Thirty-two lactating Saanen goats were used in a 10-wk lactation trial, confinement each goat in individual metabolism cage. As it is shown in Figure 1, goats were randomly assigned to each of four different nitrogen (N) source diets (8 goats per treatment) and each animal was fed individually on a daily basis. Treatments were: 1) control (canola meal as a N source); 2) Urea (0.5% urea); 3) Optigen (Alltech. Inc., Lexington, KY) (0.55% Optigen) and 4) Polymer-coated urea (0.7% PCU) (international patent number: A01K5/00, inventor: Mitra Mazinani).

Figure 1. Graphical
abstract of experimental treatments.

In Table 1, diets were formulated to satisfy the nutrient requirements of Saanen goats using Small Ruminants NRC version 1.9.4468 (Tedeschi, Callaway, Muir, & Anderson, 2011). Goat’s average weight was 38.85 ± 2.14 kg, and 1979 ± 0.25 g d^-1 milk yield, and fed ad libitum twice daily (08:00, 15:00 h) isonitrogenous total mixed ration (15% crude protein) and they were housed in individual pens (Animal Care Committee Protocol: 37756).

Daily amounts of feed distributed, and feed refusal were weighed. Fecal and feed refusal samples were taken weekly. Apparent nutrient digestibility was calculated by measuring the difference between the consumed feed and feces as a percentage of goats feed intake and considering lignin as an internal marker (Cochran, Adams, Wallace, & Galyean, 1986).

Table 1
Diet composition and chemical analysis of different dietary treatments (% of dry matter)¹.

¹Control (canola meal as dietary N source); Urea (0.5% urea); Optigen (Alltech. Inc., Lexington, KY) (0.55% Optigen), and Polymer-Coated Urea (PCU, 0.7% - patent number: A01K5/00). ²Each kg contained: Vit A, 500000IU; Vit D3, 100000 IU; Vit E, 100mg; Ca, 190000mg; P, 90000mg; Na, 50000mg; Mg, 19000 mg; Fe, 3000 mg; Cu, 300 mg; Mn, 2000 mg; Zn, 3000 mg; Co, 100 mg; I 100 mg; Se, 1 mg; Antioxidant (B.H.T) 3000 mg.

Feed, feed refusal and feces were analyzed for crud protein (CP) by using copper catalyst and steam distillation into boric acid according the Kjeldahl method (method 973.18) and for ash (method 942.05), acid detergent fiber (ADF, method 973.18) as initial marker according AOAC methods (ASSOCIATION OF OFFICIAL ANALYTICAL CHEMISTIS [AOAC], 2005). Neutral detergent fiber (NDF) was estimated according to method of Soest, Robertson and Lewis (1991). Feeds refusal and feces were collected and dried for 24 h at 105°C in oven to measure dry matter content (method 934.01) and then samples were grounded with 1-mm screen of Wiley mill. Complete fecal collection of each goat was done in collection periods (once each week and every day in collection week).

Urine samples were taken according to Chen, Mejia, Kyle and Ørskov (1995) to determine microbial crude protein. Urine was collected hourly with using a fraction collector. In order to pH decrease under 3, an initial amount of H₂SO₄ was added in 30 ml bottles before collecting. If the final pH of samples was greater than 3, more H₂SO₄ solution was added to adjust it. In practice, urinary excretion of purine derivatives is an indicator of microbial protein and it was measured as the sum of allantoin, uric acid, xanthine and hypoxanthine using a method based on the AutoAnalyzer (Chen & Gomes, 1992).

The blood samples were taken fortnightly 2 hours after first feeding in morning, from the jugular vein of goats by using vacutainer tubes containing heparin and centrifuged at 2,500 RPM for 20 min. Plasma was harvested and frozen at -20°C. Total protein, albumin, cholesterol, urea, triglyceride, ALT and AST were measured by AutoAnalyzer (Biosystems A 15; 08030 Barcelona, Spain- Pars azma kits).

Goats were milked twice daily with machine (GEA portable model, USA) at 7 am and 3 pm during feeding time, and daily milk yield was recorded throughout the trial period. Milk samples were collected weekly. In the sampling week (last 7 days), 50 ml milk of each goat was thoroughly homogenized, and milk samples were analyzed for protein, fat, lactose and total solids-non-fat content using an automatic Milkoscan 605 analyser (Foss Electric, Hillerød, Denmark, AOAC International, 2005). Milk energy (MJ kg^-1) was estimated using fat, lactose and CP content of the milk for each goat (Tyrrell & Reid, 1965).

Data were analyzed by one-way ANOVA analysis of variance using the general linear models (GLM) procedure of SAS 9.1. Nitrogen treatments were considered the only sources of variation. The significance of differences between control and experimental treatments was estimated by using one-way ANOVA, with Duncan's post-hoc test, and alpha level of p < 0.05 was used to assess the significance among means. The statistical models used in this study were as follows:

$Y_{ij} = \mu + T_i+ e_{ij}$

where: Yij = observed response, μ = overall mean, T_i = treatment effect (I = 1 to 4), E_ijk = residual error

It was observed due to supplementing slow-release urea in a high-forage diets, dry matter intake decreased (Neal, Eun, Young, Mjoun, & Hall, 2014). Cherdthong, Wanapat and Wachirapakorn (2011) found that the urea and slow-release urea (urea-calcium sulfate and urea-calcium chloride) did not influence dry matter intake. However, Pinos-Rodríguez, Peña, González-Muñoz, Bárcena and Salem (2010) observed that consuming slow-release urea did not affect dry matter intake. The results of this research are consistent with the results of the present exp. The mechanism of supplementing slow-release urea on feed intake is difficult to explain but can be related to forage content and synchronization with non-starch carbohydrates of diet and slow-release urea percent in diet. Feed intake and digestibility are intimately related to each other and may affect N availability in rumen (Köster et al., 1996). Some studies reported positive influences on digestibility and feed intake when ruminants consuming slow-release urea instead to feed grade urea. Cherdthong et al. (2011) conducted a study on supplementing calcium-urea in diet of dairy cows that fed rice straw and results showed calcium-urea mixture increased organic matter intake and digestibility, compared to feed grade urea. Owens, Lusby, Mizwicki, and Forero (1980) compared effect of supplementing feed grade urea and lipid coated slow-release urea on steers fed cottonseed hulls and found slow-release urea increased cottonseed hull intake but digestibility did not change. In contrast, Taylor-Edwards et al. (2009) and Galo, Emanuele, Sniffen, White and Knapp (2003) fed polymer-coated urea to dairy cows using corn silage base diet and did not observe any improvement in dry matter intake or digestibility. However, the effect of urea and non-protein nitrogen sources on digestibility depends on their concentration, physical structure and adaptation period. Increased digestibility of Optigen and coated urea could be due to these treatments supplied enough nutrients for microorganisms in the rumen.

The role of supplementing ruminant diets with non-protein nitrogen is supplying ammonia for ruminal bacteria as a nitrogen source for synthesizing amino acids. Although, high releasing of ammonia can exceed the nitrogen utilization capacity of bacteria and reduce productive performance (Calomeni et al., 2015). It has been reported, oils can defaunation the rumen (Puniya, Singh, & Kamra, 2015), so maybe due to this fact, existence of paraffin (oil) as a coated material in the polymer-coated urea which increased microbial protein synthesis by eliminating protozoa. The synthesis of microbial nitrogen for ‘coated urea’ was greater than other treatments and this difference was also observed in other parameters of microbial nitrogen synthesis. Optigen and control treatments had the greater microbial synthesis, respectively, which indicates using non-protein nitrogen may increase the synthesis of microbial protein, although this difference was not significant. The purpose of any nutritional strategy that is designed to improve microbial protein synthesis is accomplished by improving the final production of the animals or increase efficiency of animal production. Cherdthong et al. (2011) reported an increase in total milk yield by dairy cows fed urea-calcium.

Mazinani et al. (2018) reported that supplementing urea can elevate blood urea concentration more than canola meal and it is due to rapid hydrolysis of urea to ammonia in the rumen. excess ammonia is absorbed into the blood from the rumen wall and then taken to the rumen where urea is usually detoxified. part of this urea can recycle by the rumen while the excess amount is excreted in the urine (Lapierre & Lobley, 2001). Other authors have reported that slow-release urea reduced blood urea concentration compared with feed grade urea (Cherdthong et al., 2011; Taylor-Edwards et al., 2009), that agrees with present results.

Xin, Schaefer, Liu, Axe and Meng (2010) showed different slow-release urea sources did not affect milk fat. Cameron, Klusmeyer, Lynch, Clark, and Nelson (1991) found milk production was increased by supplementing urea, whereas protein and fat concentrations were not altered. In another study, Supplementing slow-release urea increased milk protein (Tye, Yang, Eun, Young, & Hall, 2017). Dennis, Unruh-Snyder, Neary and Nennich (2012) reported that goats' milk composition was not affected by dietary protein concentration, and this agrees with the present results. Xin et al. (2010) also observed no change in milk protein with a slow-release urea treatment. It can be concluded that feed efficiency was improved with the addition of urea and other non-protein nitrogen sources and increased milk efficiency. According result, coated urea and Optigen had better feed conversion ratio grade compared with control or feed grade urea. In recent study slow-release urea increased milk production but no impact on milk composition. This results was in contract with Santos and Huber (1996) reported that replacing part of soybean meal by Optigen or feed grade urea had no impact on milk yield but agreed with Inostroza, Shaver, Cabrera and Tricárico (2010) reported of increasing milk production when Optigen partially replaced with feed grade urea.

*Author for correspondence. E-mail: mitram@uidaho.edu