INTRODUCTION

In Mexico, sheep production is an adequate alternative for the production of food and other products (skin, wool, manure, etc.) because they are small ruminant animals that adapt easily to different environments and take advantage of the resources available in each region ¹.

The main source of feed for sheep, as well as for other ruminants, is fodder; which growth and availability, as well as quality in the tropics, vary during the year, generating deficiencies in the availability of biomass and its nutritional properties, which is required by livestock ². Due to these changes, in extreme conditions there is weight losses, deaths and an appreciable decrease in the continuity of the productive process of animals, so supplementary feeding is one of the elements that determine the productivity of the system ².

As in all species, sheep need to meet nutrient requirements ³. Grain-based supplements or rations are often incorporated, which prices have increased and consequently production costs are higher ¹, which makes it necessary to look for food alternatives for livestock maintenance ^4-6.

There are several inputs that can be used to cover the nutritional requirements of sheep, since ruminants can use fibrous products and by-products that other non-ruminant animals cannot consume ^3,6.

The State of Tamaulipas is a citrus producing region. In 2012 a production of 605 thousand tons was achieved between orange, lemon and grapefruit; of the total production, about 10%, is intended for the juice extraction industry. During this process, a large amount of by-product is obtained, producing residues from 45 to 60% of the fruit's weight, which is composed from 60 to 65% of the husk, from 30 to 35% of the pulp and from 0 to 10% of the seed; said residues constitute the fresh citrus pulp (FCP), Which can be used in animal feed ^4,5.

Potentially the FCP can be considered as a source of highly fermentable and digestible fiber in the rumen, contributing quantities of energy substrates ^4-6. In addition, this by-product is relatively cheap because the only cost is the transport from the juice fountain to the production unit.

On the other hand, Caparra et al ⁷ and Villanueva et al ⁴ indicated that the citrus residue is a low protein by-product and rich in soluble carbohydrates, in neutral detergent fiber; being an available source for the growth of rumen microorganisms. Dihigio et al ⁸ mentioned that, because of its energy content, FCP can be used as a substitute for cereals.

The purpose of this study was to evaluate the effect of fresh lemon pulp (FLP) at different levels, as a source of energy and as a substitute for sorghum grain, on the parameters of weight gain, food consumption and feed conversion, as a feeding alternative for hair lambs.

MATERIALS AND METHODS

Study area. The experimental work was carried out at the Zootechnical Site "Ing. Herminio Garcia Gonzalez" of the Faculty of Engineering and Sciences (FIC) of the Autonomous University of Tamaulipas (UAT), located at km 23 of the Victoria-Monterrey highway, in the Municipality of Güemez, Tamaulipas, Mexico. It is geographically located at 23° 45' 10" North latitude and 98° 59' 05"› West longitude, at an altitude of 145 meters above sea level, with predominant semi-dry and warm climate, with an average annual rainfall of 700 mm and average annual temperature of 22°C ⁹.

The bromatological analyzes of the rations used were conducted at the Nutrition and Food Quality Laboratory of the Faculty of Agronomy of the Autonomous University of Nuevo Leon, located in Gral. Escobedo, Nuevo Leon, Mexico.

Experimental units. Twenty hair lambs were selected from the Blackbelly, Pelibuey and Katahdin breeds, with an average live weight of 22.13±1.87 kg. Later the ewes were dewormed with 0.5 mL of ivermectin. On the other hand, 2.5 mL of toxoid bacterium was applied subcutaneously, for symptomatic charcoal and malignant edema, and a dose (1 mL) of ADE vitamin was administered intramuscularly.

The ewes were randomly distributed into four groups and kept under housing conditions under a concrete floor in metal cages of approximately 2 m², with individual feeders and drinking troughs.

The work lasted for 75 days and was divided into a pre-experimental phase of 12 days of adaptation and 63 days of testing.

The treatments evaluated corresponded to four experimental rations whose percentage composition consisted of: T₁=0% of FLP and 30% of sorghum; T₂=10% of FLP and 20% of sorghum; T₃=20% of FLP and 10% of sorghum; and T₄=30% of FLP and 0% of sorghum. The rest of the mixture is shown in table 1. The FLP used in this work consisted of lemon husk, pulp and seeds, which were acquired after the processing of lemons for the extraction of juice at a plant located 1 Km away from the production unit where the experiment was performed.

Ration analysis. To evaluate the chemical composition of each one of the rations, samples of approximately 300 g were taken and transferred to the Nutrition and Food Quality Laboratory of the Autonomous University of Nuevo Leon, Faculty of Agronomy at Gral. Escobedo, Nuevo Leon, Mexico . The analyzes were determined in duplicate for each one of the characteristics. The content of dry matter (DM), ashes (C), crude protein (CP) and ethereal extract (EE) (AOAC 10). While neutral detergent fiber (NDF), acid detergent fiber (ADF), hemicellulose (HEM), cellulose (CEL) and lignin (LIG) were performed according to the protocols of Van Soest et al ¹¹.

The in vitro digestibility of the DM (IVDMD) of rations was determined using a ruminal fermenter methodology (Daisy II, Ankom Technology ^(r) , USA). The preparation of the culture medium and inoculum, as well as the determination of the digestibility of dry matter, was based on the protocol proposed by the fermentor supplier (Daisy II, Ankom Technology ^(r) , USA).

The determination of the gross energy content (GE) was calculated with the data generated in the adiabatic calorimetric pump (Parr ^(r) , 1241 USA). The content of digestible energy (DE) and metabolizable energy (ME) was calculated considering the GE and IVDMD of the samples.

Experimental design. A completely randomized design was used with four treatments and five replicates. Each treatment was applied to a group of ewes to which food and water were freely provided on a daily basis. The variables evaluated were: daily food intake (DFI), daily weight gain (DWG) and feed conversion (FC).

The DWG of animals was determined by weighing them at the beginning of the experimental phase and then with a frequency of every 7 days, with a fasting of 12 hours prior to weighing.

Consumption of DM (DFI) was estimated on a daily basis, by weighing the amount of food offered and subtracting the amount of food rejected.

The FC was calculated by dividing the food intake by the total weight gain obtained.

The productive behavior of ewes was determined using the following (covariance analysis) model:

Y_ij = μ + T_i + β X_ij + ε_ij

Where:

Y_ij = Response variable in the j-th repetition of the i-th treatment, μ = Overall average, T_i= Effect of the i-th ration, X_ij = Initial weight of the j-th ewe, β = regression coefficient that relates Y_ij to the variable X_ij and ε_ij = Random error ¹².

The chemical composition of the rations was determined using the following model:

Y_ij = µ + T_i + ε_ij

Where:

Y_ij = Response variable in the i-th treatment and j-th repetition, μ = Overall average, T_i = Effect of the i-th treatment and ε_ij = Experimental error ¹².

In the cases where there were significant statistical differences (12), Tukey's mean comparison tests were applied with a significance level of (p<0.05).

RESULTS

The results of the chemical composition of the rations used in the feeding of ewes are detailed in table 1, which shows that the ash content decreased when the level of substitution of the sorghum by FLP was increased up to 20%, observing significant differences (p<0.05). Similarly, the behavior on CP was reduced to the extent that the higher level of substitution of sorghum by FLP was reached; showing differences (p<0.05) where T₄ had a lower value (8.29% CP) than T₁ (10.35% CP).

The EE content did not show any significant difference (p>0.05), showing values of 4.90, 5.71, 6.86 and 7.44% for T₁, T₂, T₃ and T₄, respectively.

For the fiber fractions NDF, ADF and CEL, significant differences (p<0.05) were obtained with lower values of T₁, with respect to the other treatments, which were similar to each other (p>0.05). The sorghum substitution level radically affected the fiber fractions, increasing their values (48.62 to 52.51, 30.77 to 35.69 and 24.15 to 28.90%, respectively for NDF, ADF and CEL). However, the HEM content was lower in T₄, showing significant differences (p<0.05), with respect to T₂, with the values of T₁ and T₃ being intermediate and statistically similar (p>0.05). Regarding LIG, there were no significant differences (p>0.05) between treatments (Table 1).

The IVDMD of rations showed a difference (p<0.05) when raising the FLP level (20%) with a value of 68.05% in the ration replacing sorghum grain (T₁ vs. T₃). However, it showed a quadratic trend (p=0.99) by completely replacing sorghum (30% FLP), which indicated a statistical similarity for T₁, T₂ and T₄ (Figure 1).

Figure 1 Invitro digestibility in rations with different levels of substitution of sorghum for fresh lemon pulp. 

The content of GE, DE and ME of the rations was increased with respect to the level of sorghum substitution by FLP. A quadratic trend (Figure 2) was observed (by determination coefficients greater than p=0.93) in the energy level of the ration by having a higher level of substitution of sorghum by FLP. Regarding the ME for this work, the rations with the highest energy content were T₃ and T₄ (2.14 and 2.16 Mcal kg^-1 DM, respectively), being higher (p<0.05) than T₁ and T₂, which also showed significant statistical differences among themselves (Figure 2).

Figure 2 Behavior of the level of gross energy (GE), digestible energy (DE) and metabolizable energy (ME) in rations with different levels of substitution of sorghum for fresh lemon pulp. 

The Experimental rations were use to feed hair lambs, generating the following results for the parameters evaluated. DWG ranged from 97 to 108 g d^-1 with no significant differences (p>0.05) between treatments. The DFI increased (p<0.05) from 909 and 911 (T₁ and T₂) to 1026 g d^-1 by including 30% of FLP in substitution of sorghum; where T₃ was the intermediate value. However, FC was not different (p>0.05) between treatments (Table 2).

DISCUSSION

Composition of the nutritional content of diets. The nutritional content of rations was affected by the level of substitution of sorghum by FLP. The crude protein content of the rations decreased as the highest level of sorghum substitution by FLP was achieved, with T₄ showing a lower value (8.29% CP) than T₁ (10.35% CP). These results are similar to those reported by Villanueva et al ⁵ when 15.0% of CP was obtained by integrating 15% of orange residue on the total ration and 12.5% of CP by incorporating 30%. This is attributed to the low CP content of fresh orange pulp (6.0%) as compared to sorghum with 11.2% as reported by Villanueva et al ⁸. Similarly, Bampidis and Robinson ⁴, in their review of citrus fruit by-products, found that the CP content is about 9.0%; likewise, there are other texts in the literature that mention similar values ^{(6, 13)}.

Regarding the EE content, the values obtained were higher than those reported by Bampidis and Robinson ⁴ in an exhaustive review of citrus fruit by-products. Similarly, Villanueva et al ⁵ reported values of 2.1% for EE when using 15, 20, 25 and 30% of fresh orange peel pulp in rations for fattening hair lambs. However, Dadvar et al ¹³ found values of 7.06% for EE.

In this study lignin showed valueshigher than those reported by Villanueva et al ⁵ and González et al ⁶.

The results obtained for the IVDMD of the rations used increased as the level of FLP increased (Figure 1), until obtaining a value of 68.1% for T₃ (20% of FLP). Macias-Cruz et al ¹⁴ reported that increasing fresh orange pulp in rations from 30 to 40%, the IVDMD improved from 80.6 to 84.2%, so it is expected than an increase in IVDMD will raise the ME level in the ration. However, Dadvar et al ¹³ found that IVDMD was 60.45%.

The increase in the inclusion level of FLP in the ration is associated with the increase in ME, as observed in figure 1; which can be attributed to the high pectin content ¹⁵. In this regard, Villanueva et al ⁵ reported values of 2.3 and 3.2 Mcal kg^-1 for the nutritional content of sorghum and FLP, respectively, which allows improving the content of ME in the ration.

Productive behavior of ewes. The values for of DFI recorded in this study were lower than those reported by Cienfuegos-Rivas et al ¹⁶, in lambs fed with rations that included dehydrated orange pulp, in which consumption was higher as the level of residue increased. Similarly, Macias-Cruz et al ¹⁴, when including up to 30% of fresh orange pulp, obtained a quadratic behavior in consumption (1.13, 1.22, 1.34 and 1.32 kg d^-1) of diets that included 0, 15, 30 and 45% of pulp, respectively. However, the results of this study are different from those reported by Villanueva et al ⁵, with DFI of 1.35, 1.22, 1.07 and 0.97 kg by incorporating 15, 20, 25, and 30% of fresh orange pulp in the rations.

Considering the results obtained for DWG, it is possible to mention that these are lower than those reported by Pascual-Cordova et al ¹⁷ in Pelibuey lambs, with 192 g d^-1, using whole diets with a ME content of 2.8 Mcal kg^-1 and 19.8% of CP. Similarly, Cienfuegos-Rivas et al ¹⁶ found that the DWGs were higher in Dorper x Pelibuey hair lambs.

From the results of this work it can be concluded that FLP can be used, up to 30% of diet, as a replacement of sorghum in the feeding of hair lambs, without affecting the productive parameters of DWG and FC of hair lambs.

Similarly, the FLP is considered an alternative energy source to substitute sorghum grain in the feeding of hair lambs.