*2.3. Diet Composition*

HM was fed to the lambs through diets formulated to have weight gains of 300 g d−<sup>1</sup> [27]. HM (1, 2 or 3 g kg−<sup>1</sup> of diet DM basis) was premixed with minor ingredients (vitamin and mineral supplement, limestone and salt) before incorporation into complete mixed

diets. The lambs were fed a finishing diet (total mixed ration) comprised 30.3% ground corn, 24.1% ground sorghum, 8.1% soybean meal, 7.1% wheat bran, 7.4% corn gluten, 2.3% bypass fat, 19.4% oat straw, 0.5% vitamin and mineral supplement, 0.5% salt, and 0.3% limestone (DM basis). Oat straw was ground in a hammer mill (Azteca 20, Molinos Azteca, Guadalajara, México) with a 3.8 cm screen before incorporation into total mixed ration. The nutrient composition of the basal diet was 15.53% crude protein, 2.58% ether extract, 13.57% acid detergent fiber, 26.14% neutral detergent fiber, 5.47% ash and 2.8 Mcal of metabolizable energy according to NRC [27] DM basis.

#### *2.4. Animals and Experimental Design*

Thirty-six male Dorper × Katahdin lambs (23.27 ± 1.23 kg BW, 4–5 months old) were randomly distributed in four treatments: (1) basal diet without HM (CON); (2) HM1, CON + 1 g of HM kg−<sup>1</sup> dry matter (DM); (3) HM2, CON + 2 g of HM kg−<sup>1</sup> DM; and (4) HM3, CON + 3 g of HM kg−<sup>1</sup> DM. The lambs were placed in individual pens (2.6 m × 0.8 m) equipped with automatic drinkers and individual feeders. Prior to the start of the experimental phase, lambs were vaccinated against *Clostridium* and *Pasteurella* (2.5 mL lamb−1, Bobact® 8 MSD-Merck, Kenilworth, NJ, USA), and dewormed through an oral administration of Koptisin ovine® (10 mg kg−<sup>1</sup> BW, Chinoin, Labs, Mexico City, Mexico). Additionally, 1 mL lamb−<sup>1</sup> of vitamins containing 500,000 IU of vitamin A, 75,500 IU of vitamin D and 50 mg of vitamin E (Vigantol® Bayer, Mexico City, Mexico) was provided on day 1 of the adaptation period. The lambs had an adaptation period to the basal diet of 14 days, and the experimental phase lasted 56 days. During the adaptation period, the lambs received oat straw as a ruminal pH buffer, and the experimental diets were administered at increasing levels (20, 40, 60, 80 and 100% of the total ration) for 14 days (3 days per level, except for 100%), until the oat straw was reduced to 0%. The feed was provided at 09:00 and 17:00 h, and the drinking water was supplied ad libitum. Individual BW was recorded before the morning feeding on days 1, 14, 28, 42 and 56, of the experimental phase. The amount of diet offered and refused was recorded daily to estimate dry matter intake (DMI, kg <sup>d</sup>−1). The amount of feed offered was always 10% higher than the previous intake to ensure ad libitum intake. Daily weight gain (DWG, kg <sup>d</sup>−1) was calculated between feeding period intervals. The feed conversion ratio (FCR) was expressed as feed consumption per unit of body weight gain. Figure 1 shows the experimental procedure.

**Figure 1.** Completely randomized design and sampling times of lambs supplemented with a polyherbal mixture (HM) during the final fattening period; *n* = 9—indicate the number of animals sampled in each treatment; Control—basal diet without HM; HM1—basal diet + 1 g of HM kg−<sup>1</sup> of dry matter (DM); HM2—basal diet + 2 g of HM kg−<sup>1</sup> of DM; HM3—basal diet + 3 g of HM kg−<sup>1</sup> of DM; BFT—backfat thickness; LMA—*longissimus* muscle area.

#### *2.5. Sampling and Analyses of Feeds*

Samples of feed provided and rejected were collected daily to determine the chemical composition. Prior to the analysis, the food samples were dried at 55 ◦C in a forced air oven and then ground in a Wiley mill (model 4, Arthur Thomas Co. Philadelphia, PA, USA). The variables determined were dry matter, crude protein, ether extract and ash [28]. Acid

detergent fibre and neutral detergent fibre was determined using the procedures described by Van Soest et al. [29].

#### *2.6. Apparent Dry Matter Digestibility*

Faecal samples were collected from each animal during five consecutive days (in the morning at 08:00 a.m. and in the afternoon at 16:00 p.m. before feed delivery) starting on day 51, directly from the rectum [5]. Feed and orts were collected daily during the same period. Acid-insoluble ash was employed as a marker of internal tract digestibility to analyse the apparent total tract DM digestibility [30].
