Pasture characteristics of Italian ryegrass and milk production under different management strategies Características da pastagem de azevém anual e produção de leite sob diferentes estratégias de manejo

– The objective of this work was to assess the effects of the sward structure of Italian ryegrass ( Lolium multiflorum ), during the first grazing cycle, on its morphological and bromatological characteristics throughout the growing season, and on the performance of dairy cows. The treatments consisted of two structures obtained as a function of canopy-light interception: high-light interception (HLI) and low-light interception (LLI), with different pre‑grazing heights in the first grazing cycle. Pasture was managed under rotational grazing with a herbage allowance not below 30 kg dry matter (DM) per cow per day. Three grazing cycles, with a grazing interval of 30 days, were evaluated. Pre‑grazing herbage mass was greater (2,240 vs. 1,656 kg ha -1 DM), but the proportion of leaf blades was smaller (0.35 vs. 0.43) for HLI swards. Neutral detergent fiber (NDF) content and organic matter digestibility (OMD) were similar between treatments in the first grazing cycle, but in the second and third ones NDF was greater, and OMD lower, for the HLI swards. Milk yields were greater for cows grazing LLI swards (19.4 vs. 21.1 kg per day). Initial grazing with 90% of light interception promotes greater nutritional value in the subsequent cycles.


Introduction
The efficiency of production systems for grazing animals is a function of the ingested herbage quality and quantity, which are strongly correlated to the adopted forage species and management practices (Hopkins & Wilkins, 2006). In terms of quantity, the ratio between herbage allowance and daily intake is well known (Peyraud et al., 1996). However, the pasture management designed to supply forage, without limiting intake, usually implies on reduced grazing efficiency of accumulated aerial biomass, which may decrease pasture quality throughout its growing season (Purcell et al., 2011). Pesq. agropec. bras., Brasília, v.47, n.6, p.863-868, jun. 2012 Some management strategies have been studied, mainly those involving perennial ryegrass pastures in temperate climate regions, in order to achieve an optimal herbage ingestions per cow and per hectare (Delaby & Peyraud, 2009). Among these, are the hasting date of first grazing (O'Donovan et al., 2004;Kennedy et al., 2006Kennedy et al., , 2007 and the increased defoliating frequencies with reduced intervals between grazing cycles and increasing number of cycles (McEvoy et al., 2009;Curran et al., 2010;Roca-Fernandez et al., 2011). In both cases, milk yield can increase up to 2.0 kg per cow and more than 1,300 kg ha -1 . However, little is still known about the effects of aerial biomass availability before the first grazing cycle on milk yield during the growing cycle of the most common winter forage species, such as Italian ryegrass (Lolium multiflorum Lam.), in subtropical climate regions. Therefore, it is relevant to know if Italian ryegrass pastures with larger proportion of leaf blades at the beginning of the first grazing cycle would allow for the maintenance of this parameter throughout sucessive cycles and if it has positive consequences on herbage intake and fat-corrected milk yields.
The objective of this work was to assess the effects of the sward structure of Italian ryegrass (Lolium multiflorum) during the first grazing cycle on its morphological and bromatological characteristics throughout the growing season, and on the performance of dairy cows.

Materials and Methods
The experiment was carried out at the Universidade do Estado de Santa Catarina, Lages, SC, Brazil (27°47'4"S and 50°18'26"W, at 920-m altitude), during the 2010 ryegrass growing season (from August to October). The soil was classified as a fine-loamy typical Haplumbrept. Two uniform 1.2 ha paddocks were used for each treatment and were split into three 0.4 ha replicates. It was used the rotational grazing method with electric fence-enclosed areas, inside each replicate slice, for four-day occupancy periods, and a herbage allowance not below 30 kg of dry matter (DM) per cow.
The treatments consisted of two structures obtained as a function of canopy light interception in the first grazing cycle: high-light interception (HLI) and low-light interception (LLI). HLI swards were first grazed when light interception reached 95%, whereas LLI swards were first grazed when light interception had reached 90%. Light interception was measured with a ceptometer Accupar LP-80, (Decagon Devices, Inc., Pullman, WA, USA). In order to simultaneously graze both treatments, HLI swards were fertilized with 50 kg ha -1 of N, as ammonium nitrate, as soon as tillering had began. Seven days later, LLI swards received the same amount of nitrogen. LLI and HLI swards were first grazed 21 and 28 days after N fertilization, respectively.
Twelve Holstein dairy cows were assigned to two uniform six-cow groups (two cows per replicate per area), according to milk yield (23.1±6.7 kg per day), lactation stage (114±70 days), body weight (569.3±40.3 kg), and amount of lactations (2.5±1.1). Three grazing cycles were monitored during the experiment, with a rest period of 30 days. For each cycle, animals stayed in the experimental area for 12 days, without any supplementation. Measurements were taken during the four last days of each cycle. Each area was fertilized immediately after each cycle, with 50 kg ha -1 of N supplied as ammonium nitrate. In the grazing intervals, all animals stayed, in one single group, in an overseeded ryegrass paddock: ryegrass was overseeded with a mix of temperate climate forage species (Festuca arundinacea Schreb., Trifolium repens L., and Trifolium pretense L.).
Pre-grazing biomass was estimated by the compressed height, measured with a F200 rising plate meter (Farmworks, Feilding, New Zealand), and by the DM amount present at the disk area (0.1 m 2 ). Before and after each grazing cycle, regression equations were obtained for biomass estimation (kg ha -1 DM) as a function of grass height (cm). For this, five points by treatment were cut with scissors at ground level. After manual removal of soil and roots, the samples were oven-dried for 72 hours at 60ºC.
Extended tiller and sheath heights were measured before and after grazing in the last four days of each cycle. Representative forage samples were taken at soil level from the same paddocks before grazing. Samples were separated into two subsamples: one was cut at the average post-grazing extended tiller height, with its upper portion oven-dried for 72 hours at 60ºC, and was stored for chemical composition assessment; and the other was used for morphological classification and assessment of the mass ratios of leaf blades, stems (stems and pseudostems), dead material, and others.
The protocol for forage sampling in laboratory was performed according to Ribeiro Filho et al. (2003). The offered biomass underestimated the nutritional value of forage intake. As described, the fraction selected for analysis was always higher than the residual tiller height, which was identified in individual paddocks.
Animals were milked twice a day. Individual milk yield was measured daily for both milking times (at 7h30 a.m. and 4h30 p.m.). Samples were taken in order to assess milk fat and protein contents during the last four days of each evaluated cycle for both milking times. Milk samples were pooled by day and stored at 4ºC with the preservative bromopol tablet, (D & F Control Systems, Inc., San Ramon, CA, USA) before being analyzed for fat and protein contents using infrared analysis, method n o 972.160 (Horwitz, 2000). Individual forage intake was assessed from body weight and milk yield and composition, as proposed by Baker (2004). Animals were weighed once a week.
Mineral matter content was assessed by a 4-hour muffle furnace calcination at 550ºC. Forage and feces samples were used for crude protein (CP) measurements using the Kjeldahl method. Acid detergent fiber (ADF) was measured according to Robertson & Van Soest (1981), and neutral detergent fiber (NDF) to Van Soest et al. (1991). NDF and ADF contents included ash trace residues. Forage organic matter digestibility was estimated from CP content of feces and forage and ADF contents of feces, as described by Ribeiro Filho et al. (2003).
Data was subjected to analysis of variance according to a mixed model with repeated measurements in time, using the SAS package (Littell et al., 1998). The paddock was the experimental unit chosen for assessments and calculations. Effects from the experimental treatment, grazing cycle, and treatment x cycle interactions were treated as fixed, whereas the paddock was considered as random data. Measurements taken from animals (herbage intake, milk yield, milk fat, and protein contents) used initial milk yield as a covariate.

Results and Discussion
There was no interaction between sward types and grazing cycles for pre-grazing herbage mass, pre-grazing sward height, and morphological composition (Table 1). Therefore, these results were presented and discussed based on the averages of each factor (type of pasture and cycle).
Sward height was higher for HLI than for LLI swards and increased in the second and third cycles in comparison to the first cycle. Regardless of extended tiller height and sheath, the responses observed were indirectly related to the proportion of blades in the canopy. The proportion of live laminae was higher and the proportion of live sheathes and stems was lower for LLI than for HLI swards. Similarly, the proportion of live laminae decreased and the proportion of live sheathes and stems increased from the first to the third cycle. Total herbage mass was higher for HLI than for LLI swards, but the live laminae of herbage mass was similar between them. The CP ratio was 38 g kg -1 higher, and the ADF ratio was 21 g kg -1 lower for LLI swards, when compared to HLI swards. In the third cycle, CP decreased 77 g kg -1 DM, whereas ADF increased 100 g kg -1 DM in comparison to the first cycle. However, the proportion of NDF and organic matter (OM) digestibility were similar between swards for the first cycle, but for the second and third ones NDF was lower and OM digestibility was higher for the LLI swards.
The differences observed in chemical composition between treatments can be attributed to the larger amount of aerial biomass per hectare, as well as to the smaller proportion of laminae and the higher postgrazing height for HLI swards, in comparison to the LLI swards. Larger biomass values at pre grazing implies on a reduced quality of forage (Purcell et al., 2011), which could be associated to a lower proportion of leaf blades and larger proportion of stems and dead material in the canopy (McEvoy et al., 2009;Roca-Fernandes et al., 2011). Furthermore, these results confirm that younger and leafy plants have larger N content and better digestible OM content, and that more mature plants present higher NDF and ADF values (Hopkins & Wilkins, 2006). For perennial ryegrass pastures, increases in CP content and decreases in NDF and ADF have already been observed (O'Donovan et al., 2004;Kennedy et al., 2006) as consequences for hastening date of the first grazing cycle.
Total herbage allowance did not change between swards, but increased drastically in the third cycle in comparison to the first and second ones (Table 2). Live laminae herbage allowance was similar between HLI and LLI swards in the first and third cycles, but it was higher for LLI swards in the second cycle. Post-grazing sward height was lower for LLI than for HLI swards in the first cycle and similar between swards in the second and third ones. The extended tillers and sheaths heights were lower for LLI than for HLI swards in the first and second cycles, but did not differ in the third one. Pastures with larger proportion of leaf blades in the first grazing cycle tend to maintain this characteristic and to have better digestibility in the second grazing cycle (Kennedy et al., 2007;Curran et al., 2010), whereas the quality of forage shows a greater decrease in the next grazing cycle for pastures with larger proportion of stems and dead material close to soil surface (O'Donovan et al., 2004). In terms of post-grazing height, Curran et al. (2010) found that forage quality is enhanced at subsequent grazing cycles due to lower post-grazing heights and less dead material from residual pasture left from the previous cycle.
HLI and LLI herbage intake showed no variation with sward type, but herbage digestible OM intake and metabolisable energy intake decreased as grazing cycles progressed. Milk yield was higher for LLI than for HLI swards (1.7 kg per day) and decreased throughout the cycles. Fat content of milk did not vary with treatments or grazing cycles. Live weight also did not change with treatments or grazing cycles, but protein yield was higher for LLI than for HLI in the second cycle, and it did not vary with sward type in the first and third cycles.
The similarity in herbage intake can be attributed to the fact that, for both treatments, severity of defoliation was always lower than 50% of pre-grazing height at the beginning of grazing. At this condition, animals always ingest only the higher strata of the canopy, avoiding stems, which act as a barrier to defoliation and reduce both instantaneous and daily herbage intake (Drescher et al., 2006). However, DM herbage allowance as live lamina (Table 2), during the second and third grazing cycles for HLI swards, was lower than the estimated DM intake (Table 3). This result indicates larger proportion of stems in the diets of cows in the HLI pastures.
Therefore, the larger milk yield under the LLI treatment is clearly associated to the quality of herbage intake, when compared to the HLI treatment. These results confirm the findings of O' Donovan et al. (2004), who attributed larger milk yield by cows grazing forage with larger proportion of leaf blades to a better feeding, even when the animals ingested about 1.0 kg DM less per cow with high requirement. Curran et al. (2010) also found increasing milk yield per kg of ingested herbage for decreasing pre-grazing herbage mass. Moreover, the larger protein yield by cows in LLI pastures in the second grazing cycle is closely related to animal energy balances. The effects of energy supply on protein synthesis for milk is widely known, since protein synthesis in the mammary glands is an energy-expensive process (Coulon & Rémond, 1991  2. Better quality of herbage throughout the pasture growth cycle allows for increasing fat-corrected milk yields by dairy cows, regardless of herbage intake variations. (1) Means inside each cycle followed by equal letters do not differ by the t test at 5% probability.