Existing feeding practices
The farmers of study area were maintaining their sheep by grazing them on the common property resources/forest/wastelands and. They were offering cultivated fodders like Hybrid Napier, sorghum green, or local grasses or a mixture of sorghum stovers, paddy straw, rice bran and concentrate mix at their barn after grazing. Majority (87.3%) were not supplementing sheep with common salt or mineral mixture.
Micro-mineral content of the soil
The 23.0, 2.6, 13.3 and 28.3% of soil samples of the study area were falling under the category of below critical level for Zn, Cu, Mn and Fe, respectively (Table 2, Fig 2). The reported critical levels of Zn, Cu, Mn and Fe deficiency in soils fall in the range of 0.2-6.0 ppm
(Bünemann
et_al2018 and
Czekała et al., 2020) which may vary with edaphic or agronomic factors
(Fenetahun et al., 2020 and Bayu 2020). Enough range was available for copper and manganese minerals in the soil samples, however, the soil was deficient to the extent of 23.0-28.3% for zinc and iron. It revealed that the status of Zn in soil and fodder are falling in a range of marginal to deficient level. Since, the soil is unable to give adequate trace elements to the plants that sheep graze, a shortage is inevitable, which is exacerbated when the ration consists primarily of grazed grass with limited usage of concentrates or minerals in the diet.
Micro-minerals content of forage
In the present study, pasture samples were having higher concentration of Cu, Mn, Fe and Zn than the recommended level (NRC 2007). Zinc content in dry fodder was very poor (8.12-18.28 ppm), however, the level was good in green fodder (21.15-23.15 ppm) (Fig 3a). Concentrate mix (28.26 ppm) and rice bran (63.41 ppm) were found to be a good source of zinc to the sheep unlike rice bran.
The copper content in paddy straw (7.56 ppm), sorghum stove (13.12 ppm), concentrate mix (12.56) was low as compared to other greens (14.95-17.03 ppm). Cu levels were found to be poor in paddy straw and moderate in grasses, stovers and concentrate mix unlike rice bran (21.23 ppm) used in the area (Fig 3b).
The Mn levels in the study area ranged from 33.56-48.25 ppm in green fodder and 91.12-98.22 in rice bran and concentrate mix. However, dry fodders predominantly used had high levels of Mn (paddy straw 164.22 ppm, sorghum stovers 512.23 ppm) which enabled sheep to maintain the level (Fig 3c).
Average Fe content was adequate in all feed samples (536.2-945.4 ppm in green roughages, 168.23 ppm in sorghum stovers, 521.12 ppm in paddy straw, 522.1-654.3 ppm in rice bran and conc. mix (Fig 3d). Unlike Mn, Fe levels were very poor in sorghum stovers as compared to another source of feed ingredients used in the study area.
As reported earlier
(Garg et al., 2005; Udar et al., 2003; Agegnehu and Amede, 2017;
Giannoulis et al., 2017 and
Mutwedu et al., 2020), these forages have Zn and Mn levels below the critical level in many of the semi-arid zones of India.
Micro-mineral levels in sheep
The average serum Zn, Cu, Mn and Fe content in sheep were 0.56, 0.49, 0.62 and 2.23 ppm, respectively (Table 2, Fig 4). More than 50% of the sheep screened showed low levels of Zn and Cu as compared to the critical level of Zn in blood serum (Herdt and Hoff, 2011;
Pankaj et al., 2017 and Smith and Akinbamijo, 2000). They had enough level of Fe, however, Mn level was slightly deficient in a few of them which matches with earlier reports
(Bhandari et al., 2018 and
Mukangango et al., 2020). The critical concentration of these micro-minerals in the forages might have resulted in their lowered levels
(Walsh et al., 2018 and
Sileshi et al., 2017) in sheep.
The study area showed low mineral utilization, necessitating the use of area-specific mineral supplementation to help ameliorate these deficiencies. Supplementing them improved their trace element status, as shown by greater serum micro-mineral status and body growth rate (Table 4).
Sheep faecal samples, being highly concentrated form, have high levels of all micro-mineral content (Zn-64.53, Cu-51.36, Mn-153.68 and Fe-783.25 ppm), suggesting that they may be contributing significantly to micro-mineral recycling in nature
via soil and plants (Fig 5).
Soil-plant-animal relationship
The relationship between faeces and serum concentration was significant for all micronutrients, showing that this is a non-invasive way for assessing micronutrient insufficiency in sheep (Table 3). The soil-plant and soil-animal relationships were both negative for Zn and Mn, but the plant-animal relationship was positive (Table 3). This highlights the importance of several factors in determining the availability of micro-minerals from soil to plants and then to animals. Forage micro-mineral concentration varies depending on soil type
(Garg et al., 2005) and growing conditions (Agegnehu and Amide, 2017;
Giannoulis et al., 2017 and
Mutwedu et al., 2020). Only a small percentage of trace elements are present in the soil utilised by plants, according to the physio-chemical constitution of the soil (Herdt and Hoff, 2011). Low quantities of a trace element in the soil can affect its presence in plants
(Pankaj et al., 2017); but, due to a regulated absorption process in the roots, higher levels may not be reflected in the plant
(Bhandari et al., 2018 and
Zhou et al., 2017).
Although there was a strong association between fodder and sheep serum, no comparable correlations were found between micro-mineral levels in sheep and soil (Table 3). There was no significant correlation between soil micro-mineral content and animal (serum) variables. Plant (feed and fodder) factor was, however, substantially connected with soil micro-mineral concentration in Cu and Fe, but not in Zn and Mn.
Serum micro-mineral content had a highly significant (p<0.01) correlation with all faecal micro-mineral content in sheep (Table 2), implying that even if we are not taking blood from sheep, which is difficult in field conditions, faecal samples (non-pricking) can suggest an animal’s micro-mineral status as well as in pasture. Significant association values between mineral levels in soil, pasture and sheep were not found in this study, which could be due to the development of complexes with organic matter and trace elements
(Garg et al., 2005). Most trace element concentrations in animals are essentially independent of consumption and can only be changed by dietary intervention at higher concentrations, which was also investigated in this study.
Effect of area-specific mineral mixture supplementation
After 60 days of oral supplementation, area-specific mineral combination was found to be effective in significantly (p<0.05) enhancing all micro-minerals under study (Table 4) and improvement in body weight growth rate (10.84%). Mineral supplementation was more effective in animals because they were underfed at the farmer level and couldn’t meet their nutrient requirements, particularly micronutrients
(Sahoo et al., 2017).
Deficiency of these elements in soil or plants, is impacting the mineral status of sheep, which is widespread even under natural grazing conditions in semi-arid locations. A complete image of this continuum, as well as natural solutions such as nutrient recycling
via faecal (organics) treatment
(Maruthi et al., 2019) and area-specific mineral mixture supplementation, can provide a feasible answer.
