Witjaksana Darmosarkoro
The quality of forage available to grazing livestock has a major impact on animal performance (Crampton, 1957). Grazing management practices such as species used, grazing period, and fertilization affect forage quality, and, therefore, indirectly affect animal performance.
To optimize production from grassland requires proper management practices. Species should be selected which are adapted to the ecological and environmental conditions in which they are grown to produce optimal nutrient value and forage availability. There is no single decision that can be used to solve all problems in management practices, but there are some principles that can be applied to a successful grassland.
Choosing a species is a basic step which determines the forage quality of a grazing system. Nutritive value depends on the species. One species may have a high nutritive value and another species may have a lower value. Fisher et al., (1991) conducted research on several grass species, including tall fescue (Festuca arundinacea Schreb.), switchgrass (Panicum virgatum L.), flacidgrass (Pennisetum flaccidum Griseb.), and bermuda grass (Cynodon dactylon L. Pers.), in pastures. They concluded that bermuda grass and switchgrass have a high stem to leaf proportion, resulting in lower quality than the other speceies. Bermuda grass provides herbage lowest in in vitro dry matter disappearance (IVDMD) than that of switchgrass, tall fescue, and flacidgrass. Study on protein degradation of switchgrass and smooth bromegrass showed that switchgrass (C4) had a percentage of whole-plant escape protein greater than that of smooth bromegrass (Mullahey et al., 1992). Maturity of plant is also of some concern. It is interesting that the escape protein concentration, in that study, declined with maturity, which was because escape protein in leaves was higher than that in stems, and stem/leaf ratio increased with maturation.
In order to provide an appropriate amount of forage, Jung et al., (1985) tried to use a mixture of switchgrass and big bluestem and cool-season grass (orchardgrass and timothy). This resulted in that forage availability in May being mainly provided by orchardgrass and timothy, while in the summer and fall was provided by a mixture of switchgrass and orchardgrass/timothy. Yield distribution of switchgrass is 15, 55 and 30% in May, July-August and October, respectively.
As plants are grazed, they need time to recover to produce forage regrowth. Discussing the effect of grazing period on the quality of some forages is generally discussed as growth per period of time. Research on continuos grazing of alfalfa had been done (Wolf and Allen, 1990) with the following treatments: T1 was spring grazing (height of alfalfa 10 cm) with three hay harvests, T2 was two hay harvests and then summer grazing (height of alfalfa 15 cm) plus one hay harvest, T3 was summer and spring grazing with two hay harvests, and T4 was no grazing. The results indicated that yield treatment T1 and T3 were lower than that in T2 and T4. In terms of forage quality, which includes nutritive value and its intake, it can be concluded that period of grazing will affect the amount of herbage that can be supplied for animal intake. This implies that managing the time of grazing is a priority. A previous experiment on prolonged grazing (Stewart and Clark, 1944) reported that grazing until late spring resulted in subsequent summer hay harvests which had lower yield and higher protein content. This was mostly because the hay was more immature at harvest compared to the early and mid spring grazing treatments.
Another problem of managing grazing period is providing grasses high in nutritive value at the desired time of grazing. George and Obermann (1989) conducted research on improving summer supply and quality of switchgrass by defoliation treatments. The result showed that 1) Leaf or stem IVDMD had little or no differences among the treatments and 2) switchgrass can be delay onset of yield until early - mid June by implied partial spring defoliation. This experiment fulfilled the demand for forage in the time it was needed.
Wheat-grasses (Agropyron and Thinopyrum spp.), sainfoin (Onobrychis vicifolia Scop.), and their mixture under different grazing initiation periods have been used in order to minimize seasonal growth distribution (Griggs and Matches, 1991). Yield of the latest initiation period was the highest, but using the mixture increased consumption up to 29%.
There are two basic systems that can be used in grazing, the continuous system and the rotational grazing system. Research on comparing these two systems had been conducted to evaluate forage quality, production, and animal performance (Heitschmidt et al., 1987a and 1987b, and Jung et al., 1985b). The results indicated that there was no significant difference between the two systems; however, under Short Duration Grazing (SDG, rotational system), the availibility of forage and crude protein content tended to be greater (Jung et al., 1985a). Research conducted by Heitschmidt et al., (1987b) had similar results, in which the rotational grazing system had crude protein and organic matter digestibility greater than that in the continuous grazing system. They explained that the differences were because of the differences in stocking rates.
Application of grazing system still requires experimentation in order to know its affect on livestock performance due to forage availibility. Deciding on the number of paddocks to use is one of the problems of a rotational grazing system. Numbers used on the research scale were 8 paddocks (Jung et al., 1985), 16 paddocks (Heitschmidt et al., 1987b) and 14 and 42 paddocks (Heitschmidt et al., 1987a). The results indicated that the quality of forage did not show a significant difference between the number of paddocks used; however, a study on SDG cells which was conducted by Olsen et al., (1989), had different results. They used 10-paddocks of SDG cells to evaluate ingestion rate, biting rate, and grazing time as an indication of forage quality of grazing. The results showed that diet quality declines with increasing days of sequential grazing in a paddock, and they concluded that the best grazing period in SDG paddocks is no more than 2 days.
Fertilizing And Balanced Nutrients In Soil
It is definitely important to control the forage environment, especially water availability and soil fertility, in order to have a high quality forage. Low soil fertility will produce low grade forages in term of yield and its nutritive value, and this condition could be solved by using fertilizers. The use of fertilizers in grass management in order to improve forage quality is often needed because the availability of nutrients in the soil and demand for nutrients by the forage are sometimes not in balance. Nitrogen fertilization increased crude protein concentration of big bluestem and switchgrass grown in a fine, mixed, mesic Aquic Hapludalf soil by 10 and 26% fertilized with 45 and 90 kg N ha-1, respectively (Jung et al., 1985). The case of high concentration of potassium has an adverse effect on plant magnesium absorption, and on the other hand, balanced nutrients will increase forage yield and in turn will increase its quality. Miyasaka and Grunes (1990) reported that based on their research, as root temperature increased, concentration of potassium and nitrogen in forage increased, and so did the ratio K/(Ca+Mg). This ratio is used to indicate forage that has a greater tetany hazard, which is because of lack of magnesium in animal forage intake.
Phosphorus, which is very important as part of skeleton and other organic constituents, it should come from P available in the forage (Buxton and Fales, 1994), so maintaining sufficiency of phosphorus in the plant to be grazed will result in maintaining phosphorus supplied to the animal.
Although effects of soil nutrients on chemical compositions of grasses are relatively small (Buxton and Fales, 1994), the existence of balanced soil nutrients or balanced fertilizer application can increase yield and in turn give better quality of forage.
Other Factors
One other factor of maintaining grasslands which affects whole grazing management is maintaining productivity by avoiding overgrazing. Wilson and Macleod (1990) stated that with overgrazing "there is concomitant vegetation change and loss of animal productivity from grazing of land by herbivores".
The concept of declining quality of grasslands is sometimes associated with soil erosion. Bare soil is subject to high soil erosion. It was reported that land under livestock grazing exposed larger areas of uncovered soil (Wood et al., 1989). This condition resulted in phosphorus and total nitrogen levels in the soil that were less than that in fully covered soil. Problems also occurred on hilly and erodible grassland. Hofmann et al.(1993) suggested that on land that had been reseeded to cool-season grasses, season-long grazing may be useful, especially in the northern Great Plains.
Based on the items discussed above, quality of forage is effected by the management of the grassland. One of the most important things to consider is choosing the right species of plant in the right location (related to soil type and local climate). In addition to this, manage the plant to produce high quality forage by providing proper nutrients, perhaps through fertilizer, and by choosing the right grazing practices to achieve best forage utilization efficiency.
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