trabalho completo - 52ª Reunião Anual da Sociedade Brasileira de

52a Reunião Anual da Sociedade Brasileira de
Zootecnia
Zootecnia: Otimizando Recursos e Potencialidades
Belo Horizonte – MG, 19 a 23 de Julho de 2015
Light interception and structure of MG4 grass grazed by beef cattle under rotational stocking1
Jean Carlos Steinmacher Lourenço2, Larissa Goltz2, Ricardo Davi Kliemann2, Eduardo Michelon do
Nascimento2, Dayanna Pastal2, Sergio Rodrigo Fernandes2, Américo Fróes Garcez Neto2
1
Research financed by CNPq
Grupo de Estudo em Nutrição de Ruminantes e Forragicultura (GENFOR), Federal University of Paraná (UFPR), Palotina, Paraná State, Brazil.
e-mail: [email protected]
2
Abstract: The aim of this study was evaluate the structure and light interception of the canopy on a pasture of MG4
grass (Brachiaria brizantha) under rotational stocking with a breeding herd. The pasture under study represents a
module of traditional grazing (without intercropping) formed by 16.8 ha and grazed for 10 days by a herd of 60
cows and their calves. The structure and light interception of the pasture was evaluated in two times of the grazing
cycle, representing two treatments (Pre-grazing vs. Post-grazing). The data was analyzed following a complete
randomized design with 12 replicates. Mean canopy height and forage mass during the grazing cycle showed
similar values (P>0.05) with 30.8 cm and 3305 kg DM/ha, respectively. Despite on high stocking rate, the low
forage mass variation may be explained by the high forage accumulation rate (FAR) of the pasture. Leaf area index
(LAI) and light interception (LI) decreased (P<0.05) during the grazing cycle. These results indicate a decrease of
leaf mass of the pasture, which may be associated to the high intake of leaves by the animals. The rotational
stocking management used in the farm during the study allowed the intake of high quality forage by the breeding
herd.
Keywords: Brachiaria, canopy height, forage mass, leaf area index
Estrutura e interceptação luminosa de pasto de capim MG4 pastejado por bovinos sob lotação rotacionada
Resumo: Objetivou-se com este estudo avaliar a estrutura e a interceptação luminosa de um pasto de capim MG4
(Brachiaria brizantha) manejado sob lotação rotacionada com bovinos na fase de cria. O pasto em estudo
representa um módulo de pastejo tradicional (sem integração à rotação de culturas) com área de 16,8 ha e pastejado
durante 10 dias por um lote de 60 vacas e seus bezerros. Foram avaliadas as características estruturais e de
interceptação luminosa do pasto em dois momentos do ciclo de pastejo, representando dois tratamentos (Pré-pastejo
vs. Pós-pastejo). Os dados foram analisados em um delineamento inteiramente casualizado com 12 repetições. A
altura média do dossel e a massa de forragem antes e após o pastejo foram semelhantes (P>0,05) e apresentaram
valores médios de 30,8 cm e 3305 kg de MS/ha. Apesar da lotação elevada, a baixa taxa de desaparecimento de
forragem pode ser explicada pela elevada taxa de acúmulo de matéria seca (TAMS) do pasto. Houve redução
(P<0,05) do índice de área foliar (IAF) e da interceptação luminosa (IL) no período de pastejo. Isso mostra que
houve redução da massa de folhas do pasto, que foram potencialmente consumidas pelos animais. Pode-se inferir
que o manejo sob lotação rotacionada adotado no período de estudo permitiu a colheita de forragem de alta
qualidade pelos bovinos na fase de cria.
Palavras-chave: altura do dossel, Brachiaria, índice de área foliar, massa de forragem
Introduction
Brazil is one of the main countries in the global meat and dairy markets using grazing systems. Part of this
success is due to a large area of grasslands covering almost one third of all agricultural lands of the country.
However, when that production is analyzed in terms of yield per animal and per hectare, it is clear that our livestock
systems still present low efficiency causing harmful effects to different ecosystems reducing their sustainability.
When the Brazilian cattle herd is distributed into these large areas of grasslands, it is realized that the general
stocking rate is low compared to the yield potential of tropical regions with good availability of natural resources
like water and radiation. In a broad view, this stocking rate could be justified to explain a higher animal individual
performance. However, the true picture for these systems is related to low performance, both as in animal as in land
basis, depicting common mistakes in the grassland management resulting in a widespread degradation process
(Flores et al., 2008; Macedo, 2009).
Regarding that scenario, a study was carried out to characterize how a pasture under a traditional rotational
stocking has its structure and light interception changed from pre-grazing to post-grazing condition.
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52a Reunião Anual da Sociedade Brasileira de
Zootecnia
Zootecnia: Otimizando Recursos e Potencialidades
Belo Horizonte – MG, 19 a 23 de Julho de 2015
Material and Methods
The study was carried out at the Fazenda Água da Lontra (24⁰00’29.75” South, 53⁰56’11.72” West and 299
m altitude) located at the city of Iporã, Paraná State, Brazil. The climate is a Humid Subtropical with hot summer
and regular rainfall distribution during the year, represented by Cfa classification according to Köppen.
The grazing area (23º59’53.79” South, 53º56’13.05” West) was established with Brachiaria brizantha cv.
MG4, representing 16.8 ha of pasture grazed by breeding herd under rotational stocking. A herd of 60 Nellore cows
(mean body weight of 404.5 kg) and their calves (mean body weight of 101.1 kg) grazed the area for ten days
during May/2014. The stocking rate during that period corresponded to four animal units (AU)/ha/day.
The experiment followed a completely randomized design with two treatments and twelve replicates. The
treatments were the first and the last day of grazing period in a single paddock (entrance and exit of the animals
from the paddock) during the rotational grazing cycle, and the experimental units were the sampling points in the
pasture. The light interception (LI), leaf area index (LAI), canopy height (CH) and forage mass (FM) were
measured in twelve points distributed along two transects (six points per transect) in the paddock. Each sampling
point represented the mean pasture visual condition. LI, LAI and CH were measured before cutting the forage
samples. LI and LAI measurements were performed using the SunScan® (Delta Devices Ltda., Cambridge, United
Kingdom), with one measurement above and five measurements below the canopy (ground level). The CH was
measured with a ruler. Forage samples were taken by cutting the forage at ground level using a portable mower and
a square frame of 1.0 m². The forage samples were dried in a forced-air oven at 65ºC until achieve constant weight
to calculate the FM (kg DM/ha).
The data was submitted to analysis of variance by the General Linear Model procedure (PROC GLM), and
the means were compared by Tukey test at 5% significance. The analyses were performed in the SAS software
(Statistical Analysis System, version 9.0).
Results and Discussion
The means for CH and FM before and after grazing were similar (Table 1), presenting general values of 30.8
cm and 3305 kg DM/ha, respectively. Although at high stocking rate (4.0 AU/ha/day), the forage disappearance rate
at the paddock was low. This can be related to high forage accumulation rate (FAR) during the grazing period. In
May, during the period when the measurements were made (10 days), the region presented 240 mm of rainfall
regime and a sequence of days with high radiation (Mean air temperature = 21.2ºC, minimum air temperature =
11.8ºC, and maximum air temperature = 32.8ºC). This climatic condition associated to high natural soil fertility is
determinant to increase the yield of tropical forages.
Although the FAR had not been measured, some works with cultivars of Brachiaria brizantha (Xaraés and
Marandu) under similar conditions have shown that yields around 83.5 and 121.1 kg DM/ha/day are potentially
reached on favorable climatic conditions and high soil fertility (Pedreira et al., 2007; Flores et al., 2008). It is likely
that there was a balance between the rates of forage accumulation and disappearance, which may have caused the
maintenance of the CH and FM during the period that the paddock was under grazing.
The LAI in the pre-grazing condition was 4.25, with a LI of 94% (Table 1). The LI was close to 95%, value
in which the highest net growth of the pasture is reached with best balance in favor to leaf production instead
production of senescent material and stems (Pedreira et al., 2009). The LAI and LI in the pre-grazing condition
point out that the animals had an offer of forage of high quality during the grazing period, which was characterized
by the high availability of new leaves for intake.
Table 1. Means for structural characteristics and light interception of MG4 grass (Brachiaria brizantha cv.
MG4) pasture in pre-grazing and post-grazing condition.
Measurement
Pre-grazing
Post-grazing
P value
Canopy height (cm)
31.5 (1.3)
30.0 (1.0)
0.3842
Forage mass (kg DM/ha)
3321 (128)
3289 (331)
0.9296
Leaf area index
4.25 (0.18) a
1.47 (0.29) b
<0.0001
Light interception (%)
93.93 (0.58) a
89.42 (1.74) b
0.0227
Numbers inside parenthesis indicate the standard error of mean; Means followed by different lowercase letters
in the same row differs by Tukey test (P<0.05)
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52a Reunião Anual da Sociedade Brasileira de
Zootecnia
Zootecnia: Otimizando Recursos e Potencialidades
Belo Horizonte – MG, 19 a 23 de Julho de 2015
LAI and LI were reduced during the grazing period (Table 1). These results suggest that there was a
reduction in the mass of leaves during the grazing, likely due to the selection by the animals, but which were
quickly replaced by new leaves with the fast growth of the forage. The high availability of leaves during the grazing
makes easier the selection of this morphological component (Trindade et al., 2007). Thus, the structural condition
of the pasture before and after grazing during the study allowed animals select a high quality diet which could result
in better animal performance.
Conclusions
Changes in structural characteristics and light interception during the grazing period do not necessarily
imply on changes in the total forage availability when the pasture growth is well supported by favorable climatic
conditions. Although the results point out to the compatibility of the management to the herd demands, it is
recommended the study of how climatic variables as rainfall and environmental temperatures could be
systematically included into the farm forage planning.
Acknowledgements
Mr. Luiz Moesh, owner of the Fazenda Água da Lontra, to gently make available his property to carry out
that work.
Literature Cited
Flores, R. S.; Euclides, V. P. B.; Abrão, M. P. C.; Galbeiro, S.; Difante, G. S. and Barbosa R. A. 2008. Desempenho
animal, produção de forragem e características estruturais dos capins marandu e xaraés submetidos a
intensidades de pastejo. Revista Brasileira de Zootecnia 37:1355-1365.
Pedreira, B. C.; Pedreira, C. G. S. and Silva, S. C. 2007. Estrutura do dossel e acúmulo de forragem de Brachiaria
brizantha cultivar Xaraés em resposta a estratégias de pastejo. Pesquisa Agropecuária Brasileira 42:281-287.
Pedreira, B. C.; Pedreira, C. G. S. and Silva, S. C. 2009. Acúmulo de forragem durante a rebrotação de capimxaraés submetido a três estratégias de desfolhação. Revista Brasileira de Zootecnia 38:618-625.
Macedo, M. C. M. 2009. Integração lavoura e pecuária: o estado da arte e inovações tecnológicas. Revista
Brasileira de Zootecnia 38:133-146.
Trindade, J. K.; Silva, S. C.; Souza Jr., S. J.; Giacomini, A. A.; Zeferino, C. V.; Guarda, V. A. and Carvalho, P. C.
F. 2007. Composição morfológica da forragem consumida por bovinos de corte durante o rebaixamento do
capim-marandu submetido a estratégias de pastejo rotativo. Pesquisa Agropecuária Brasileira 42:883-890.
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