Influence of forage quality on ruminal degradation kinetics of feedlot diets for lambs formulated according to neutral detergent fiber

Constantino Raymundo-Hernández, Jaime Salinas-Chavira, Miguel Ángel Domínguez-Muñoz and Miguel Ángel Guevara-Guerrero

Facultad de Medicina Veterinaria y Zootecnia, Universidad Autónoma de Tamaulipas, México. Carretera Victoria-Mante Km 5, Ciudad Victoria CP 87000, México
jsalinasc@hotmail.com

Abstract

This study evaluated fattening diets for lambs formulated with forages of different quality at similar neutral detergent fiber from the forage. Ruminal kinetics of dry and organic matter was determined using three non-castrated crossbred hair lambs (20.6±1.8 kg) fitted with permanent rumen cannulae assigned in a 3 x 3 Latin Square experiment. Treatment diets contained 6.5% NDF from forage (eNDF), which was supplied by low quality forage (sorghum stover, SS); a mix of low quality plus high quality forage (sorghum stover and alfalfa hay, SS-ALF); and high quality forage (alfalfa hay, ALF). Diets were incubated at 0, 4, 8, 12, 24, 48, and 72 hours; pH in ruminal liquid was measured 4 h after feeding. Rapidly soluble fraction (a), rate constant (c) and effective degradability modeled at 5%/h ruminal turnover were similar among all diets (p >0.05). Degradable (b) and potentially degradable (a+b) fractions were more degraded (p <0.05) in the rumen in ALF than SS; diet SS-ALF was similar (p >0.05) to SS and ALF. There was no effect of treatment on in situ organic matter at any ruminal incubation time (p >0.05). Ruminal pH was not influenced by dietary treatments (p >0.05). In conclusion, the good quality forage may have minimal advantage over the low quality forage on ruminal degradation kinetics when they are included at similar NDF from forage in fattening diets for lambs.

Keywords: cell wall, digestibility, forage, rumen function, sheep

Introduction

In the dry tropical areas of México, large quantities of sorghum stover are available as a roughage source that can be included in diets for feedlot lambs (Salinas-Chavira et al 2013a). In terms of crude protein and net energy (NE) value, the nutritive quality of sorghum stover is low (NRC 2000) compared with forages of high nutritive value. In contrast, high quality forages like alfalfa, are scarce in the subtropical areas of México.

Improved productive performance has been obtained reducing forage level in the diet of feedlot lambs (Papi et al 2011). However, forages are included in finishing diets largely on the basis of their functional role as roughage, i.e., buffering ruminal acids. Higher ruminal pH and saliva production has been observed with higher forage level in cattle (Chibisa et al 2016) and lambs (Fimbres et al 2002). In this regard, forages are included in finishing diets by their function and not by their crude protein concentration or nutrient digestibility. Galyean and Defoor (2003) with feedlot cattle observed that fiber function is given by its NDF concentration, and this constraint is a better approach than diet formulation considering forage level, per se.

Salinas-Chavira et al (2013b) observed minimal effect on the productive performance of feedlot steers fed finishing diets with forages of different nutritive value (alfalfa, Sudan-grass and rice straw) when they were incorporated into an equivalent proportion of NDF from forage (similar functional fiber). Comparable results were reported by Swanson et al (2017) in feedlot steers fed diets with different forage sources (alfalfa hay, maize silage, wheat straw, and maize stover) when forage sources were at the same percentage of NDF from forage. Conversely, Galvani et al (2014) observed greater energetic efficiency of feedlot lambs fed a finishing diet using a low-quality forage (sugarcane bagasse) than a medium-quality forage (coast cross hay) when forages were at similar physically effective NDF in the diet. Also, Flores-Mar et al (2017) observed better productive performance of feedlot lambs fed diets with sorghum stover than with diets containing alfalfa hay, when diets were formulated at equivalent forage NDF level from forages. Research results on ruminal degradation kinetics of feedlot diets for lambs balanced according to NDF with different forage quality are limited. The aim of this study was to determine the ruminal degradability of dry matter (DM), organic matter (OM), and pH response of feedlot diets for lambs formulated according to NDF from low and high quality forages.

Materials and methods

Procedures including animals were performed according to the protocol for animal welfare and management of the Facultad de Medicina Veterinaria y Zootecnia (FMVZ) of the Universidad Autonoma de Tamaulipas (UAT).

This study was carried out at FMVZ-UAT, in Cd. Victoria, Tamaulipas (subtropical area in northeastern Mexico), located at 23°44’06” N and 97°09’50” W, at an altitude of 340 m. The mean annual rainfall is 900 mm and the average temperature is 25 ˚C (INEGI 2017).

In situ dry and organic matter digestibility of diets

Three 5-month-old non-castrated cross-bred (Dorper-Pelibuey-Black Belly) male lambs (20.6±1.8 kg initial weight), fitted with permanent ruminal cannulae (40 mm in diameter) were used. Before the trial, lambs were treated for parasites with a preparation of Ivermectin which also contained vitamins (Iverfull-macrovit ADE®, Laboratorios Aranda, SA de CV, Mexico). During this period each animal received a single dose of 1 ml of a preparation that contained 40 mg trimethoprim plus 200 mg sulfamethoxazole (Fioprim®, Laboratorios FIORI, SA de CV, Mexico). The animals were immunized against Clostridium spp. and Pasteurella spp. (Denkabac 8®, Laboratorios Tornel, SA, Mexico).

Lambs were housed in individual well-ventilated draft-free pens and fed ad libitum three dietary treatments (Table 1) in twTo equal meals at 9:00 and 15:00 h. Treatment diets had similar NDF content (6.5%) that was supplied by sorghum stover (SS); sorghum stover and alfalfa (SS-ALF); or alfalfa hay (ALF). Diets were formulated to fulfill growing lamb requirements according to NRC (2007) and contained 14% CP, 11.0 MJ/kg ME (metabolizable energy) and 6.5% NDF from forage. Diet formulation was on DM basis. Water was made available permanently throughout the experiment.

During the Latin square design experiment an adaptation period of 10 days was given prior to the incubation of bags in the rumen. For rumen incubation, 5 x 10 cm and 53-μm mesh nylon bags were used (ANKOM Technology, Macedon NY, USA). Bags were incubated in duplicates with 6 g per bag of the diet offered to the lambs. Feed samples were ground in a No. 4 Wiley mill equipped with a 2.0-mm retaining screen. Bags with feed-sample were tied to a 20-cm nylon cord with a metal weight at the end to ensure that samples were immersed in the ventral sac of the rumen. Bags were incubated for 0, 4, 8, 12, 24, 48 and 72 h. The bags were removed, washed with tap water at low pressure until clear water came out of the bag, and then they were dried in an air-forced oven at 60^oC for 48 h to a constant weight and analyzed for OM by ashing at 600 °C for at least 8 h (AOAC 2003).

In situ dry and organic matter degradability

The in situ dry and organic matter degradability for samples on each incubation time was calculated by the weight difference between samples in bags before and after ruminal incubation, following Ørskov and McDonald’s (1979) model, which was modified by McDonald (1981): P = a + b (1 – e^-c*t). Where P = degradation of DM or OM (%), a = rapidly soluble fraction or washing loss (%), b = degradable fraction (%), c = fractional degradation rate/h, a + b = potential degradability (%; representing the quantity of feed that can be degraded in rumen if the time is not a limiting factor), t = time (h).

Ruminal turnover constants (k) at 5%/h were used to calculate effective degradation (ED; Ørskov and McDonald 1979): ED (DM or OM) = a + (b x c)/(c + k). Where ED (DM or OM) = effective degradation of DM or OM.

Ruminal pH was measured from ruminal liquid obtained 4 h post feeding at the end of each sampling period. The pH was measured using a portable pH meter (model Waterproof pH Testr 3®, Oakton Instruments).

Statistical analysis

Data from this study were analyzed using 3 x 3 Latin square models. Turkey´s test was used for mean comparison. Statistically significant differences were declared at p <0.05. GLM procedures of SAS (2007) were used.

Results

Results of in situ DM digestibility are shown in Table 2. There was no treatment effect on in situ dry matter digestibility at 0, 8, 12, 24 and 48 h of ruminal fermentation (p >0.05). Rapidly soluble fraction (a), rate constant (c) and effective degradability modeled at 5%/h ruminal turnover were similar between diets (p >0.05). Degradable (b) and potentially degradable (a+b) fractions were more degraded (p <0.05) in rumen in the diet with higher level of alfalfa hay (ALF) than in the diet with higher level of sorghum stover (SS). The diet with sorghum stover plus alfalfa hay (SS-ALF) was similar ( p >0.05) to both SS and ALF.

Results of ruminal organic matter degradability of diets with different forages at similar NDF in lambs are shown in Table 3. There was no effect of treatment on in situ organic matter at any ruminal incubation time (p >0.05). Consistently, ruminal parameters of organic matter degradability were not influenced by treatment diets. The observed values of ruminal pH were 5.58, 5.69 and 5.87 (SEM=0.05) for SS to ALF, respectively. It was not influenced by dietary treatments (p >0.05).

Discussion

In agreement with the present study, Salinas-Chavira et al (2011b) in lambs fed high-concentrate diets did not find an effect on ruminal dry matter degradability when whole ear of maize replaced sorghum stover in their diets. However, another study showed an increase in the rapidly soluble fraction (a) by the substitution of low by high quality forage in feedlot diets for lambs (Salinas-Chavira et al 2011a). This may be explained by less NDF in the high-quality forage than in low-quality forage. In contrast, in the present study all diets had similar NDF content from forage. This indicates that a ration with high or low quality forage can be used similarly during the first hours of ruminal incubation when forages are in proper amounts and do not have negative effect on ruminal function. Forages prevent the fast ruminal degradation of diet that induces ruminal acidosis. In this study the ruminal pH values approached subclinical acidosis (Nagaraja and Titgemeyer 2007); nevertheless, Gonzalez-Momita et al (2009) reported values of ruminal pH equal to the present investigation.

In feedlot diets for lambs with rice polishing, Salinas-Chavira et al (2013c) observed ruminal dry matter degradability during the early hours of incubation similar to the present study. However, these degradabilities were higher than observed by Salinas Chavira et al (2012) in high-concentrate diets. These differences are due to different percentages and ingredients of the diets in each study. Ingredients such as urea and molasses are rapidly degraded in the rumen (Salinas-Chavira, 2013c). Besides, Salinas-Chavira et al (2011c) observed that a diet with rice polishing had higher digestible fraction during the first hours of incubation in rumen (0 and 4 h) than diets with lower NDF concentration.

Kozloski et al (2006) reported that the inclusion of about 30% NDF was adequate for feedlot lambs on diets with sorghum silage. They considered total NDF, using soybean hulls in different levels to reach the proposed NDF levels; however, soybean hulls are highly digestible, are of small particle size and do not meet the effective fiber requirements of cattle to stimulate chewing and salivation to keep rumen pH at acceptable levels to maintain adequate feed intake.

Conclusions

In the present study is concluded that alfalfa hay (good quality forage) may have minimal advantage over sorghum stover (low quality forage) on ruminal degradation kinetics when they are included at similar NDF from forage (6.5%) in fattening diets for lambs.

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Received 14 May 2020; Accepted 15 May 2020; Published 1 July 2020

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