No abstract

Discussion of limits to ruminant digestion of plant cell walls largely focuses on the iignification and chemical nature of these walls. We examined the anatomical limitations to digestion of thick-walled fiber particles in grasses. Estimates were made of wall surface area to cell wall volume ratio (SA/CWV), rate of bacterial digestion, and accessibility to bacteria for different types of cell walls. The analysis reveals the following: (i) Bacterial digestion of fiber calls can progress only from the interior (lumen) surface because their middle lamellaprimary wall region is consistently found to be indigestible. (li) Because of secondary wall thickness (c. 1-5 pro), we calculate that at best only 0.45 to 0.60 pm of wall thickness (as little as 20% of the wall in some cells) would be digested within the average residence time of fiber particles in the rumen assuming digestion of wall at the fast rate of 0.02 pm h-1. (iii) This potential rate of wall digestion overestimates that of typical fiber particles in the rumen because these particles are comprised of many hundreds of cells and relatively few will be disrupted by chewing to give bacteria immediate access to cell lumens. (iv) Digestion of thick-walled calls by bacteria is surface-based and sclerenchyma cells have a particularly low SA/CWV ratio. Calculated ratios for single cells are sclerenchyma (1:5) < stem parenchyma (1.9: 1) < mesophyil (6.7 to 13.3:1). (v) During digestion of secondary walls, an accumulation of toxic levds of phenolic monomers in call lumens and at the digesting surface is unlikely, but more slowly diffusing phenolic-carbohydrate complexes could reach concentrations toxic to bacteria. The structural limitations described are discussed in relation to future research directions. I NTAKE AND DIGESTIBILITY of forage dry matter are key attributes that determine the production of grazing animals (Minson, 1990). Both factors are influenced the proportion of cell wall material and the resistance of forage and fibrous structures to breakdown to small particles during mastication and digestion. The broad influence of anatomical structure of leaf and stem on dry matter digestibility and physical breakdown has been covered in recent reviews (Akin, 1989; Wilson, 1990, 1991, 1993). These reviews, and many research papers, have focused attention on the detrimental effect of lignified tissues. The walls of lignified cells, especially those that stain strongly with acid phloroglucinol, are regarded as virtually indigestible (Akin, 1989), and it is these thick-walled, lignified cells that dominate the undegraded particles in ruminant feces (Sakurai, 1963). Sectioning of such particles indicates that much potentially degradable material has escaped exposure and hence digestion (Pond et al., 1987). In grasses, these

Effects of artificial shading to 50% sunlight of nitrogen (N) limited tropical pastures of different grass species on a high (clay loam) and low (granitic loam) fertility soil type were evaluated in a semi-arid. subtropical environment over 3 years. The hypothesis was tested that shade can stimulate shoot growth by providing a modified environment more conducive to organic matter breakdown leading to increased mineralisation and availability of soil N, and the ability of tropical grasses to take advantage of this effect was examined. Unfertilised pastures of green panic (Panicum maximum var. trichoglume), buffel (Cenchrus ciliaris). rhodes (Chloris gayana), and speargrass (Heteropogon contortus) in full sun or shaded by sarlon cloth were sampled on 9 occasions. Additional green panic plots on both soils were irrigated for the first 2 years, and all other plots were dependent on natural rainfall. Shoot and root dry matter and N yield, and soil nitrate and ammonia N, were measured. In one set of green panic plots on each soil, canopy. litter, and surface soil temperatures were monitored continuously, and soil moisture at different depths was measured fortnightly. Shade stimulated shoot dry matter yield over the 3 years by up to 37% in green panic. 22% in rhodes, and 9% in speargrass. Shade decreased buffel yield on the clay soil but had no effect on the granitic soil. Relative increases in yield of shoot N were similar to those for shoot dry matter, except for buffel on the granitic soil where N yield was increased by 39% with no increase in shoot growth. Positive shade responses occurred in all 3 years but were reduced by extreme drought in year 3, particularly on the clay soil. Irrigation gave a greater shade response on the clay but not on the granitic soil. Root mass was lower under shade than in full sun. but there was no long-term trend of progressive decrease. and the change in N yield of roots did not appear to explain the gain in shoot N of the shaded pastures. Nitrogen percentage in the youngest expanded leaf was higher in the shade than the sun leaves only after about 2 to 2 5 months of shading. Surface soil nitrate and ammonia concentrations tended to be higher under shade for most harvests. Shade lowered temperature extremes of surface soil and litter by up to 10-12�C, and improved soil water status. compared with the sun plots. Soil water data were analysed to separate effects on plant water stress and soil microbial activity. The consistent positive response of shoot N yield to shade across grass species. weeds, and soil type. the delay in it becoming evident, and its longevity all support the hypothesis that shade enhances organic matter breakdown and N cycling. Harsh surface temperatures and low soil moisture in open sun pastures appear inimical to high microbial activity. Implications for pasture management are discussed. with the caveat that the outlined benefits of artificial shade may not necessarily arise with tree canopies.

The water relations characteristics of three grass species (Panicum maximum var. trichoglume, Cenchrus ciliaris, Heteropogon contortus), and a legume (Macroptilium atropurpureum) grown in the field were measured using both a modified pressure/volume technique with pressure bomb measurements on single leaves and a dewpoint hygrometry technique applied to fresh and to frozen and thawed leaf discs.The two techniques agreed well in the estimates of osmotic potential at full turgor and the water potential at zero turgor. However, for parameters such as the relative water content at zero turgor, bound water and bulk modulus of elasticity there was a poor correlation between the estimates from the two methods. The pressure/volume technique gave less variable results and is more convenient for field use than the hygrometry technique. The determination of the modulus of elasticity from various functions relating pressure potential to relative water content is discussed.

Thirteen tropical and 11 temperate grasses were grown in controlled environment under daylnight temperatures of 21/13, 27/19 and 32124°C. Neutral detergent fibre (NDF) was prepared from the fifth leaf on the main stem of each plant, 2 days after it had reached maximum length. Panicum maximum var. trichoglume (tropical) and Loliumperenne cv. S24 (temperate) were also harvested at 4,8, and 12 days after this stage of development. For the tropical grasses NDF values, corrected for starch and protein, decreased with increasing growth temperature, whereas in the temperate species they increased. In the tropical group the decrease in NDF was due to a lower cellulose content, whereas hemicellulose and lignin levels generally tended to increase slightly with increasing temperature. In the temperate grasses, lignin, hemicellulose and cellulose levels all showed a consistent increase with growth temperature. At each growth temperature the lignin content of the species in the tropical group had a significant negative correlation with in vitro digestibility, and lignin appeared to be more closely associated with hemicellulose than with cellulose. In contrast, at each temperature, variation in digestibility between species of the temperate group was not correlated with lignin. Levels of cell wall components in the later-harvested material from Lolium varied in a similar manner to that of the younger growth stage, whereas older Panicum cell wall constituents showed more variability.