Implications
Predicted impacts of climate change will negatively affect extensive livestock production
in subtropical grasslands and savannas unless proactive strategies are developed to
mitigate negative impacts.
Livestock adaptation, via breeding for future environments, is key to ensuring livestock
health and performance from animals adapted to extensive grazing in hot and unpredictable
environments.
Grassland and savanna grazing management to ensure an adequate supply of the highest-quality
wet season forage (quality) and adequate volumes of dry season forage (quantity) is
critically important for extensive livestock production under adverse conditions.
Breeding (small adapted animals) and feeding (wet season quality and dry season quantity)
are key strategies for future-proofing livestock production in extensive conditions.
Introduction
Grasslands and savannas cover a majority of the Earth’s surface and have been heavily
impacted, fragmented, and transformed by anthropogenic activities (Buisson et al.,
2021). In many parts of the world, extensive livestock production is the most sustainable
agricultural option due to biophysical constraints on cropping. Well-managed livestock
grazing not only directly produces animal protein but supports the delivery of a breadth
of ecosystem services from grasslands (Lemaire et al., 2011).
Extensive livestock production relies upon a constant forage supply (Rust, 2019) throughout
the year, with quality meeting the nutritional requirements of growing or lactating
animals. In the future, managing subtropical grasslands and savannas for livestock
production will be complicated by global change factors including reduced or erratic
rainfall patterns and increased temperatures (Giridhar and Samiredddypalle, 2015).
Managers must also cope with greater scrutiny of the environmental impacts of livestock
production and animal welfare, along with concerns over biodiversity impacts and greenhouse
gas emissions.
Livestock and climate stress
The most important direct effects of increasing temperatures on livestock are reduced
nutrient intake and heat stress (Mariara, 2008). Higher ambient temperature can elevate
body temperature, to which cows respond by decreasing feed intake by 3% to 5% per
additional degree of temperature (Collier and Gebremedhin, 2015). Heat stress increases
respiration and mortality, reduces fertility, modifies animal behavior, and suppresses
immune and endocrine systems, thereby increasing disease susceptibility (Silankove,
2000). At the same time, exposure to vector-borne diseases—which can be strongly influenced
by climate change—and transmission of wild-borne diseases like foot and mouth disease
is likely to increase under predicted climate change scenarios (Savasni et al., 2015).
Breeding and cross-breeding might improve heat tolerance, as there are substantial
differences among breeds’ inability to cope with heat stress, even among high-yielding
genotypes. Small bodied cattle breeds are more heat tolerant (Elayadeth-Meethal et
al., 2018). The adoption of drought-tolerant ruminant livestock species and/or breeds
that are capable of efficiently utilizing poor quality roughages needs to be undertaken.
This would entail exploiting local or indigenous breeds of cattle, sheep, and goats
(Moyo and Nsahlai, 2021).
Smaller body size of tropical indigenous cattle breeds is recognized as being beneficial
for surviving in harsh environments, due, in part, to the smaller animals’ lower feed
and water requirements. High heat tolerance may, in part relate, to a greater ratio
of surface area to BW, (Tadessea et al., 2019) with smaller animals having a greater
relative surface area to lose heat from, relative to larger animals (Figure 1).
Figure 1.
N’dama cattle in equatorial savanna in Gabon. Mature weight ~250 kg, well adapted
to local forage quality, heat, and diseases. Photo by K P Kirkman.
Biological efficiencies of cow and calf to weaning or yearling weights were superior
for small cows (Morris and Wilton, 1976). Indigenous small framed breeds in Namibia
produced more kg weaner mass produced/100 kg cow mated than larger framed breeds (Lepen,
1996). Also in Namibia Els (2002) showed that frame size was related to productivity,
when stocked at the same biological weight per ha, with small framed animals had a
higher production per ha than large framed animals. Environmentally, semi-arid cattle
producing regions can very effectively take advantage of the lower production cost
and increased pasture-carrying capacity associated with maintaining cows of a smaller
frame size that will result in greater net return per ha per cow (Senturklu et al.,
2021).
Strategies to reduce the impact of hot conditions include ensuring optimal quality
forage to compensate for decreased intake, reducing walking during the hottest time
of the day as it increases a cow’s heat load, allowing full access to grazing at night,
and providing shade throughout the grazing unit. Cows will graze up to 70% of their
daily grazing at night in hot weather (Savasni et al., 2015). The shade provided by
“silvopasture”—a practice in which animals graze under intentionally-managed stands
of shade-providing trees that in turn deliver ecosystem services (Figure 2)—substantially
reduces ambient air temperatures for livestock across Latin America and Africa (Zeppetello
et al., 2022). Silvopasture also has a long history in the southeastern USA, and is
undergoing a resurgence as part of regional efforts to adapt to global temperature
change (Smith et al., 2022).
Figure 2.
Cattle grazing in a pine plantation silvopasture in the southeastern United States.
Photo by Richard Straight, USDA FS National Agroforestry Center, CC BY-NC.
Forage resources
In general, grazing natural grasslands is an economical feed source for livestock,
requiring little expenditure on additional resources that would otherwise increase
cost of production as well as environmental impact. Converting diverse natural agroecosystems
to low-diversity, intensive crop or pasture systems can substantially reduce the delivery
of supporting ecosystem services and greatly diminish soil carbon pools (Knops and
Tilman, 2000).
Rainfall amount and seasonal distribution, climate, and soil influence the quantity
and quality of grassland forage (Owen-Smith, 2008), which interacts with grazing effects
on grassland structure and maturation (Hempson et al., 2015). In the dry season, quality
decline is driven by changes between carbon assimilation and soil nutrient supply
(Ellery et al., 1995).
Reduced soil nitrogen caused by more rapid plant growth over extended growing seasons
in a warmer, carbon enriched environment are likely to cause a decline in forage quality
(Morris et al., 2022). Increasing temperatures are further projected to reduce forage
quality by lowering digestibility and crude protein content of feeds (Polley et al.,
2013). For every 1 °C rise in ambient temperature, the neutral detergent fiber (NDF)
content of feeds increases by 0.4% (Moyo and Nsahlai, 2021). Because NDF correlates
with forage dry matter digestibility (DMD), each 1% increase in NDF can drive a 0.6%
decline in DMD (Lee et al., 2017). Similarly, Moyo and Nsahlai (2021) suggest that,
the rumen degradability of feeds would most likely decrease by 0.6% for every 1 °C
increase in ambient temperature.
Lower-latitude regions are projected to experience greater livestock production loss
than temperate regions (Moyo and Nsahlai, 2021; Thornton et al., 2022). Sub-Saharan
Africa is particularly hard-hit by temperature-driven impacts on meat and milk production,
while projected negative impacts on beef production in central America are also high
(Thornton et al., 2022). Heat stress is consistently projected to become a serious
challenge in cattle production systems through this century, leading to decreases
in milk and meat production (Thornton et al., 2022).
Grassland and savanna management
The livestock heat stress and health issues outlined above require targeted interventions
that are likely to involve adapting livestock via breeding for future scenarios. Equally
important is ensuring a constant, high-quality supply of forage in a sustainable manner
without degrading the resource and lowering productive capacity as has happened in
many areas of the world.
As such, managers must simultaneously consider both the grassland resource (supply)
as well as the livestock (demand), which can impact both the livestock production
enterprise as well as the health of the grassland ecosystem. Grassland management
options include the use of fire (Fuhlendorf et al., 2017) and the manipulation of
livestock grazing patterns and intensity. In the case of livestock manipulation, variables
for manipulation include livestock type, livestock numbers (stocking rate), livestock
density (spatial distribution), and livestock movement (intra- and interseasonal movement
or temporal distribution), in which density and movement are coupled. Stocking rate
influences livestock performance (Jones and Sandland, 1974) as well as the impact
of grazing on the ecosystem (Gibson, 2009), with excessive stocking rates having the
potential to lead to desertification and loss of productive capacity. Manipulating
livestock density involves restricting livestock to grazing specific areas at higher
density for various periods of time, with corresponding periods of absence in the
grazing cycle. Interseasonal movement can influence supply of forage through the year,
while intraseasonal movement influences the quality and quantity of intake. Both inter-
and intraseasonal movement have the potential to influence defoliation patterns, vegetation
structure and maturity, and selectivity on the part of the grazers, which in turn
may influence livestock performance as well as livestock impact on the grassland ecosystem
(Briske et al., 2008; Teague et al., 2013).
A key constraint for animal performance, in mesic grasslands especially, is the decline
in forage quality as grassland increases in biomass and matures during the growing
season. To address this constraint, Venter and Drewes (1969) presented a flexible
or open rotation grazing management system, incorporating fire, periodic full season
rest, and flexible movement of livestock through variable numbers of paddocks utilized,
in an attempt to maintain grass in a short, high-quality state during the summer growing
season. In years of abundant growth, fewer paddocks may be used to maintain short,
high-quality forage (Figure 3), while in dry years with poor growth, more or all paddocks
in the planned grazing cycle are used. When livestock are returned to a paddock is
based on the grass growth stage rather than time or a rigid cycle. A proportion of
paddocks (priority paddocks) will be repeatedly heavily grazed in a nonselective manner
(facilitated by burning in the spring where necessary to ensure all grass species
start in a short, immature, and high-quality state), maintaining high quality. A proportion
of paddocks (in the planned grazing cycle) will be lightly grazed in a more selective
manner (depending on rainfall and growth patterns during the season). Unused (have
come through a full wet season rest) or lightly grazed paddocks may be prioritized
for burning, while those heavily grazed are prioritized for full season rest. They
also highlight that, because a large proportion of the paddocks are grazed short and
nonselectively, the requirement for fire to remove unpalatable, ungrazed grass is
reduced. While not specifically highlighted originally, periodic full growing season
rests have a positive impact on grass vigour, Kirkman (2002) while at the same time
providing forage for winter use (Kirkman and Moore, 1995; Figure 4). While the specific
system proposed by Venter and Drews (1969) has seen little adoption or reference in
published literature, the general concept of adaptive, multipaddock grazing systems
has been widely embraced (Teague et al. 2013).
Figure 3.
Cattle concentrated on a wet season priority paddock in a flexible grazing management
system in a mesic grassland. Photo by K P Kirkman.
Figure 4.
Wet season grazing on the right of the fence, and rested grassland on the left of
the fence in preparation for dry season grazing. Photo by K P Kirkman.
The role of periodic, full season rests is not often adequately described in various
meta-analyses and evaluations of grazing systems, probably because most management
focuses on grazer movements within a single grazing season, rather than across years
(Barnes, 1992; O’Reagain and Turner, 1992; Briske et al., 2008; Teague et al., 2013;
Hawkins, 2017; McDonald et al., 2019). Evaluating the impacts of periodic full season
rests on grassland ecosystem health and livestock performance logically requires a
multiyear study, which might explain the lack of clear differentiation between the
outcomes of experiments examining continuous and rotational grazing.
Provision of forage throughout the year
Many grazing systems have only a minor focus on providing winter forage, particularly
where the provision of alternate forage sources such as pastures and the preservation
of forage in the form of silage or hay is economically viable. Hence early grazing
systems were focused mainly on utilization of grassland during the wet season (intraseasonal)
and mostly ignore interseasonal movement, apart from some recommendations for resting
a proportion of grassland either for a period within a growing season or for a full
growing season. This was particularly true in mesic areas where winter grass quality
is typically inadequate for livestock maintenance in the absence of some form of supplementation
(Kirkman and Moore, 1995). However, even in more arid regions where winter grass quality
can maintain livestock condition, grazing system recommendations sometimes largely
ignore the requirement for wet season resting to provide winter forage.
Many grazing management strategies ignore the different impacts that grazing has on
grassland during the active growing season (summer) and the dormant season (winter).
In general, summer grazing has been shown to impact the vigour and regrowth potential
of the more palatable (more frequently and intensively grazed grasses) to a greater
degree than the less palatable grass species which are grazed less frequently and
less intensively (Barnes and Dempsey, 1992; Kirkman, 2002). Apart from any direct
impacts on individual grass tufts, this influences the competitive interactions of
the multispecies grass sward during regrowth (Fynn et al., 2005). Tufts that are less
severely grazed and have some photosynthetic material remaining are able to regrow
faster, and consequently are able to commandeer resources (nutrients, water, and light),
giving them a competitive advantage. This creates a self-reinforcing feedback, which,
if not interrupted, can result in rapid species composition change tending towards
the less palatable, less preferred species with grazing pressure on the remnant palatable
species consequently increasing rapidly.
Grazing during the dormant season essentially comprises removal of senesced material,
and provided defoliation levels are not excessive, has negligible negative impact
on regrowth at the beginning of the following growing season, as old senesced leaves
are not photosynthetically active. Dormant season grazing has been reported to have
no detrimental effect on production, cover or seed production (Rethman et al., 1971)
while positive impacts may be realized by reduced shading, although few studies report
explicitly on the impacts of winter grazing. These seasonal differences in grazing
impacts imply that grazing management should also vary seasonally.
Forage quality and quantity (standing biomass) are usually inversely related during
the growing season (Fuhlendorf et al., 2017). Short, rapidly growing grass is highly
palatable, nutritious and easily digestible. Tall, slowly growing or senescing grass
(typically at or after flowering stage) has a lower concentration of nutrients and
higher fiber content, resulting in lower palatability and quality (Poppi, 2011). During
the dormant season, the quality is reduced and the degree of quality reduction is
dependent on environmental factors, with high rainfall having a large influence on
this reduction (Ellery et al., 1995). Again, this implies that management for forage
quality should vary between the growing season and the dormant season and should vary
depending on climate, environmental variables, and livestock requirements.
The Serengeti migratory system is an example of a natural system adapted to forage
quality and quantity provision throughout the year for large numbers of grazing animals
(Murray and Illius, 1996). During the wet season, migratory animals are concentrated
in the short-grass plains in the Southeast region of the Serengeti, where they graze
short, actively growing green grass (grazing nonselectively at a relatively high density)
during a time of high forage quality demand for lactation, young animal growth, and
conditioning males and breeding females for the imminent breeding season (McNaughton,
1985). During this period, animals move around, keeping the grass short and actively
growing (Hopcraft et al., 2014). During the dry season, the migratory animals congregate
in the Northwest regions where much of the forage has grown unchecked during the wet
season and is available as a high biomass/low-quality forage during a time of reduced
forage quality demand. Here, animals graze the standing biomass at a relatively lower
density with a higher degree of selectivity (McNaughton, 1985), with regrowth commencing
during the wet season after the migratory animals have moved on. In the Serengeti,
there are many grazing animals that do not migrate, including buffalo. Typically,
these move from high catenal positions (short, high-quality grass) during the wet
season to lower positions during the dry season (taller grass grown out during the
wet season) in what has been described as a “mini-migration” (Hopcraft et al., 2014).
Another relevant example from unmanaged natural systems is the grazing patterns of
the white rhinoceros. During the growing season, white rhino grazing is usually confined
to grazing lawns, being patches of very short grass. They maintain these short, actively
growing, high-quality forage patches by repeated grazing for the duration of the wet
season (Owen-Smith, 1988). During the dormant season, the white rhinos move to areas
of taller grass that have grown relatively unchecked during the growing season, where
they trade-off quality for quantity (Owen-Smith, 1988).
Parallels between the Serengeti migratory system, the “mini-migration” in the Serengeti
and white rhino grazing patterns are obvious, where the focus is on wet season quality
and dry season quantity. Within the wet season, movement is focused on ensuring enough
quantity of high-quality forage, with repeated grazing ensuring an adequate amount
of short, actively growing grass. The term “surfing the green wave” is an elegant
description of this phenomenon (Middleton et al., 2018). Within the dry season, movement
is focused on accessing enough quantity. Without regrowth during the dry season, movement
is likely to be sequential i.e., an area grazed once before animals move on. These
examples of natural systems have strong parallels with the flexible grazing management
system proposed by Venter and Drewes (1969).
Fire and grazing interaction
In the absence of fire in mesic grasslands, grazing tends to become more selective
over time, due to the inherent differences in palatability and acceptability between
grass species. Periodic fire serves to remove all accumulated unpalatable forage,
and resets the grazing selectivity patterns, as most grass species tend to be acceptable
after fire (Briske, 1996). In addition, mesic grasslands are generally highly adapted
to fire and regular fire usually has a positive impact on species richness and diversity
(Fuhlendorf et al., 2017).
Fire, in mesic grasslands, increases forage quality and animal performance, particularly
in the early part of the growing season, while potentially reducing above ground net
primary productivity over the same period. This can present a trade-off between quality
and quantity, however it is likely that the reduced biomass will be offset by the
increased spectrum of species being grazed after fire (Mentis and Tainton, 1984) with
associated advantages for maintaining cover of palatable grasses when unpalatable
neighbors are grazed. Incorporation of fire into a grazing management system should
thus depend on the objectives of the grazing system, the potential consequences of
either including or excluding fire, and importantly, post fire management.
Philosophy and principles
The first and most crucial step in grassland and savanna management for livestock
production is to set aside a large enough area of grassland for season-long resting
to produce adequate forage for winter, which also provides a whole growing season
for recovery of grasses after grazing for sustainability purposes (maintaining high-quality
grassland). The second step is to identify the growing season area and develop grazing
plans for the growing season as well as plans for physical separation of the areas
(i.e., keep animals out of the winter area during summer). In the higher rainfall
areas where the rainy season is around six months, the summer and winter grazing areas
are likely to be similar in size.
During the summer, if, for example, animals are grazing on half of the available area,
this results in an automatic doubling of the density relative to the stocking rate.
Additional subdivision can serve to increase the density further if required, bearing
in mind that maintaining a short, actively growing grass sward will enhance quality
and animal performance. During the winter, density becomes less important although
subdivisions allow for effective rationing of forage for the winter period and may
alleviate chronic shortages towards the end of winter.
Total animal numbers, or stocking rate, remain important in the relationship between
forage production and forage consumption. The suggested approach provides a useful
means of assessing this relationship in the manner of an early warning system. If,
in a normal rainfall year, forage becomes depleted prior to the planned move to winter
or summer grazing area, then forage consumption is higher than production, and animal
numbers should be reduced accordingly before forage shortages impact livestock production.
If there is excess forage at the time of moving to the winter or summer area, then
forage production exceeds consumption, and animal numbers could be increased. This
relationship between forage production and consumption is likely to vary from year
to year based on rainfall. In years of excess production, fire could be incorporated
in the management, with consequent improvement of forage quality, reduction of shrub
and bush encroachment and benefits to plant species that are dependent on fire. In
years where consumption equals or exceeds production, there will be reduced need for
fire. During years of forage shortage, fire consumes plant biomass that could be grazed
and should be avoided.
Management practices
It is unlikely that any management approach or system is adopted on-farm entirely
from research results. In reality, principles adopted from research are commonly combined
with practical experience to develop grazing management approaches applied by livestock
managers (Stuart-Hill, 1989). This is likely to exacerbate the lack of clear differentiation
in the outcomes between various grazing management approaches when assessing impacts
at farm scale.
Local circumstances influence management practices. For example, in Australia the
large ranch and paddock sizes, coupled with high costs of labor preclude most forms
of management intensification. This results in a strong focus on stocking rate as
the main management factor under control of the manager, and manipulating stock numbers
depending on rainfall (O’Reagain et al., 2014). While wet season resting is strongly
promoted, it is not widely practiced. Nonetheless, wet season spelling requires low
management intervention, and provides significant benefits.
Wet season grazing management
In practice, a combination of judiciously timed fire (depending on rainfall) with
confinement by herding or subdivision is likely to be the most viable option for providing
optimal quality and quantity of forage, while minimizing grazing selectivity during
the growing season (Figure 5).
Figure 5.
Example of short, high-quality grazing following fire in a mesic grassland. Photo
by K P Kirkman.
Maintaining short, actively growing grass to optimize quality requires relatively
high grazing pressure. If or when animals are moved to fresh grazing, it is important
to move back to the first grazed area before it gets too tall, so that quality remains
high. This implies a strategically, adaptively managed, irregular grazing cycle in
the growing season, with a gradient from intensively grazed short, high-quality grass
(priority planned high-quality grazing area) to taller, lower quality, less intensively
grazed grass (reserve grazing area). In seasons of below average rainfall, there should
be little or no areas that grow tall and lose quality. In seasons of high rainfall,
there may be significant areas that grow tall and lose quality. This should not affect
animal performance if grazing is concentrated on the short, high-quality (priority)
areas.
Dry season grazing management
During the dry season, there is no regrowth following grazing. Quality is likely to
decline gradually throughout the dry season, following the normal senescence patterns
of the grass. Under these circumstances the most logical approach is to move animals
systematically through the dry season grazing area, managing for quantity (intake)
and not quality. The ability to store sufficient forage for the dry season will depend
on how much area is rested during the growing season, which will also be influenced
by stocking rate.
Examples
Long-term monitoring of rangeland condition on several ranches of the Ghanzi region
of Botswana (mean annual rainfall ~395 mm) show that seasonal resting and grazing
of grassland as outlined above can result in an increase in the most desirable high-quality
grasses over time, only if they occur above a specific abundance threshold where selective
grazing is minimal (R. Fynn, unpublished data). . The findings show that when the
abundances of the high-quality grasses are low (<20% to 40%) then they are selectively
grazed giving taller, lower quality grasses a competitive advantage, which increase
at the expense of the high-quality grasses. These results demonstrate that a priority
paddock approach is required to rehabilitate grassland where the high-quality grass
species occur at low abundance. The priority paddock approach of burning a paddock
at the start of the growing season and then ensuring it is kept short all season will
ensure that the shorter grasses are not selectively grazed and thereby ensure that
the taller grasses do not get a competitive advantage. The priority paddock approach,
as described for the Venter–Drewes system, has been demonstrated in the Dundee region
of KwaZulu-Natal, South Africa (mean annual rainfall ~840 mm) to result in a large
increase in short, high-quality grasses and a decrease in taller grass species, while
facilitating improved livestock performance at greater stocking rates.
Conclusions
The grazing management principles outlined above closely resemble several natural
wildlife systems, with adaptations for different livestock farming scenarios (Figure
6). The principles can be applied across commercial (fencing) and communal (herding)
grazing systems, with unique adaptations under different scenarios. It is envisaged
that effective grazing management (feeding), along with using adapted animals (breeding)
should comprise the primary strategy for future-proofing livestock production in the
face of climate change. Once the primary strategy is in place, secondary strategies
comprising animal health and veterinary programmes and targeted supplementary feeding
should receive focus.
Figure 6.
Schematic outline of grassland and livestock management pointers for ecologically
and economically sustainable extensive livestock production from grasslands and savannas.
Conflict of interest statement. None declared.