4.1. Nutrient Intake, Body Weight, Milk Production, and Composition
All animals completed the experiment without any noticeable health problems or disorders. In agreement with our results, previous studies have shown that the use of AF did not affect the health status of the animals [
6,
7,
8,
9,
10]. In the current study, no effect on DM, CP, and ADF intake was found. The increased intake of NDF for the CON diet can be attributed to the higher NDF content of this diet compared to the AF200 and AF400 diets, because of the higher level of NDF from wheat straw in the CON group. Consistent with our study, OC [
14], CARP [
15], and BB [
16] had no effect on the intake of DM, OM, CP, NDF, and ADF in Awassi sheep in previous studies. Consistently, Kleinschmit et al. [
17] and Schingoethe et al. [
18] reported that DM intake was not reduced by including DDGS in the diet until the incorporation rate exceeded 200 to 300 g/kg of dietary DM. Moreover, Felix et al. [
19] reported that increased dietary DDGS diets (0, 200, 400, and 600 g/kg DM) did not affect DM intake in feedlot lambs.
In contrast, Hindiyeh et al. [
20] found that different levels of BB (100, 200, and 300 g/kg DM) decreased DM, OM, CP, NDF, and metabolizable energy (ME) intake in a fattening diet for Awassi lamb compared to a control diet. Hence, the different response to the BB intake could be attributed to the components of this by-product, animal species, basal diet composition, and inclusion level [
16]. The similarity in nutrient intake (apart from NDF intake) in all groups in our study could be attributed to the fact that the inclusion of AF at 200 or 400 g/kg DM did not change the nutrient composition of all diets. Our experiment feed intake results are in agreement with previous studies indicating that these AF are palatable and easy to be consumed by the animals [
14,
15,
16,
17,
18,
20].
Ewes’ BW changes and the lambs’ average daily gain (ADG) did not differ between treatments. Similarly, Belibasakis [
21] and Omar et al. [
22] reported no negative effect on the ADG and BW of lambs with the inclusion of 150 to 250 g/kg DM of OC. Obeidat et al. [
15] also found that different levels of CARP inclusion (0, 150, and 250 g/kg DM) did not affect BW change or ADG. Moreover, BW change and ADG of nursing Awassi ewes and their lambs did not differ between treatment diets with 100, 150, and 200 g/kg DM of BB [
16]. Alshdaifat and Obeidat [
9] reported no impact on BW change of nursing ewes or ADG of their suckling lambs by replacing soybean meal with DDGS. Morricall [
23] also found no difference in the BW of ewes and lamb total gain when nursing ewes were fed DDGS diets compared to basal diets.
Similar to our results, Obeidat et al. [
8]; CARP), Obeidat et al. [
16] (BB), and Janicek et al. [
24] (DDGS) observed no effects on milk production and composition when AF were included in the diet. Recently, Keles et al. [
25] supplemented dairy Saanen goats with OC silage at 0, 100, and 200 g/kg of dietary DM and observed no effect on milk yield, CP, lactose, energy corrected milk (ECM), fat corrected milk (FCM), or feed efficiency, while fat content and fat production increased. Diets containing OC silage may have produced milk with a higher fat content due to its high level of fiber (NDF and ADF) and, subsequently, more volatile fatty acids were generated (mainly acetate; the major precursor for fatty acid synthesis).
Moreover, Hassan et al. [
26] reported that lactating Zaraibi goats fed diets containing CARP at levels of 25 and 50 g/kg DM had greater milk production and protein and fat percentages than when supplemented with 100 g/kg of CARP. This could be linked to an increase in the metabolizable protein supply associated with condensed tannins (CT). Protein-CT complex is preserved from microbial degradation and, subsequently, digested and absorbed as amino acids in the small intestine [
27].
Kleinschmit et al. [
28] found that DDGS contains an average 200–300 g CP/kg, of which about 50–55% is rumen undegraded protein (RUP). Consequently, Kleinchmit et al. [
17] and Pamp et al. [
29] reported that milk yield increased with the inclusion of DDGS in diets. Leonardi et al. [
30] also reported a linear increase in milk yield and milk protein yield with an increasing dietary DDGS (at 0, 50, 100, and 150 g/kg of dietary DM) in dairy cows. Owen and Larson [
31] found that milk production in early lactating cows increased when DDGS was included at 188 g/kg DM but decreased when DDGS was included at 358 g/kg of the diet DM. These findings may be due to lower protein absorbability and lysine deficiencies.
4.3. Intake, Digestibility, and N Balance
Similarly to our results (1 and 2), Tufarelli et al. [
32] did not find any differences in intake of DM and CP when lambs were fed diets with different levels of partly destoned exhausted OC (0, 100, or 200 g/kg DM). Furthermore, Silanikove et al. [
33] observed no change in DM intake when feeding 520 g/kg DM CARP to Anglo-Nubian goats, and Benchaar et al. [
34] reported similar DM intake when supplementing dairy cows with 150 g CT/kg. Reed [
35] and Silanikove et al. [
36], however, reported that feed intake decreased due to the high levels of CT present in the CARP reducing palatability. In contrast to these finding, Afzalzadeh et al. [
37] reported that diets containing BB at different levels (0, 60, 120, and 250 g/kg of dietary DM) had no effect on DM intake in fattening Zandi lambs. Similarly, Felix et al. [
19] reported that increased dietary DDGS (0, 200, 400, and 600 g/kg DM) did not affect DM intake in feedlot lambs, and McKeown et al. [
38] reported that DDGS from different sources of cereals had no effect on ADG of feedlot lambs.
The nutrient digestibility results were consistent with those of Awawdeh and Obeidat [
14], who reported no difference in the digestibility of DM, OM, CP, and NDF with the dietary inclusion of OC. Tufarelli et al. [
32], however, reported that DM and fiber (NDF and ADF) digestibility diminished in diets containing partly destoned exhausted OC (100 or 200 g/kg of dietary DM) compared to a control diet. This finding could be explained by Feggeros and Kalaisakes [
39], who reported that digestibility of OC was very low because of the greater levels of lignin and tannin. Moreover, Filya et al. [
40] observed that poor digestibility values are a consequence of the close association of proteins with lignocellulosic compounds.
In contrast to these findings, Awassi lambs that were fed a diet containing 125 and 250 g/kg DM of CARP showed no negative effects on nutrient digestibility [
15]. Similarly, Benchaar et al. [
34] found that a diet supplemented with 150 g/kg CT produced no differences in apparent DM, OM, CP, NDF, and ADF digestibility. Silanikove et al. [
33] and Obeidat et al. [
8], however, found that CARP had an adverse effect on DM digestibility and the digestibility of other nutrients compared to a control diet. These effects could be attributed to the high CT levels in CARP, which can diminish proteolysis rate, restraining the development of rumen proteolytic microorganisms and reducing the digestibility of nutrients, especially CP. Provenza [
41] showed that a high level of tannins in animal feed could potentially reduce the nutritional value as tannins bind to proteins and reduce their availability to the ruminal microflora. Obeidat et al. [
16], however, reported that DM, OM, CP, NDF, and ADF digestibility was similar across treatments with partial replacement of barley with BB at 0, 100, 150, and 200 g/kg of dietary DM in nursing Awassi ewes diets. Moreover, Felix et al. [
19] found that diets containing DDGS (0, 200, 400, and 600 of dietary DM) did not affect ether extract (EE), NDF, and ADF digestibility of lambs.
In contrast, Winterholler et al. [
42] reported that when spring-calving beef cows were fed DDGS at different levels (0.77, 1.54, and 2.31 kg/d), the digestibility of NDF, ADF, CP, and fat increased. This result could be explained by Lemenager et al. [
43], who found that DDGS led to the high digestibility of nutrients, including NDF and ADF. The use of isonitrogenous and isocaloric diets in our study may have prevented the negative effects on production parameters that were previously observed in other studies, but further studies are needed in this area to validate these findings.
Our N balance results are in line with Awawdeh and Obeidat [
14], who found that diets containing OC did not affect the N intake or the N excreted in the feces and urine of Awassi lambs. Consequently, retained N was not different among treatments. This is similar to Almira et al. [
44], who found that N intake was not affected by the ratio of BB to cornmeal and observed no influence of BB on the excretion of urinary and fecal N or on N balance. In contrast, Felix et al. [
19] reported that increasing DDGS levels in lamb diets increased N intake, N digestibility, and urinary N output, while N retention decreased. In addition, Benchaar et al. [
45] observed that N intake, fecal N, urinary N, total N excretion, and retained N increased with increasing DDGS (0, 100, 200, and 300 g/kg DM) in dairy cows. Gurung et al. [
46] also found that when goats were fed diets with increasing DDGS levels (0, 127, 254, 381 g/kg DM), urinary N increased because DDGS is a good source of RUP, while fecal N, urinary N, retention N, and retained N were similar across treatments.