Environmental Sustainability of Sheep and Goat Production System
Ruminant livestock produce about 80 million tons of methane (CH4) accounting for about 28 % of anthropomorphic emissions each year (Beauchemin et al. 2008). CH4 is physiologically produced as a by-product of digestion; at farm level CH4 is emitted both from direct ruminant fermentation and from manure. Methane production by ruminants is influenced by animal size, dry matter intake, carbohydrates, and other components of the diet (Wilkerson et al. 1994). The increasing level of concentrate in the diet of ruminants causes changes in the fermented substrate from fiber to starch and a decline in ruminant pH with a reduction of the proportion of dietary energy converted to CH4. However, the increased level of concentrate in ruminant diet should be limited at 50 % of the diet to avoid negative effects on milk quality. A number of nutritional management strategies to reduce enteric CH4 production have been reported, among which are increasing the level of grain and lipids, and supplementation with ionophores in the diet. Furthermore, a number of feeding strategies to reduce CH4 emissions in animal production systems have been proposed, such as feeding maize and cereal silages instead of grass silage, improved pasture management, inclusion of legumes, yeast and enzyme, feed additives, or plants characterized by secondary compounds with potential CH4 suppressing properties (Beauchemin et al. 2008).
Nitrogen losses originate mainly from the imbalance and the asynchrony between the rate at which carbohydrates and proteins are degraded at rumen level (Tamminga 1996). The imbalance between the simultaneous availability of net energy and amino acids in the rumen is the main cause for NH3 ruminal losses; ammonia, after its conversion into urea in the liver, is found in blood and milk, and is excreted in urine. Apart from the contradictory results on the relationship between CP level of diet and milk yield and composition, the concentration of urea in the milk is always regarded as a good indicator of efficient use of dietary protein by the animals, and it is currently used to evaluate and to adjust the diet (Cannas 2002). The efficiency of dietary N utilization for milk protein synthesis is rather low in dairy sheep and goat (15–35 %) (NRC 1988), and the use of high protein level diets for sustaining milk production in lactating animals is widely studied. High protein level diet can result in a number of deleterious events: reducing environmental sustainability of sheep and goat farming, causing an increase of N output to the environment, and high levels of ammonia pollution in animal houses, which originates from animals' urine and feces and can be injurious to both livestock and stock persons' health. An increased N excretion in urine and feces, as a consequence of poor dietary N absorption, can result also in high growth of microorganisms. Sevi et al. (1999, 2006) suggest that the efficiency of utilization of dietary N in the lactating ewe increases with decreasing protein levels in the diet from 16 to 13 %, especially when limiting amino acids (lysine and methionine) in the ration are encapsulated to prevent bacterial deamination in the rumen. Sheep fed with a moderate protein level excrete more N than sheep fed with low protein level of the diet. On the contrary, the low protein level of the diet results in lower amount of feces, in less wet feces, and in higher NH3 release in the urine. The choice of a proper ventilation regimen in small ruminant housing is critical for the control of environment and for removing aerial pollutants, which originate from animals and their excreta. In particular, low ventilation rates can fail in removing efficiently the moisture and gases, which originate from the respiratory activity of animals and the decomposition and fermentation of manure, resulting in increased relative humidity and higher air concentrations of ammonia and carbon dioxide. Very high ventilation rates, instead, can result in higher air dust concentrations, probably due to reduced humidity levels and to turbulent air currents maintaining dust particles suspended in the air for a longer time. Ventilation rate is based on the length of ventilation cycles and on air speed. During summer, dairy sheep need an average ventilation rate of about 65 m3/h/head achieved by giving most ventilation cycles during the hottest hours of the day (Sevi et al. 2002, 2003b). However, also overnight air exchange for removing gases, mostly ammonia, originated from excreta decomposition and fermentation are of great importance. Furthermore, combining a moderate protein level of the diet (16 %) with low ventilation rates result in excretion of 40–64 % of higher volumes of urine and 40–79 % greater amounts of total water.
Litter management by using paraformaldehyde and bentonite is an effective strategy for reducing emissions from manure. Paraformaldehyde is a polyoxymethylene containing 90 to 98 % formaldehyde, which has a recognized bacteriostatic effect on the microorganisms naturally present in droppings. Litter treatment with paraformaldehyde was shown to influence milk hygienic quality reducing somatic cell count and microbial cell load of 10 and 15 %, respectively. The use of paraformaldehyde for litter disinfection markedly increased protein (10 %) and fat (20 %) content in sheep milk with a consequent improvement of milk coagulation performance. Addition of paraformaldehyde to the bedding is a procedure that may be applied at intervals suitably far apart to be not economically prohibitive. Its use, however, would remain limited to circumstances in which there are difficulties in processing or marketing milk of low hygienic quality which could not be easily resolved using other cheaper methods (Sevi et al. 2000). In a subsequent study (Sevi et al. 2003a), the effects of litter renewal intervals on the yield and quality of ewe milk were evaluated as an alternative strategy to litter treatment with antimicrobial products. Bentonite has been used to reduce the levels of airborne particulates in livestock housing and improve the hygienic quality of ewe milk. Bentonite is mainly composed of clay minerals and has a high waterabsorbing capacity. It is relatively inexpensive and there are no reports of it having adverse effects on the health of either ewes or people. Litter renewal of litter at 4 weeks intervals can sustain health status of the mammary gland and improve ewe performance at the same levels as litter treatment with bentonite.