The exclosure cage technique revisited: Pasture responses under continuous stocking and their relationship with estimates of forage accumulation in grazing experiments

Abstract

Forage accumulation (FA) is a key response in grazing experiments, and estimates of FA in continuously stocked pastures are often obtained using exclosure cages. This study evaluated how 14-, 21-, and 28-d exclusion periods using cages affected morphological, physiological, and morphogenetic responses of continuously stocked 'Mulato II' hybrid brachiariagrass (Brachiaria spp. syn. Urochloa spp.) pastures maintained at 20-and 30-cm canopy height and how this affected estimates of FA. Grazing exclusion resulted in greater rates of leaf elongation (LER; 1.62 vs. 1.37 cm tiller −1 d −1), stem elongation (SER; 0.16 vs. 0.07 cm tiller −1 d −1), leaf growth (101 vs. 86 kg dry matter [DM] ha −1 d −1), and stem growth (3.4 vs. 1.8 kg DM ha −1 d −1) inside the cages compared with outside. Greater LER resulted in greater final leaf length and leaf area index (LAI) in longer exclusion periods. With greater LAI, photosynthesis was greater within the exclosure cages than on pasture (54.5 vs. 43.9 μmol CO 2 m −2 s −1). Greater leaf growth rate, together with same tiller population density and leaf senescence rate (LSR) across exclusion periods, resulted in overestimated FA by 14, 26, and 24% for 14-, 21-, and 28-d exclusions, respectively. There was no effect of canopy height on FA, because greater LER and SER values in 30-cm canopies were compensated by greater LSR. Choice of sites for cage placement by visual appraisal and reduced number of sampling sites resulted on "negative" values of calculated FA for the 14-d exclusion, making the choice of sampling sites a major source of error in estimation of actual FA. Increasing the number of sampling sites and using indirect measurements of forage mass (FM) may increase the accuracy of FM and FA estimates.

Defoliation management and nitrogen fertiliser rate affect canopy structural traits of grazed guineagrass (Megathyrsus maximus) cv. Zuri under rotational stocking

“…Taller canopies (H75) have lower TPD (Table 7) with bigger and heavier tillers (H75) (Table 4). Tropical grasses managed at greater height have taller plants (higher ATW) and lower TPD (Yasuoka et al . 2021), as registered in the present study.…”

Intensive Production and Management of Marandu Palisadegrass (Urochloa brizantha ‘Marandu’) Accelerates Leaf Turnover but Does Not Change Herbage Mass

“…Nonetheless, the lack of differences in herbage accumulation rate between semiintensive and intensive systems, but with lower stocking rate in semi-intensive than intensive systems, might suggest an under grazing condition in semi-intensive levels of pasture intensification. On the other hand, a previous study discussed the limitations of exclosure cage techniques in continuously stocked, with variable stocking rate, pastures and concluded that the cage technique tended to overestimate forage accumulation and to result in high variability in forage accumulation rate calculations, depending on the number and choice of sampling sites, and the measurement of initial forage mass [39]. When the number of sampling sites is small, as in the present study, to avoid excessive destructive sampling, the accuracy of forage mass estimative tended to decrease [39].…”

“…On the other hand, a previous study discussed the limitations of exclosure cage techniques in continuously stocked, with variable stocking rate, pastures and concluded that the cage technique tended to overestimate forage accumulation and to result in high variability in forage accumulation rate calculations, depending on the number and choice of sampling sites, and the measurement of initial forage mass [39]. When the number of sampling sites is small, as in the present study, to avoid excessive destructive sampling, the accuracy of forage mass estimative tended to decrease [39]. Then, the lack of differences in herbage accumulation rates between semi-intensive and intensive systems might have resulted from the high variability and limitations inherent to the method.…”

Stocking method effects on forage accumulation, canopy attributes, and nutritive value of grazed Mulato II brachiariagrass pastures

“…The FA was calculated as the difference between FM inside two exclosure cages (0.64 m 2 basal area and 1.2 m in height) at the end of each 21 days period and the FM at the time of cage placement in pastures under CS (Yasuoka et al, 2021). The anchoring locations of exclusion cages in each FA periods were in representative relation to the average FM.…”

“…& C. M. Evrard) Crins] was one of the first Brachiaria hybrids commercially released and has been considered a good option for diversification of Brazilian pastures (Pequeno et al, 2015; Silva et al, 2016a). It has shown a good potential of forage accumulation (FA) (Paraiso et al, 2019; Silva et al, 2016a; Vendramini et al, 2012; Yasuoka et al, 2021), nutritive value (Paraiso et al, 2019; Pequeno et al, 2015; Silva et al, 2016a), and response to nitrogen fertilisation (Silva et al, 2016b; Silva et al, 2020; Yasuoka et al, 2017).…”