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Abstract
In order to optimize the regrowth and harvest of signal grass (Urochloa decumbens) cv. Basilisk pastures it is necessary to establish more precise grazing management guidelines. The objective of this study was to evaluate herbage accumulation, plant-part composition and nutritive value of signal grass managed under contrasting levels of steady-state canopy heights. Treatments included 3 canopy height targets, i.e. 10 (S-short), 17.5 (M-medium) and 25 cm (T-tall), in a completely randomized design with 4 replications. Experimental units were 144-m2 plots which were grazed by groups of steers for short periods in an endeavor to keep canopy heights at the 3 desired targets. On average, herbage accumulation rate (HAR) in T pastures was greater than in M and S pastures, including the dry-wet season transition period in spring (September‒November). The S pastures had higher crude protein and lower acid detergent fiber concentrations than M and T pastures, especially in the first half of the calendar year. However, in vitro organic matter digestibility was similar for all treatments (612 g/kg). As S and M pastures had lower HARs than T pastures in the spring, it appears advantageous to maintain the signal grass canopy at ~25 cm in order to ensure quick regrowth with the return of the wet season. However, longer-term studies are needed with recording of animal performance before these initial findings can be promoted widely.
Highlights
The calibration curves between rising plate meter (RPM) and canopy height did not differ across treatments and dates of calibration (P>0.05)
For herbage mass (HM) prediction there was no effect of treatment (P>0.05), but there was an effect of calibration date (P
This process peaked at the end of the wet season likely due to increased tissue senescence triggered by the imminent soil water deficit during this period (Figure 1)
Summary
Plant growth and harvest efficiency in grazed pastures are closely related to canopy structure (Bircham and Hodgson 1983). Excessive elongation of stem has a negative effect on pasture utilization (Hodgson and Silva 2002) due to increasing proportion of rejected patches and plant lodging. Grazing management, based on the maintenance of specific canopy structure by controlling LAI, canopy height or herbage mass, may avoid excessive stem growth and allow more predictable levels of herbage accumulation and animal performance (Silva et al 2013). Canopy height ranges recommended for grazing management are associated with the canopy architecture, as well as the plant-part (leaf, stem and dead material) accumulation dynamics under grazing. Keeping canopy height of Marandu palisade grass (Urochloa brizantha) at 10 cm by heavy grazing intensity negatively affected herbage accumulation (Silva et al 2013). Optimum canopy height seems to be around 15‒25 cm under continuous stocking (Santos et al 2013), with a pregrazing target under rotational stocking of 18‒30 cm (Pedreira et al 2017)
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