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Forage Temperature Interaction on Feed Intake During the summer months grazing ruminants often show signs of distress with only short periods of grazing from mid-morning to late afternoon. The restricted grazing is usually attributed to the direct effects of temperature and solar radiation on the animal, but this is not necessarily the case as climate-forage interaction also contributes to animal distress. TEMPERATURE EFFECTS High ambient temperatures bring about rapid rate of maturity of forages and a rise in cell wall content (cwc), as illustrated in Figure 10. This occurs in the stems and leaves of both temperate and tropical grasses, but the change is more pronounced in Topical grasses. The rate of plant maturation rises with temperature, e.g., alfalfa grown at 17°C took 52 days to reach early bloom but only 21 days at 32°C (Faix, 1974~. As forages mature, there is an in- crease in cwc and a decrease in the digestibility of the cell wall (DCW). As ambient temperature increases, the digestibility of the dry matter (DDM) of a forage decreases due to a rise in cwc and decrease in DCW as illustrated in Figure 11 (Minson and McLeod, 1970~. LIGHT INTENSITY High light intensity increases the content of water-soluble carbohydrates (wsc), whereas, high temperature decreases wsc (Deinum, 1966~. With high light intensity cwc decreases (Deinum et al., 1968~. When there is heavy 35
36 FARM ANIMALS AND THE ENVIRONMENT 70 60 - - o o- - c: 50 40 . _ 30 _ - - _ _ - ~ropical Grass Stems - Tropical Grass Leaves _ _ Temperate Grass Stems __ _ _ - T~mnerate Grass Leaves 10 15 20 25 30 TEMPERATURE ( C) FIGURE 10. The effect of temperature on the cell wall content (cwc) of grasses of the same maturity (adapted from Van Soest, 1981). 80 70 - m 60 in 50 _ 40 ~1 1 1 1 1 10 15 20 25 30 TEMPERATURE ( C) FIGURE 11. Changes in the digestibility percentage by ruminants of plant digestibility of dry matter (DDM) and digestibility of cell wall (DcW) with changes in ambient temperature from 10 to 28°C (adapted from Deinum, 1976; Minson and McLeod, 1970). _~ - \ \ - Ad\
Forage-Temperature Interaction on Feed Intake 37 cloud cover, such as occurs in the United States during summer or in tropical areas during the rainy season, both temperature and light contribute to high cwc, which results in lower DDM and lowered intake by grazing ruminants. FORAGE QUALITY Seasonal variations in temperature and light intensity markedly affect the DDM and cwc of both grasses and legumes. During the spring, alfalfa grows under longer daylength and lower temperature, thus its cwc is lowest and DDM highest (Kalu, 19761. The higher temperature during mid-summer causes an increase in cwc with a corresponding decrease in DDM; therefore, alfalfa harvested at this time is at its lowest quality. Grasses tend to follow a similar pattern (Jolliff et al., 1979~. The percent cwc of forages has a negative correlation ~-0.83) to daily dry matter intake (DMI), as illustrated for temperate forages in Figure 12 (Os- bourn et al., 1974; Van Soest, 1971~. Another important relationship is the positive association of DM! and DDM; however, this relationship declines for forages grown under- high temperature conditions (Laredo and Minson, 19731. \ 80 - ~ 70 en - ' 60 50 Y = 95 - 0.73 x 20 30 40 50 60 CWC (%) FIGURE 12. Relationship between daily dry matter intake (DMI) by ru- minants and cell wall with increasing percentage of cell wall content (cwc) of temperate forages (adapted from Osbourn et al., 1974).
38 FARM ANIMALS AND THE ENVIRONMENT Forages grown under high temperatures have a higher stem to leaf ratio. Grazing animals prefer leaves (McDowell, 19721. When the animals select leaves instead of consuming the whole plant, bite size and the rate of intake are decreased; hence, morphological characteristics of forages, such as leaf to stem ratio, are negatively related to cwc. A higher cwc due to high tem- perature has a negative influence on DM! and DDM of plants. Also, feeds that have high cwc have higher heat increment even at similar lE; thus, the higher cwc of forages grown during summer-or,.in~tropical areas results in poorer uti- lization of energy (Van Soest, 19711.~The reasons for the decline in effi- ciency as cell wall intake rises is not~entirely clear but it is associated with a linear increase in rumination time (Welch and Smith, 1969~3. This implies an increase in the energy costs associated with.the digestion of:.cwc, but the ef- ficiency of utilization of the end products of cwc digestion, li.R., increased acetate, is an alternative ex~planad~on. The decrease in forage quality resulting from -seasonal changes in tempera- ture has a negative effect on intake., digestibility, and efficiency of utilization of ME. As a result, the interactic~n~f.fects of climatic conditions on forages complicates deriving estimates of the.direct effects of temperature on intake of grazing ruminants.