Annual cool-season pasture in rotation with corn/soybean at the summer in crop-livestock
systems is a very important strategy in southern Brazil since at winter, there
are few economically viable crops (Brum et al.,
2005; Balbinot et al., 2009).
Among the annual cool-season forage species, black oat (Avena strigosa Schreb.)
and annual ryegrass (Lolium multiflorum Lam.) are the most used species
due to its adaptability to soil and climatic conditions of southern Brazil and
due to its high productivity, high quality and potential for animal production
(Assmann et al., 2004; Aguinaga
et al., 2008).
Regarding to the system management, grazing intensities represented by different
sward canopy heights of pasture management is the most important issue, once
it determines the primary and secondary production and/or the success or failure
of the crop-livestock system. Considering these aspects, the challenge is to
find a sward canopy height that allows at the same time both forage production
with good animal performance maintaining soil fertility in order to obtain high
grain yield in the subsequent row crop.
Pastures kept at very low sward canopy height or high grazing intensities can
affect forage production (Da Silva and Pedreira, 1997).
Moreover, in this limited condition, animals bite size is affected and to compensate
and maintain the level of consumption, the animal increase the number of bites,
the number of visited feeding station and the total displacement on the area
(Baggio et al., 2009). As a result, soil physicochemical
and biological traits may be affected, resulting or not in lower yield of crops
grown in sequence depending on the used grazing intensity (Albuquerque
et al., 2001; Salton et al., 2002;
Nicoloso et al., 2006; Flores
et al., 2007; Lopes et al., 2009;
De Souza et al., 2009).
In this context, critical level of residual biomass after grazing period is
a major issue for the security of the crop-livestock system, since the addition
and maintenance of plant residues as a soil coverage is extremely important
to increase water infiltration and storage in the soil, reducing, within certain
limits, runoff and erosion (Panachuki et al., 2011).
Moreover, besides the amount of residual biomass, its decomposition rate plays
a major role on the system management due to its soil protection and nutrient
Thus, the aim of this study was to evaluate the effects of pre and post-grazing
sward canopy height management of Common black oat (Avena strigosa Schreb.)+common
ryegrass (Lolium multiflorum Lam.) in rotational stocking system over
the biomass production and its residual biomass decomposition rate over 270
days of field incubation.
MATERIALS AND METHODS
Research was carried out on a farm at Coronel Vivida, PR (25°
07 south, 52°
41 east; average altitude 730
m). Climate of the region is subtropical humid, according to the Köppen
classification and the soil at the experimental site is classified as an Oxisol.
The meteorological data of the experimental period are show in Fig.
The experiment was laid out as a randomized block design with four replications.
Treatments consisted of three sward canopy heights of pasture management (black
oat+annual ryegrass) on rotational stocking and one treatment without grazing.
Sward canopy height management were characterized by the entrance of animal
at the paddocks at 25, 30 and 35 cm of sward canopy height and exit at 5, 10
and 15 cm, respectively for high, medium and low grazing intensity.
Black oat was sown with a fertilizer-seeder (100 kg ha-1of seed)
on May 05th and ryegrass obtained through natural reseeding. The area has been
managed under no-tillage system since 2004 with crop-livestock system with soybean/corn
at summer and oat+ryegrass at winter for grazing animals being soybean the crop
cultivated before the experiment. Was applied 100 kg ha-1 of N thirty
days after the pasture sowing.
The experimental units (184 m2 per paddock) were separated by two
strands of electric fence. Were used Holstein dairy cows (500 kg live weight)
and to adjust the heights proposed the animals stayed into the paddocks for
different periods (from 5 up to 8 h). After each grazing period, ungrazed sites
were mowed to uniform height of pasture according to treatment and the biomass
taken out from the paddock.
Sward canopy height assessment were done along the experimental period to determine
the time to entry and exit with the animals from paddocks and before and after
each grazing period using a sward stick. The distance from the soil level to
the touch of the stick marker in the first leaf was considered as the plant
height being 20 places measured in each paddock.
All the treatments provided five grazing events up to September 09th
totaling 133 days of pasture evaluation. After the grazing period, the area
was desiccated (09/17 with 740 g ha-1 i. a. glyphosate).
Forage dry matter accumulation (kg ha-1 day-1) was evaluated
by cutting the pasture in a square area of 0.25 m2 (0.5-0.5 m). Before
each grazing period, two sites (0.25 m2) each paddock, representing
the average sward canopy height were cut to determine the pasture rate of growth.
The cut samples were dried in a forced-air oven at 60°C until constant weight
and then converted to kg ha-1 of dry matter.
The average accumulation rate over the experimental period was obtained by
dividing the forage production of the period by the number of days of the period.
Total forage production was obtained by the initial forage mass plus the forage
production of each period and expressed in kg of dry mass per hectare.
To determine the amount of residual dry matter, three sites of 0.25 m2
were cut each paddock after the last grazing period. Residual biomass was collected
and dried at 55°C until achieving constant weight to determine the amount
of dry biomass. Pasture residual dry matter decomposition rate of the treatments
were evaluated by the litter bag technique.
The material collected after the last grazing was homogenized and samples taken
out and placed into litter bags (15 g bag-1) having a 1 mm mesh size
and measuring 20x20 cm in size. Litter bags were sewn by a stitch machine and
placed on the soil surface of their respective treatments where remained for
different incubation periods from October 18th, up to June, totaling 270 days
of field incubation.
Average, minimum and maximum temperatures
and rainfall data observed through the experimental period
Dry matter decomposition rate was evaluated along the time, performing ten
samples with three replicates (three bags per treatment to each period) collected
at: 15, 30, 60, 90, 120, 150, 180, 210, 240 and 270 days after field incubation.
The percentage of remaining biomass was calculated on the basis of the total
biomass at the start and end of the incubation periods. Using the weight difference
between the incubation periods, dry matter decomposition rate was determined
and converted to a percentage. Results are expressed in percentage of the initial
weight in grams considering 15g equal to 100%.
Data were subjected to ANOVA by the Statigraphic plus 4.1 statistical program
being conducted comparison of means by Tukey test at 5% probability. Decomposition
rate of the residual dry matter were adjusted by nonlinear models to fit the
decay curves. Before choosing the model, the data from each replication were
plotted (p>0.05) to observe the pattern of distribution. Single and double
exponential models are described by Eq. 1 and 2
(Weider and Lang, 1982):
where, RDM is proportion of remaining dry matter at time t (days), ka and kb
is decay constants of the easily decomposable compartment (A) and more recalcitrant
compartment (100-A), respectively.
From the decay constant values of each compartment, its half life (t1/2)
was calculated, or in other words, the time necessary to decompose 50% of the
compartment. To accomplish this, an equation described by Paul
and Clark (1996) was used:
t1/2 = 0,693/k(a,b)
RESULTS AND DISCUSSION
Sward canopy heights data were close to the desired ones, differing among them
(p<0.05), an essential requirement to set up the contrasts proposed (Table
Was noticed a particular difficulty in maintaining the desired sward canopy
height of the treatments since the animals do not graze uniformly due to the
selectivity and the presence feces and urine in the pasture. This fact was also
observed in other studies in which the criterion of management is the height
of the pasture (Lopes et al., 2009; Carvalho
et al., 2010). Due to it, the samples were cut at representative
sites of the wanted heights being the remaining portion of the paddock mowed
and the residue removed from the paddock.
||Black oat+ryegrass height assessed before and after each grazing
event at the different treatments (ungrazed, 35-15, 30-10 and 25-05 cm)
Carvalho et al. (2011)
|Means in the same column followed by different uppercase letters
differ (p<0.05) by Tukey test
At the last grazing period, the sward height did not differ among treatments
because was chosen to standardize the last grazing on the same dates in order
to end the grazing period on the same day for all treatments and avoid a possible
regrowth of the pasture. On Fig. 2a, b and
c are shown respectively the results of the dry matter accumulation
rate (kg ha-1 day-1), total dry matter production (kg
ha-1) and residual dry matter (kg ha-1) and it is possible
to observe that the different sward height of management significantly influenced
all the evaluated parameters.
Average dry matter accumulation rates (Fig. 2a) were of 56,
42, 41 and 32 kg ha-1 day-1 corresponding to a total dry
matter production of 7,548; 5,531; 5,494 and 4,322 kg ha-1, respectively,
for treatments without grazing, 35-15, 30-10 and 25-05 cm (Fig.
2b). It is also possible to observe that the treatments 35-15 and 30-10
cm, despite producing less than the ungrazed treatment, did not differ from
each other, showing however higher production than the 25-5 cm treatment.
According to Da Silva and Pedreira (1997), sward height
kept very low or high grazing intensities affects the plant leaf area and canopy
radiation interception, which, in turn, affect the photosynthetic rates and
the ability to produce new leaves. These changes in the photosynthetic process,
determined by variations in grazing intensity affect the accumulation rate and
overall productivity of dry matter. Thus, the difference on forage production
can be explained by the lower leaf area index and thus lower photosynthetic
activity of the 25-05 cm treatment.
Black oat+ryegrass dry matter
, (a) Accumulation
rate, (b) Total production and (c) Remaining dry matter
in relation to
the different height of management (ungrazed, 35-15, 30-10 and 25-05 cm)
Moreover, it is important to consider that common Black oat at the grazed treatments
decreased its contribution in total production from August on, while at the
ungrazed treatment, black oat represented a large proportion of the total forage
production. This fact may have occurred because at the early period of grazing,
tillers of the black oat were higher than the ryegrass and therefore grazed
more often, which associated with low height of pasture management result on
tiller population reduction and dead, which negatively affect the pasture production.
Aguinaga et al. (2008) also reported that common
black oat has a fast initial development and decrease its production in the
later periods of development. Associated with this aspect, Carvalho
et al. (2011) also observed a reduction in the black oat population
density of tillers when subjected to intense grazing (10 cm). The author also
reports that this may result in higher unprotected soil subject to erosion and
compaction of the soil surface, initiating the degradation process.
Moreover, grazing period in this experiment was finished earlier (09/17) than
usually it is when soybean is grown, simulating a rotation with corn and due
to it ryegrass could not contribute much to the total dry matter production
of the grazed treatments, resulting in lower production when compared to the
Assmann et al. (2004) evaluating the performance
of improved Black oat+ryegrass with 100 kg ha-1 of N under continuous
stocking grazing system (93 days of grazing) management at 14 cm of height reported
dry matter accumulation rate of 37 kg ha-1 day-1 and total
dry matter production of 4,706 kg ha-1. This data are similar to
those found in this study when compared to the similar sward heights.
Canto et al. (1997) evaluating common black
oat fertilized with 100 kg ha-1 of N reported total dry matter production
of 4,545 kg ha-1. Furthermore, Moreira et
al. (2001) evaluating Black oat IAPAR 61 by cuts and with 100 kg ha-1
of N, reported total dry matter production of 4,993 kg ha-1. These
results are similar to the ones found in this experiment.
Cassol (2003) in a similar work, however, working with
continuous stocking rate grazing system, found DM accumulation rates of 46,
43 and 35 kg ha-1 day-1 and total production of 7,542;
7,118 and 5,973 kg ha-1, respectively, to the Black oat+ryegrass
managed at 30, 20 and 10 cm of sward canopy height. It is important to highlight
that although there was no statistical difference, there was a DM difference
of 1,569 kg between treatment 30 and 10 cm of height, a value similar to the
difference found in this study between the treatment 35-15 and the lowest height
management (25-05 cm). The lower height of pasture management and the length
of the evaluation period may have contributed to the differences between this
study and the data reported by Cassol (2003).
Lopes et al. (2009), evaluating Black oat+ryegrass
production on a continuous stocking rate grazing system managed at different
sward canopy heights (12, 19, 28 and 32) and a treatment without grazing (40
cm) reported no difference to the dry matter accumulation rate between treatments
with an average of 50 kg ha-1 day-1. The lower height
of the ungrazed treatment and similar height among treatment at the first grazing
period may have benefited the treatment managed at the lowest height, thus reducing
possible differences between them.
Grazing intensity affected not only the total biomass produced, but also the
amount of remaining biomass at the end of the grazing period as well as its
decay rate constants. Residual biomass increased linearly (p<0.05) as the
sward height increased, showing that the taller is the pasture canopy, the greater
is the amount of biomass that will remain on the soil surface, contributing
to water infiltration and water storage in the soil, reducing, within certain
limits, runoff and erosion (Panachuki et al., 2011)
and increasing soil organic matter content (De Souza et
Residual biomass values found were: 7,548; 1,950; 1,610 and 757 kg ha-1,
respectively to the treatments without grazing, 35-15, 30-10 and 25-5 cm, which
showed real heights of 80, 18, 13 and 7 cm after the last grazing period.
Cassol (2003) also reported a great difference (more
than 4 t ha-1) on the black oat+ryegrass residual biomass after the
grazing period between the treatments without grazing and the treatment managed
at 10 cm. The author also reports residual biomass values of 2,120 and 622 kg
ha-1 of DM for the sward heights management of 20 and 10 cm, respectively.
Flores et al. (2007), working under the same
experimental conditions, reported soil surface residual biomass ranging from
1,850 up to 6,050 kg ha-1 respectively for the treatment managed
at 10 cm of sward height and the ungrazed treatment. Differences between wanted
sward height and real sward height may explain these differences.
Considering that for the success of No-tillage System (NTS), the annual addition
of dry matter biomass to the soil must not be less than 8 t ha-1
(Lovato et al., 2004; Nicoloso
et al., 2006), it can be inferred that low sward height of management
may compromise the NTS.
Beyond soil biological characteristics, its physical characteristics are very
important to the crop-pasture integration development. Flores
et al. (2007) evaluating different grazing intensities and remaining
biomass reported that even with lower levels of residual biomass (2,000 kg ha-1
of DM) there were no differences on the physical attributes related to soil
compaction when compared to the ungrazed treatments and consequently there was
no reduction on the soybeans production cultivated in sequence, although, negative
effects may appears after few years if low height of management or high grazing
intensities are kept (Carvalho et al., 2011).
In this context, De Souza et al. (2009) working
under the same experimental conditions but after several years of evaluation,
reported that the treatment managed at 10 cm is not able to maintain an adequate
level of straw to establish the no-tillage system and that after the third year
of evaluation, carbon stocks losses are in the order of 0.33 mg ha-1
Carvalho et al. (2011) in a summary about the
effects of the grazing intensities on continuous stocking rate report that pasture
management at 10 cm of height adversely affects soil porosity, water infiltration
into the soil, the stock of carbon and nitrogen in the soil, the aboveground
biomass after grazing, the time and displacement of grazing animal, the average
daily gain, carcass quality and finally the soil quality and sustainability
of the system.
On the other hand, the same authors reported that sward heights (black oat+ryegrass)
managed between 20 and 40 cm have been able to add to the soil carbon amounts
higher than 9.0 t ha-1 of DM when pasture roots and shoots are add
up to the soybean residual biomass, what resulted in an increase on the soil
carbon content of these treatments over time. The authors conclude by saying
that the best management of black oat+ryegrass mixtures on continuous stocking
rate grazing system, in a way that best benefits the crop-livestock system management
corresponds to the plant height at 20 cm, where several chemical, physical and
biological soil properties are enhanced by the action of grazing.
In addition, an alternative to increase the supply of plant biomass in areas
with intensive use of cool-season pastures would be the summer crop rotation
with corn, due to its high residual biomass production and slow decay constants
(Bertol et al., 2004) as well as the use of
nitrogen, both on cool-season pasture and in corn crops (Assmann
et al., 2003).
Nicoloso et al. (2006) evaluating the residual
biomass in areas with and without grazing observed at the plots without grazing
(cool-season pasture used as a cover crop only), residual biomass higher than
10 t ha-1. Grazed areas although, with rotational grazing each 14
and 28 days added to the soil at the end of the grazing period, 2,640 and 3,420
kg ha-1 of DM, respectively, however, this area produced more than
300 kg ha-1 of animal live weight. In this context, it is important
to consider that at the grazed areas, in addition to residual biomass, a large
part of the nutrients consumed by the animals (70 to 95%) return to pasture
as excrement (feces and urine) readily decomposable for use by the next crop
(Haynes and Williams, 1993).
As important as the amount of the remaining material after grazing is its decay
rate over time. Pasture biomass dry matter decomposition followed a double exponential
model (Eq. 2), were both compartments of nutrients decrease
exponentially at a constant rate (ka and kb) being the first fraction (A) decomposable
at higher rates than the second one (100-A), which is more difficult to decompose
There was a rapid decomposition at the initial periods followed by slower one.
In a short term, decomposition rates are high due to the high content of fast
decomposable components such as sugars, aminoacids and proteins. In the later
stages, decomposition rates tend to decrease due to the accumulation of recalcitrant
components such as lignin, tannins and cellulose (Giacomini
et al., 2003; Lupwayi et al., 2007).
Residual biomass from the ungrazed treatment showed the lowest A compartment
and the lowest decay constants in relation to the other treatments. The percentage
of material present in the A compartment is highly dependent on the quality
of the residual biomass, which is dependent on the pasture sward. Carvalho
et al. (2010) reported that usually, pasture managed at low sward
canopy height represent a small amount of residual biomass but of greater nutritional
value, while pastures managed at high sward canopy heights turn out in greater
amount of residual biomass but of lower nutritional value. As noticed in Table
1, treatment without grazing showed at the end of the grazing period, sward
canopy height of 80 cm. This residual biomass was made up of old material, composed
mainly of shoots with high amount of structural material such as lignin, cellulose
||Parameters of double exponential decay model fitted to the
measured values of the residual biomass after grazing in relation to the
different sward canopy heights of management
|decay constants (ka and kb), half-life
(t1/2) coefficient of determination R2
These factors resulted in the reduction of material present in the more easily
decomposable compartment (A), smaller decay constants (ka and kb), longer half-life
and therefore lower rates of decomposition (Table 2).
On the other hand, the residual biomass from the 25-05 cm treatment had the
highest A compartment and the highest decay constants what resulted in shorter
half life and therefore higher rates of decomposition (Table 2).
Higher decay constants found at this treatment may be explained because the
pasture sward at this treatment was composed of younger shoots and large quantity
of leaves and therefore better quality.
Holland and Detling (1990) reported that herbivory
can influence the organic matter decomposition and nutrient cycling rates by
altering the quality of the remaining plant biomass both above and below ground
and also by altering the soil environment for decomposition. After grazing,
the regrowth of plants often have higher nutrient concentrations in the aerial
tissues what can increase the decomposition rate of these tissues (Holland
et al., 1992). Thus, lower carbon/nitrogen ratio (C/N) at the grazed
plants can result in greater N mineralization on the soil and reduce the demand
for N during microbial decomposition of plant biomass and N immobilization problems
(Dubeux et al., 2006).
At the end of the first month after the litter bags incubation on the field,
73% of the initial Dry Matter (DM) of the ungrazed treatment was still on the
soil surface in the decomposition bags. The presence of grazing increased the
residual biomass decomposition rate, whereas in the same period, there was a
remaining percentage of 62, 57 and 52, respectively for the treatments 35-15,
30-10 and 25-05 cm.
It can be noticed on Fig. 3 that the decomposition rates
of the residual biomass increase as the sward canopy height of pasture management
decrease or residual biomass decrease, being the 25-05 cm the treatment with
the highest decomposition rates.
Remaining dry matter
of the residual
biomass from the (a) Ungrazed treatment , (b) 35-15 , (c) 30-10, and (d)
25-05 cm and along 270 of litter bags field incubation
Aita and Giacomini (2003), studying pastures decomposition
rate reported a dry matter residual of 81% to the black oat (4,390 kg ha-1)
and 57% for vetch after 30 days of litter incubation at the field. The highest
rate of decomposition observed in this experiment compared to the data reported
by Aita and Giacomini (2003) may have occurred because
of the nitrogen fertilization used (100 kg ha-1) in association with
203 mm of rainfall occurred within the first 30 days after the bags incubation
on the field against 130 mm reported by the authors.
It is also possible to infer that the highest rates of decomposition occur
in the first 90 days after field incubation. After three months of field incubation,
the percentage of residual dry matter to the treatments without grazing, 35-15,
30-10 and 25-05 were of 52, 43, 37 and 33%. These values decreased to 41, 33,
32 and 29% after another 60 days of field incubation showing higher rates of
decomposition on the 90 first days after field incubation.
After 270 days of field incubation, residual biomass dry matter percentage
was of 35, 27, 24 and 21%, respectively to the treatments without grazing, 35-15,
30-10 and 25-05 cm confirming the lower rates of decomposition of the ungrazed
From these data it is possible to infer that besides affecting the forage production,
the 25-5 treatment also shows the fastest rates of decomposition what may compromise
the crop-livestock production in a long term due to its negative effects on
the carbon inputs to the system and consequently to the soil productive potential.
It is also possible to infer that pasture management at higher sward canopy
height (30-10 and 35-15) allow better forage production and lower decay constants
and due to it, are more sustainable in relation to the 25-5 cm treatment, once
its rate of decomposition maintains a better synchrony between soil protection
and nutrient release.
Ungrazed treatment showed the highest biomass production, greatest residual
biomass and the lowest decomposition rate. Black oat+ryegrass pastures managed
at 30-10 and 35-15 cm provide greater forage production, greater residual biomass
and lower decomposition rate than the 25-5 cm treatment. Results indicate that
the mixture of black oat+ryegrass should not be managed at sward canopy height
lower than 30x10 on rotational grazing.