Abstract
The objective of this work was to evaluate the sward structure and nutritive value of Panicum maximum Jacq. cv. Tanzânia subjected to rotational stocking managements characterised by a common pre-grazing condition of 95% canopy light interception (LI) and two post-grazing residues, 25 and 50 cm. Treatments (95/25, 95/50 - LI/residue) were assigned to experimental units (groups of six 2500 m² paddocks per treatment) according to a complete randomised block design, with two replications. The variables measured corresponded to: canopy light interception, pre and post-grazing sward height, herbage mass and pre and post-grazing morphological composition, herbage bulk density, herbage accumulation and nutritive value (including to IVOMD) of the morphological components. Pre-grazing herbage mass did not differ between residues, although the herbage accumulation rate was higher for the 50 than the 25 cm (164.9 and 90.6 kg/ha.day DM, respectively). Post-grazing herbage mass values were higher for the 50 cm residue and were characterised by a higher proportion of leaf blade in relation to the 25 cm treatment, which presented a higher proportion of dead material. On average, the contents of crude protein (CP), neutral detergent fibre (NDF) and lignin in acid detergent (LAD) as well as the values of the "in vitro" organic matter digestibility (IVOMD) were similar for both treatments. Crude protein and IVOMD decreased and NDF and LAD increased from top to the bottom of the sward, indicating grazing intensity as an important variable for promoting adjustments in the grazing efficiency and nutritive value of the consumed herbage by the grazing animals.
Highlights
Tropical forage grasses are characterized by a high potential of herbage accumulation, the general productivity and efficiency indexes of tropical pastoral systems are relatively low (Nascimento Jr. et al, 2004)
Results for Mombaça (Carnevalli et al, 2006) and Tanzânia (Barbosa et al, 2007) grass subjected to rotational grazing managements characterized by pre-grazing conditions determined by levels of sward canopy light interception during regrowth have demonstrated that there is an overlap between the conditions necessary for the efficient production of herbage mass and those for producing herbage with high nutritive value
Grazing swards down to 25 cm, sward height and light interception showed a minor reduction with successive grazing cycles (Figure 3A)
Summary
Tropical forage grasses are characterized by a high potential of herbage accumulation, the general productivity and efficiency indexes of tropical pastoral systems are relatively low (Nascimento Jr. et al, 2004) In such systems, there is no single solution, and the manipulation of a simple and/or isolated variable does not result in immediate and effective modifications in the overall process and animal production efficiency. The magnitude and absolute value of these responses can vary according to the morphophysiological characteristics of the plant and animal species involved, but follow similar functional response patterns This opens a vast new horizon for managing tropical pastures in which the use of sward target-based grazing management practices becomes a reality and allows for generalization across a wide range of environments, respecting the rhythm of plant growth and development (Hodgson & Da Silva, 2002; Da Silva, 2004)
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