to 500 kg DM ha–1 and maintained between the low and high HM swards. Nitrogen fertilizer was applied after each paddock was grazed from April to August, with a total amount of 250 kg nitrogen ha–1 applied to each farmlet (60-50-50-50-40 kg nitrogen ha–1). Sward measurements HM determination and DM yield Pasture HM (>4.0 cm) was determined twice weekly by cutting two strips per paddock (1.2 m wide ×10 m long) per treatment with an Agria machine (Etesia UK Ltd., Warwick, UK). Ten grass height measurements were recorded before and after harvesting on each cut strip using an electronic plate meter (Urban and Caudal, 1990) with a plastic plate (30 ×30 cm and 4.5 kg m–1; Agrosystèmes, Choiselle, France). DM yield below 4.0 cm was measured to ground level, within two of the four strips cut using the Agria, per treatment area, by harvesting with a scissors the residual herbage within a 0.5 ×0.2 m quadrant. Soil and roots were manually removed and the sample was weighed and dried overnight at 80°C in a drying oven to determine DM content below 4.0 cm. Grass growth The grass growth (kg DM ha–1 day–1) was calculated by dividing the grass production by the number of days regrowth. The effect of treatment on grazing management (rotation length, stocking rate, milk output and milk solids yield per ha) were calculated by rotation according to the methodology described by Hoden et al. (1986). Preand post-grazing sward heights The pre-grazing sward height was measured daily throughout the experimental period in each plot by recording 30 measurements per treatment across the two diagonals of each paddock by using the electronic plate meter. The DHA for each herd was calculated by multiplying pre-grazing pasture height (>4.0 cm) by the sward density. Post-grazing sward heights were measured immediately after daily grazing. Herbage utilization Herbage mass utilization (> 4.0 cm) was calculated according to the method described by Delaby and Peyraud (1998) and it was further used to evaluate the HM produced in each sward. Sward structure Sward structural characteristics describe the proportion and relative vertical distribution of leaf, stem and dead material in the sward profile and it was determined weekly for the duration of the experiment. Approximately twenty sward samples were taken at random by cutting with a scissor at ground level in the Effect of herbage mass and allowance on perennial ryegrass swards and milk performance 89 area to be grazed the following day to derive the morphological composition of the sward. The samples were laid in a plastic bag to maintain the vertical structure of the sward. A 150 g sub-sample was cut into two fractions, above and below 4.0 cm. Each individual layer was then manually separated into leaf, stem and dead material (including flower head and seeds if present) and weighed. Each sward constituent was oven dried overnight at 80°C for DM determination. Criteria for determining when to defoliate pastures have been based on rotation length, sward height and HM following the assumptions of Mayne et al. (2000). Pasture dry matter intake Individual total PDMI were estimated four times during the experiment in weeks 6, 12, 21 and 28 using the n-alkane technique (Mayes et al., 1986) as modified by Dillon and Stakelum (1989). All cows were dosed twice daily before milking for 12 consecutive days with a paper bung (Carl Roth, GmbH and Co. KG, Karlesruhe, Germany) containing 500 mg of dotriacontane (C32). From day 7 of dosing, fecal grab samples were collected from each cow twice daily for the remaining 6 days and stored at –20°C until analysis. The grab fecal samples of each cow from the total collection were then bulked (10 g of each collected sample) to obtain one sample per cow. This was dried for 48 h at 40°C, milled and analyzed by gas chromatography. In conjunction with the fecal collection, the diet of the animals was also sampled. Herbage representative of that grazed (following close observation of the grazing animals’previous defoliation) was manually collected from each paddock before morning grazing on days 6 to 11 (inclusive) of the intake measurement period. Two samples of approximately 25 individual grass snips were taken from each paddock with a Gardena hand shears. The ratio of herbage C33 (tritriacontane) to dosed C32 was used to estimate PDMI. The n-alkane concentration was determined as described by Dillon (1993). Animal measurements Milk yield and composition Milking took place at 07.00 h and 16.00 h daily. Individual milk yields (kg) were recorded at each milking (Dairymaster, Causeway, Co. Kerry, Ireland). Milk fat and milk protein content were determined from two successive evening (Tuesday) and morning (Wednesday) weekly milking samples collected and pooled together for later analysis. The concentrations of these two milk constituents were determined using MilkoScan 203 (DK-3400, Foss Electric, Hillerød, Denmark). Solids-corrected milk yield were also calculated using the equation of Tyrrell and Reid (1965). Body weight and body condition score Body weight (BW) was recorded weekly electronically using a portable weighing scale and the Winweigh software package (Tru-Test Limited, Auckland, New Zealand). Body condition score (BCS) was scored weekly by one experienced independent observer throughout the experiment on a 1 to 5 scale (1=emaciated, 5= extremely fat) with 0.25 increments (Lowman et al., 1976). Statistical analyses All statistical analyses were carried out using SAS (SAS Institute, 2005). Sward measurements were analyzed using analysis of variance by the following model: Yijkl =µ+H i+D j+R k+W l(Rk)+H i×Dj +e ijkl where: Yijk represents the response of sward kto HM i and DHA j; µ is the mean; Hiis the HM (i= 1 to 2); Dj is the DHA (j= 1 to 2); Rkis the rotation (k= 1 to 10); Wl(Rk) is the week within rotation (l= 1 to 30); Hi×Dj is the interaction between HM and DHA; and eijkl is the residual error term. Animal variables (daily milk yield, milk composition, milk constituent yield, BW and BCS) were analyzed using the model as follows: Yijk =µ+H i+D j+H i×Dj+b1Xijk +e ijk where: Yijk represents the response of animal koffered HM iand DHA j; b1Xijk is the respective pre-experimental variable; and eijk is the residual error term. Results Grazing management The effect of HM and DHA on grazing management and milk output per ha for each of the four grazing 90 A. I. Roca-Fernández et al. / Span J Agric Res (2011) 9(1), 86-99 treatments is shown in Table 1. On average, the low HM treatments had three more grazing rotations than the high HM treatments with a difference accumulated in rotations length from an extra 10 days per rotation for the high HM (32 days) compared to the low HM treatments. The low HM treatments were allocated a greater (p<0.001, +34 m2cow–1 day–1) mean grazing area per day compared to the high HM treatments (74 m2cow–1 day–1). Offering high DHA increased the mean grazing area per day (p< 0.001, +19 m2cow–1 day–1) over the treatments offered low DHA (79 m2 cow–1 day–1). Daily grass growth rate for the low DHA swards (66 kg DM ha–1 day–1) was 9.5 kg DM ha–1 day–1 lower than the high DHA swards. The total area conserved for silage was 5.26 ha (LL), 6.23 ha (LH), 6.78 ha (HL) and 5.77 ha (HH). The effective mean grazing stocking rate (SR) was similar (3.95 cows ha–1) across both HM. The high DHA SR (4.01 cows ha–1) was 0.12 cows ha–1 higher than the low DHA system. Total milk output and milk solids per ha were greater (p< 0.001) for the low HM treatments (16,020 and 1,202 kg ha–1, respectively) compared to the high HM treatments (14,658 and 1,115 kg ha–1, respectively). The LH had the highest milk output and milk solids per ha compared to the lowest milk output and milk solids per ha obtained from the HL. Sward structural characteristics The effect of treatment on sward measurements is shown in Table 2. The mean HM for the low treatments (1,597 kg DM ha–1) was significantly lower (p< 0.001, –793 kg DM ha–1) compared to the high HM treatments. Effect of herbage mass and allowance on perennial ryegrass swards and milk performance 91 Table 1. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) on grazing management and output per hectare (average 206 days grazing season) Treatments1 L H SED Significance L H L H HM DHA HM*DHA Number of rotations 9a10a6b7b0.9 *** NS NS Rotation length (days) 23a21a34b29b2.96 *** NS NS Area offered (m2cow–1 day–1)98 a119a66b83b11.3 *** *** NS Grass growth (kg DM ha–1 day–1)68 a77b64a74b2.9 NS *** NS Stocking rate (cows ha–1) 4.00a3.85b4.01a3.93b0.037 NS *** NS Milk output (kg ha–1) 15,057a16,983b13,876c15,440a641.1 *** *** *** Milk solids (kg ha–1) 1,135ab 1,268c1,072a1,157b40.9 *** *** *** 1LL (low HM, 1,600 kg DM ha–1 and low DHA, 15 kg DM cow–1 day–1), LH (low HM, 1,600 kg DM ha–1 and high DHA, 20 kg DM cow–1 day–1 ), HL (high HM, 2,400 kg DM ha–1 and low DHA, 15 kg DM cow–1 day–1) and HH (high HM, 2,400 kg DM ha–1 and high DHA, 20 kg DM cow–1 day–1). a-c Means within a row with different superscripts differ (p<0.05). NS: not significant (p≥0.05). ***: p< 0.001. **: p< 0.01. *: p< 0.05. SEM: standard error of the difference. Table 2. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) and their interaction on preand post-grazing sward height and herbage utilization during the experimental period Treatments1 L H SED Significance L H L H HM DHA HM*DHA HM (kg DM ha–1) 1,601 1,593 2,376 2,403 168.7 *** NS NS DHA (kg DM cow–1 day–1) 14.8a19.5b14.8a19.6b0.256 NS *** NS Pre-grazing height (cm) 11.9a11.5a14.4b14.3b0.595 *** NS NS Post-grazing height (cm) 4.2a4.7b4.2a5.2c0.098 *** *** *** Herbage utilization (%) 97.7a91.3b98.3a88.4c 1.66 NS *** NS 1See Table 1. a-c Means within a row with different superscripts differ (p< 0.05). NS: not significant (p ≥ 0.05). ***: p< 0.001. **: p< 0.01. *: p< 0.05. SED: standard error of the difference. The mean DHA for the low treatments (14.8 kg DM cow–1 day–1) was significantly lower (p<0.001, –4.8 kg DM ha–1) compared to the high DHA treatments. Pre-grazing sward height was significantly higher (p<0.001, +2.7 cm) for the high HM swards (14.4 cm) compared to the low HM swards. There was an interaction in post-grazing swards heights between HM and DHA, the HH treatment had a higher residual than the LH treatment and the two low DHA treatments. The low HM swards (4.5 cm) had a lower post-grazing height (–0.2 cm) compared to the high HM swards. Treatments offered the low DHA (4.2 cm) showed a significantly lower post-grazing sward height (p< 0.001, –0.8 cm) than treatments offered the high DHA. Herbage utilization was significantly greater (p<0.001, +8.1%) for the low DHA treatments (98.0%) compared to the high DHA treatments. The low HM treatments (94.5%) showed higher herbage utilization (+1.1%) than the high HM treatments. The highest herbage utilization was observed in the HL and the lowest was obtained in the HH with the rest of treatments in between. The effect of HM and DHA on swards morphological composition is shown in Table 3. The mean leaf yield (> 4.0 cm) for the low HM treatments (1,206 kg DM ha–1) was significantly lower (p< 0.001, –493 kg DM ha–1) compared to the high HM treatments. The mean stem and dead DM yield (> 4.0 cm) for the low HM treatments (221 and 170 kg DM ha–1, respectively) were significantly lower (p<0.001, –167 and –134 kg DM ha–1, respectively) compared to the high HM treatments. There were no significant differences in leaf, stem and dead DM yield (< 4.0 cm) sward horizons in either the low or high HM treatments. The mean stem DM yield was lower (–70 kg DM ha–1) in the swards grazed at the low HM (842 kg DM ha–1) in contrast to the high HM with a increased dead DM yield (+111 kg DM ha–1) in the low HM swards (1,047 kg DM ha–1) than in the high HM swards. Leaf and dead proportions did not differ significantly between treatments. Stem proportions were significantly (p< 0.05, –0.05) affected by the HM, the low HM treatments (0.35) showed a lower stem proportions than the high HM treatments. The LH had the lowest stem proportion during all the experimental period. Pasture dry matter intake There were four weeks PDMI measurements analyzed in two periods, P1 (summer, w-6 and w-12) and 92 A. I. Roca-Fernández et al. / Span J Agric Res (2011) 9(1), 86-99 Table 3. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) and their interaction on morphological composition of the swards greater than and less than 4.0 cm during the experimental period Treatments1 L H SED Significance L H L H HM DHA HM*DHA > 4.0 cm Leaf yield (kg DM ha–1) 1,211a1,201a1,690b1,707b122.6 *** NS NS Leaf proportion 0.75 0.74 0.71 0.72 0.031 NS NS NS Stem yield (kg DM ha–1) 216a226a392b384b61.5 *** NS NS Stem proportion 0.14 0.15 0.16 0.15 0.023 NS NS NS Dead yield (kg DM ha–1) 174a166a295b312b33.6 *** NS NS Dead proportion 0.11 0.11 0.13 0.13 0.018 NS NS NS < 4.0 cm Leaf yield (kg DM ha–1) 598 539 511 504 116.4 NS NS NS Leaf proportion 0.24 0.22 0.22 0.21 0.043 NS NS NS Stem yield (kg DM h–1) 870 813 872 952 86.2 NS NS NS Stem proportion 0.37ab 0.33b0.39ab 0.41a0.032 * NS NS Dead yield (kg DM ha–1) 960 1,134 921 954 170.4 NS NS NS Dead proportion 0.39 0.45 0.39 0.38 0.054 NS NS NS 1See Table 1. a-c Means within a row with different superscripts differ (p< 0.05). NS: not significant (p ≥ 0.05). ***: p< 0.001. **: p< 0.01. *: p< 0.05. SED: standard error of the difference. P2 (autumn, w-21 and w-28) that were presented in a previous paper by Curran et al. (2010). Results from the PDMI of sward measurements weeks are shown in Table 4. There was no interaction between HM and DHA for PDMI in both periods. HM no showed any effect on PDMI in P1 and P2. The DHA had a significant effect on PDMI in both periods (p<0.001). The mean PDMI in P1 of cows offered the high DHA swards was significantly (p<0.001, +1.8 kg DM cow–1 day–1) higher (16.5 kg DM cow–1 day–1) compared to animals offered the low DHA swards. While in P2 these differences were a bit lower (p< 0.001, +1.6 kg DM cow–1 day–1) between the animals offered the high DHA swards (15.7 kg DM cow–1 day–1) compared to the cows offered the low DHA swards. Animal performance Milk production and animal performance of all the grazing groups are shown in Table 5. Cows offered the high DHA had a significantly higher (p<0.001, +1.4 kg cow–1 day–1) mean milk yield (19.5 kg cow–1 day–1) than cows offered the low DHA. The accumulated milk yield per cow and per day during some of the experimental weeks is shown in Figure 1. The highest milk performance was achived by the LH grazing group and the lowest by the HL with the rest of grazing groups (LL and HH) in between. Milk yield of all the groups decreased progresively across the grazing season progressed and according to the lactation stage of all the spring-calving dairy cows. Milk protein and milk fat content did not differ significantly between groups. Cows offered the high DHA swards produced greater (p< 0.001, +0.09 kg cow–1) milk solids yield (1.46 kg cow–1) than cows offered the low DHA swards. The highest milk solids yield per cow was obtained by the LH grazing group and the lowest by the HL with the other two groups (LL and HH) in between. There was a significant effect of DHA (p< 0.001) on the mean BW and BCS. The high DHA treatments (523 kg) had a higher BW (p< 0.001, +12 kg) compared to the low Effect of herbage mass and allowance on perennial ryegrass swards and milk performance 93 Table 4. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) and their interaction on pasture dry matter intake during the experimental period (P1, summer and P2, autumn) Treatments1 L H SED Significance L H L H HM DHA HM*DHA P1-PDMI (kg DM cow–1 day–1) 15.1a16.6b14.2a16.4b0.54 NS *** NS P2-PDMI (kg DM cow–1 day–1) 14.1a15.8b14.0a15.5b0.071 NS *** NS 1See Table 1. a-c Means within a row with different superscripts differ (p< 0.05). NS: not significant (p ≥ 0.05). ***: p< 0.001. **: p< 0.01. *: p< 0.05. SED: standard error of the difference. Table 5. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) and their interaction on milk production and animal performance of dairy cows during the experimental period Treatments1 L H SED Significance L H L H HM DHA HM*DHA Milk yield (kg day–1) 18.2a20.1b17.9a18.9b0.56 NS *** NS Milk protein content (g kg–1) 35.0 35.5 34.5 35.2 0.63 NS NS NS Milk fat content (g kg–1) 40.8 39.9 42.4 40.6 1.28 NS NS NS Milk solids yield (kg cow–1) 1.37a1.50b1.36a1.41b0.046 NS *** NS Body weight (kg) 511a521b511a524b5.6 NS *** NS Body condition score 2.62a2.74b2.60a2.74b0.071 NS ** NS 1See Table 1. a-c Means within a row with different superscripts differ (p< 0.05). NS: not significant (p ≥ 0.05). ***: p< 0.001. **: p< 0.01. *: p< 0.05. SED: standard error of the difference. DHA treatments. The BCS was also higher (p< 0.001, +0.13) for the cows offered the high DHA swards (2.74) in contrast to the low DHA swards. Discussion In other previous experiments describing the effects of HM on milk performance of dairy cows compared treatments of early and late grazed swards, with the experiment beginning when the swards were first grazed (O’Donovan et al., 2004; Kennedy et al., 2007). However, in this study as occurred in the work reported by McEvoy et al. (2009) all swards were grazed once to a similar post-grazing sward height prior to the experiment commencement in order to ensure similar pre-experimental treatment of all swards and avoid differences arising from grazed and ungrazed swards. The experiment lasted 30 weeks across the grazing season providing an opportunity to assess the cumulative treatment effects of pre-grazing HM and DHA on swards structure, PDMI and dairy cows’ milk performance at pasture. In the current research a more approximated situation to the main effects of grazing perennial ryegrass swards with different HM and DHA has been realized with the aim of establishing the sward canopy response and milk output per ha of springcalving dairy cows throughout the grazing season, which can be really extrapolated in the future to on farm grazing management system. Grazing management Within each HM system the grazing SR difference was low (0.07 cows ha–1) between the low HM treatments and the high HM treatments due to different regrowth periods between the low DHA swards and the high DHA swards. In fact, the high DHA swards had a greater (+10 kg DM ha–1 day–1) daily growth rate when compared to the low DHA swards. Kennedy et al. (2007) reported that high HM swards can support a greater grazing SR of 6.5 cows ha–1. However, grazing at low HM with a mean SR of 4.5 cows ha–1 has showed a positive effect on swards quality, PDMI and milk performance. A lower grazing SR difference was found in this experiment (–0.12 cows ha–1) between the two DHA treatments in contrast to the results reported by McEvoy et al. (2009) with a grazing SR difference of 0.40 cows ha–1 between the low DHA and the high DHA when the mean HM was of 1,700 kg DM ha–1 for the low HM treatments and 2,200 kg DM ha–1 for the high HM treatments and the mean DHA was of 16 kg DM cow–1 day–1 for the low DHA treatments and 20 kg DM cow–1 day–1 for the high DHA treatments. Moreover, the daily growth rate was lower from the experiment of McEvoy et al. (2009) compared to the current study with the mean daily growth rate difference (+6.9 kg DM ha–1 day–1) between the high DHA swards and the low DHA swards. Swards structural characteristics The mean greater proportions of unutilised herbage in the high DHA treatments resulted in the low HM, being higher for the high DHA swards. This suggests that as a sward is more severely grazed total DM yield is reduced, whereas the leaf DM proportion (> 4.0 cm) tends to increase (Stakelum and Dillon, 2007) as occurred in the current grazing experiment and, consequently, the feeding value of the sward tends to increase. The increased regrowth interval and pre-grazing DM yield difference between the two HM treatments suggest that the age (Hoogendoorn et al., 1992) and stage of growth (Dillon et al., 2005) of the plant tissue may be responsible for decreasing the digestibility and quality for the high HM swards compared to the low HM swards. Furthermore, Terry and Tilley (1964) reported that the chemical composition and digestibility of the various plant components (leaf and stem) varies widely both between components and within components as the 94 A. I. Roca-Fernández et al. / Span J Agric Res (2011) 9(1), 86-99 10 12 14 16 18 20 22 24 26 28 30 13-Apr Date Milk yield (kg cow–1) 4-May 25-May 15-Jun 6-Jul 27-Jul 17-Aug 7-Sep 28-Sep LL, Low HM and Low DHA LH, Low HM and High DHA HL, High HM and Low DHA HH, High HM and High DHA Figure 1. Effect of pre-grazing herbage mass (HM; L1,600 or H2,400 kg DM ha–1) and daily herbage allowance (DHA; L15 or H20 kg DM cow–1 day–1) on milk yield (kg per cow) at grazing during 24-weeks of the experimental period. Treatments: see Table 1.