Demographic patterns of tillering and herbage production in mombaça grass pastures submited to regimes of intermittent stocking.

Author(s)

Marina Castro Uebele

• Document type: Master's Dissertation
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2002-06-04

Examining board members

Sila Carneiro da Silva; Moacyr Corsi; Luis Roberto de Andrade Rodrigues

• Advisor: Sila Carneiro da Silva

Abstract

The optimization of grazing systems cannot be conceived simply as a maximization of the amount of produced herbage or dry matter consumed by grazing animals. It is necessary to take into account factors that condition and determine the persistence and productivity of pastures in order to ensure the ability of plants to replace dead tillers and maintain pasture stability. Against this background, the present experiment aimed at evaluating the effects of four combinations of grazing frequency and intensity on tiller dynamics/population density and herbage production of grazed Mombaça grass pastures to provide a firm basis for planning grazing management strategies that allow for high herbage productivity and utilization, preserving pasture structure within acceptable limits of use and ensuring its persistence. The experiment was carried out at Universidade Federal de São Carlos, Araras, SP, from 8 January 2001 until 23 February 2002. Treatments comprised combinations between two grazing intensities (post-grazing heights of 30 and 50 cm) and two grazing frequencies (grazing initiated at 95% and 100% canopy light interception - LI), and were assigned to experimental units (2000 m 2 paddocks) according to a complete randomized block design, following a 2x2 factorial arrangement, with 4 replications. The average grazing interval varied from 23 to 36 days for the 95 and 100% LI treatments, respectively, throughout the spring/summer period. During the winter, these intervals ranged from 95 to 186 days. Pre-grazing pasture height was a reliable grazing management guide based on canopy light interception, since it presented a homogeneous and consistent behavior during the entire experiment. There was no difference in tiller appearance and death rates among treatments. High tiller death rates, however, were accompanied by high tiller appearance rates, with higher values recorded during spring/summer in relation to autumn/winter. High grazing frequency (95% LI) and/or intensity (30 cm post-grazing height) resulted in the highest tiller death rates, causing a high tiller turnover that generated a younger profile in tiller population density compared to the low frequency treatments (100% LI). There was no difference in tiller population density among treatments (average of 380 tillers m -2 ), indicating that differences in herbage production (18,230 to 27,220 kg DM ha -1 ) must have been consequence of differences in production per tiller, with younger tillers being more productive than older ones, which was the main reason for the highest forage yield of the 30/95 treatment (27,220 kg DM ha -1 ). Tiller population density varied with time of the year, with higher values recorded during summer in relation to autumn and spring (400, 384 e 344 tillers m -2, respectively). The highest values for pre-grazing herbage mass (8,100 kg DM ha -1 ) and pre-grazing pasture height (116 cm) were recorded for the 100% LI treatments compared to 95% LI (5,400 kg DM ha -1 and 89.5 cm, respectively), since they resulted in longer grazing intervals than the 95% LI treatments. Accumulation of stem and dead material was certainly the main reason for the difference in herbage mass, and made it difficult for grazing pastures down to the target 30 cm post-grazing height. There was a season effect on all responses studied, indicating a very seasonal growth behavior of Mombaça grass pastures. Based on the results and baring in mind the importance of a high tiller turnover for optimizing herbage production and utilization, grazing must be initiated at 90 cm pre-grazing height (95% LI) and stopped at 30 cm post-grazing height. (AU)