Whereas the prenennial Cenchrus spp. are highly palatable to farm animals through the entire year, annual species such d& Aristida funiculata and Cenchrus biflorus are avoided by sheep and cattle from August through November because of the stiff awns and burrs which these grasses have (Ahuja, 1978). From August to November, cattle relish perennial grasses. In summer season, even cattle eat green shoots of Calligonum polygonoides and Capparis aphylla which during other seasons are not liked by the animals. The now-depauperate and dwindling rangelands and pastures of the arid zone support a few nutritive and perennial grasses and annuals which can support livestock for a few months in a year only. Many of these pastures have deteriorated as a result of the havoc wrought by rodents (Gupta and Prakash, 1978).
Rodents debark saplings and young trees, burrow tunnels in the root zone thereby causing desiccation of soil in the root zone, and eat seeds, shoots and leaves of such fodder grasses as Cenchrus ciliaris, Panicum antidotale, and Lasiurus sindicus. Large populations of such rodents as Tatera indica, Gerbillus gleadowi, and Meriones hurrianae can eat the entire grass cover, leaving nothing for the cattle. It has been estimated that the merion gerbil alone, whose population size can vary from 5 to 600/ha, can dig out more than 60,000 kg of stabilized soil/day/km2 during its burrowing activities, leaving it loose at the surface in the form of mounds which are blown about by wind (Mann, 1978). It explains the association between grasslands and dominant rodents in the arid zone. According to Mann (1978), overgrazing has interfered in the natural processes of regeneration and succession of grasses and other plants in the area and as a consequence of the interplay of various adverse factors in the desert ecosystem, about 9,300 km2 of western Rajasthan has already been affected by desertification, about 1,63,000 km2 area is now highly vulnerable to desertification, and about 41,000 km2 area is somewhat less vulnerable. A.N. Lahiri has extensively studied the seed biology of desert grasses and also the effects of grazing/defoliation stress on their physiology related to growth, regeneration, protein yield and biomass production.
He has also made valuable contributions on plant adaptations to xeric environments, water use of desert trees, impact of vegetation on soil fertility, and foliar absorption of moisture. The Central Arid Zone Research Institute in Jodhpur is engaged in afforestation and other efforts to check the march of the desert and to manage it properly. An intensive tree plantation programme has been undertaken. In the deteriorating and overgrazed rangelands, intercropping of grasses with certain legumes has given significant increases in forage production and also in the secondary production and growth rate of heifers and lambs (Maim, 1978). The ecology laboratory of the Jodhpur University has done extensive work on adaptive strategies of desert plants.
Several aspects of the ecology, biology and management of weeds and of seed germination of arid zone plants have been worked out by D.N. Sen and his associates. Some of the most crucial problems in the Rajasthan desert include the following: (1) overgrazing by goats and other animals; (2) excessive felling, cutting and chopping of indigenous trees and other plants for fodder, fuel and timber use; (3) many desert mammals are herbivorous and vegetarian whereas only a few are insectivorous or carnivorous. Some estimates indicate that desert habitats provide shelter to about 38 species of mammals (Purohit, 1968) which retard the growth of vegetation. Joshi (1978) has made a valuable study of the vegetation in certain areas of the Rajasthan desert embracing Jhunjhunu and Sikar districts.
This area supports some discontinuously distributed plant communities in which the more important plants are Acacia nilotica, Anogeissus pendula, Calligonum polygonoides, Calotropis procera, Ephedra foliata, Leptadenia pyrotechnica, Prosopis cineraria, Saccharum bengalense and Zizyphus nummularia. Some species, e.g., Calligonum polygonoides, Capparis decidua, Leptadenia Pyrotechnica, Lycium europaeum and Clerodendron sp. are efficient sand binders. The area also has such perennial forbs as Crotalaria burhia, Tephrosia hamiltonii, Convolvulus microphyllus and Boerhaavia diffusa, and such grasses as Cenchrus ciliaris, Panicum antidotale, Brachiaria reptans and Desmostachya bipinnata.
Saccharum bengalense, which is not grazed by animals, has been found to be most effective in checking erosion. However, though this grass is unpalatable to herbivores, it does afford great protection and shelter to rodents and mice against carnivores. In recent years extensive plantations of such trees as Azadirachta, Melia, Dalbergia, Albizzia, Parkinsonia, Tamarix and Prosopis have been undertaken and out of these, Parkinsonia aculeata and Prosopis juliflora have been particularly successful.