## 4 Agro-Food Substrates for Anaerobic Digestion
### Livestock Manure
#### 4.1.1. Cattle Manure  
Although the anaerobic digestion of cow manure may give approximately 63% of biogas yield [60], ruminant manure, in general, and cattle manure, in particular, may be conveniently used to prompt the fermentation phase of AD, because it may provide the required methanogenic bacteria. However, the fermentation of manure as single substrate provides low methane yield, due to a moderate anaerobic biodegradability of about 45-50 % [61]. Moreover, the biogas production from cattle manure is lower than that obtained from other farm animals, since cattle manure often contains an excess of lignin complexes from fodder that are very resistant to AD [62]. Low biodegradability of animal manure is often caused by large amounts of indigestible fractions such as lignocellulosic, an element of the plant cell wall, residual from animal digestion, and composed by hemicellulose, cellulose and lignin. Lignin is a natural complex polymer and non-carbohydrate constituent of wood bond to cellulose fibers and it provides mechanical strength and structural support to the plants. Lignocellulose is very difficult to be degraded in anaerobic environments due to its rigid structure produced by lignin. Numerous studies have reported that lignin content and the efficiency of enzymatic hydrolysis have an inverse relationship [50]. Due to the different diets, dairy cow manure generally shows the most abundant amount of lignin (i.e., around 18.0 %), whereas manure from beef breeds contains a lower amount of about 13% [50]. Other authors [47] found a content of lignin in dairy manure of 7.95%, while in pig and poultry manure it was 1.83% and 1.7%, respectively. Therefore, the extent of biogas production from cattle manure may be affected by management and, in particular, by the diet of the animals as in most cases they are fed on large amounts of substrates containing structural carbohydrates (cell wall components), which result in lower biogas production [63]. On the contrary, feeding cattle diets containing high levels of concentrate increases the biogas production from manure, due to the generation of higher levels of soluble organic-C during anaerobic digestion [63; 64]. Accordingly, [PERSON] et al. (2016) [65] observed higher biogas yield in young bulls fed on a high concentrate diet (80% concentrate + 20% forage) corresponding to high levels of net energy. Comparing organic and conventional management in dairy farms, [PERSON] et al. [66] observed higher biogas yield from cow manure produced by dairy herds under the conventional production system compared to AD with cow manure under the organic production system. These differences in methane yield may be due to the different diet fed to the animals, as in the conventional management diets are based on concentrate that provided more appropriate substrates for the AD process than organic diets, mainly based on roughage. Animal nutrition also affects the composition of manure-derived bio-fertilizers, altering the levels of the macronutrients N, P, and K [64]. Although high water content and high fraction of fiber make cow manure an unsuitable substrate for the production of biogas as single substrate [67], it may be an excellent \"carrier\" substrate for the mixed digestion of wastes, allowing AD of concentrated industrial waste, which could be not treated separately [67]. Its suitability to be used as \"carrier\" substrate is due to high water content (70-90 %), acting as solvent for dry waste materials, high buffering capacity [68] with beneficial effects on the pH in the reactor, and the high level of nutrient for optimal bacteria growth [67].