Impacts on Forage Quality

13.4.1 Plant-animal interface

Animal production on rangelands, as in other grazing systems, depends on the quality as well as the quantity of forage. Key quality parameters for rangeland forage include fibre content and concentrations of crude protein, non-structural carbohydrates, minerals and secondary toxic compounds. Ruminants require forage with about 7% crude protein (as a percentage of dietary dry matter) for maintenance, 10-14% protein for growth and 15% protein for lactation (Ulyatt et al., 1980). Optimal rumen fermentation also requires a balance between ruminally available protein and energy (Dove, 1996). The rate at which digesta pass through the rumen depends on the fibre content of forage. Increasing fibre content slows passage and reduces animal intake.

13.4.2 Atmospheric [CO2]

Based on expected vegetation changes and known environmental effects on forage protein, carbohydrate and fibre contents (e.g. Wilson, 1982; Owensby et al., 1993b, 1996), both positive and negative changes in forage quality are possible as a result of atmospheric and climatic change (Table 13.1). Effects of CO2 enrichment on crude protein content of forage, for example, are likely to be negative, for plant nitrogen concentration usually declines at elevated [CO2] (Owensby et al., 1993b; Cotrufo et al., 1998). Limited evidence suggests that the decline is greater when soil nitrogen availability is low (Bowler and Press, 1996; Wilsey, 1996), implying that rising CO2 could reduce the digestibility of

Table 13.1. Potential changes in forage quality arising from atmospheric and climatic change.

Change

Examples of positive Examples of negative effects on effects on forage quality forage quality

Life-form Decrease in proportion distributions of woody shrubs and increase in grasses in areas with increased fire frequency (Ryan, 1991)

Species or Increase in C3 grasses functional group relative to C4 grasses distributions with higher CO2

(Johnson et al, 1993)

Plant biochemical properties

Increase in non-structural carbohydrates at elevated CO2 (Read et al, 1997). Increase in crude protein with reduced rainfall

Increase in proportion of woody species because of elevated CO2, increases in rainfall event sizes and longer intervals between rainfall events (Stafford Smith et al, 1995)

Increase in proportion of C4 grasses relative to C3 grasses due to higher temperatures (Campbell et al., 1996) or changes in availability of water at elevated CO2 (Owensby et al, 1997). Increase in plants poisonous to animals

Decrease in tissue nitrogen contents and increased fibre contents as result of reduced photosynthetic protein contents at elevated CO2 or higher temperatures (Sage et al., 1989; Owensby et al, 1993b, 1996; Soussana et al., 1996; Read et al., 1997). No change or decrease in crude protein in regions with more summer rainfall forages that are already of poor quality for ruminants. Such reductions in forage quality would have pronounced negative effects on animal growth, reproduction and mortality (Owensby et al., 1996) and could render livestock production unsustainable unless animal diets are supplemented with N (e.g. urea, soybean meal). Concentrations of some of the plant products that are toxic to animals may also increase in a CO2-rich environment.

13.4.3 Botanical composition and animal selectivity

Both positive and negative effects on forage quality are possible for individual species, but the total quantity of nutrients on offer to a grazing animal is determined by the relative abundances of plant species in vegetation. Carbon dioxide enrichment initially reduced crude protein content of both species in a grass-clover mixture, but the protein content of the entire sward eventually increased at elevated [CO2] because of a greater overall proportion of high-N clover (Schenk et al., 1997). Similar effects are likely on rangelands, which contain complex mixtures of species of differing ecology and forage quality.

Ultimately, the quality of livestock diets is determined both by the quality of the forage on offer and by selectivity of animals during grazing. Research efforts have focused primarily on changes in forage quality to the near exclusion of potential changes in grazing behaviour by animals. Selective grazing is a significant feature of livestock on rangelands, where utilization is much lower than in intensively managed pastures. There is a need, therefore, to determine whether higher temperatures or other global changes will alter grazing behaviour and whether changes in grazing behaviour could compensate for a general decline in forage quality.

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