Effects of salinity and drought on the productivity of ruminants and forage yield under tropical conditions

Nguyen Thiet¹, Nguyen Trong Ngu¹ and Sumpun Thammacharoen²

¹ Can Tho university, Vietnam
nthiet@ctu.edu.vn
² Chulalongkorn university, Thailand

Abstract

The Mekong Delta of Vietnam is increasingly affected by climate change–induced drought and saltwater intrusion, which reduce freshwater availability, degrade water quality, and threaten livestock and forage production. This review synthesizes research on the effects of high temperature, saline drinking water, and limited irrigation on the productivity of ruminants and commonly cultivated forage species. Goats and sheep demonstrate greater tolerance to heat stress and salinity than cattle and buffalo, owing to smaller body size, lower metabolic rates, and adaptive physiological responses. Goats can tolerate saline drinking water with NaCl concentrations up to 1.5% for short periods, though prolonged exposure can reduce feed intake and productivity. Among forages, Mombasa guinea grass and Mulato II grass maintain high yields under short-term drought, while Brachiaria mutica (para grass) sustains growth and biomass production even at salinity levels up to 20‰. These findings highlight the potential for integrating salt- and drought-tolerant livestock–forage combinations to improve the resilience and sustainability of ruminant production systems in the Mekong Delta.

Keywords: climate change, goats, forage species, Mekong Delta, sheep

Introduction

Recent studies have classified Vietnam among the countries facing water scarcity, with the total average availability of surface and groundwater resources across the territory estimated at only 4,400 m³/person/year, compared to the global average of 7,400 m³/person/year. Some studies indicate that if sea level rises by 1 meter, approximately 39% of the Mekong Delta (MD) could be at risk of saline inundation. In some Mekong Delta provinces in 2016, measured salinity levels ranged from 6‰ to 15‰ in certain localities. Saltwater intrusion reduces the availability of freshwater resources, making them insufficient for human consumption and livestock production, thereby posing serious challenges to the livestock sector. Climate change-induced salinity intrusion not only reduces freshwater quantity but also tends to degrade water quality due to high salinity and acidity. The consumption of such substandard and polluted water by livestock and poultry can lead to increased disease incidence and negatively affect animal health. Therefore, adaptive livestock production strategies are essential to cope with climate change in the Mekong Delta (Dau Van Hai, 2016). For the sustainable development of the region’s livestock sector, necessary adaptation measures include: restructuring livestock breeds, changing production systems, and selecting or breeding forage varieties tolerant to drought and salinity. Goats, with their small body size and lower feed requirements, demand less housing space and pasture area compared with cattle and buffalo. Moreover, they have a greater tolerance to heat stress than cattle or buffalo (Silanikove, 2000), making them a suitable livestock species under current climate change conditions. In the Mekong Delta, the provinces with the largest goat populations are coastal provinces such as Ben Tre, Tien Giang, and Tra Vinh (Department of Livestock Production, 2018). These provinces suffer severe saltwater intrusion during the dry season each year, which significantly impacts existing livestock herds. To date, in both Vietnam in general and the Mekong Delta in particular, there has been no systematic, foundational research assessing the salt tolerance of goats or evaluating the performance of major forage crops under the region’s hot climate conditions. The following sections summarize research findings from both domestic and international studies on the impacts of salinity and drought on the productivity of ruminants and commonly cultivated forage crops in Vietnam’s livestock systems.

Effects of heat stress on ruminants

Heat stress refers to environmental conditions that challenge an animal’s ability to dissipate excess body heat (Silanikove, 2000). It is typically characterized by increased respiratory rate and rectal temperature, leading to reduced metabolic activity (Bandaranayaka & Holmes, 1976). Body temperature, being highly sensitive to hot weather, is a reliable indicator of heat stress (Araki et al, 1984; Kadzere et al, 2002). The Temperature–Humidity Index (THI) is widely used to assess heat load, with values below 82 indicating no stress in goats and sheep, 82–84 mild stress, 84–86 severe stress, and above 86 extreme stress (LPHSI, 1990). For cattle, THI values of 70 or below indicates no stress, above 70 signal the onset of stress, and above 80 denote severe stress (West, 1994). Studies have shown that goats and sheep can withstand higher heat stress levels than cattle due to their smaller body size and lower metabolic rates (Silanikove, 2000), making them more suitable for livestock production in Vietnam’s increasingly hot and arid conditions.

Effects of saline drinking water on feed intake and productivity of ruminants

According to NSW (2014), a salinity level below 0.8 g/L total dissolved salts (TDS) is considered relatively low. Salinity between 0.8 and 2.35 g/L TDS is generally acceptable but may cause temporary, mild diarrhea in unadapted animals, without long-term effects on health or productivity. Water with 2.35–3.9 g/L TDS can be safely used for dairy and beef cattle, sheep; however, for pregnant or lactating animals, it is advisable to avoid the upper end of this range. A salinity range of 5.45–7.8 g/L TDS poses significant risks for pregnant or lactating animals, young stock, and animals under heat stress or dehydration. Salinity levels between 7.8 and 11.7 g/L TDS are only tolerable for mature sheep and goats; water with ~15 g/L TDS is toxic, with the severity of effects depending on the specific salts present. Peirce (1957) reported that 15 g/L TDS is the upper safety limit for sheep, while a total salt concentration of 2.5% is highly toxic. Gihad (1993) found that sheep consuming water with 1% NaCl experienced no adverse effects, but 1.5% caused issues in some animals, and 2% was detrimental to all. Goats can tolerate up to 1.5% NaCl in drinking water (Nassar & Moussa, 1981), whereas Boer goats refuse saline water at 1.25–1.5% NaCl and are more sensitive to salt ingestion after prolonged exposure (Runa et al, 2016). Nguyen Thiet et al (2022) observed that Boer crossbred meat goats gradually adapted to drinking water containing 15 g/L TDS, consuming more water than goats not acclimated to saline conditions. In a subsequent study, Nguyen Thiet et al (2023) demonstrated that both meat and dairy goats tolerated saline water concentrations up to 1.5% without adverse effects. However, at 2% salinity, goats refused to drink, and meat goats exhibited relatively higher water intake than dairy goats (Figure 1). Consistently, Nguyen Thiet et al (2024) found that Phan Rang sheep tolerated saline water up to 1.5% without reductions in productivity, whereas higher salinity levels resulted in decreased body weight change. For dairy cattle, NRC (2001) recommends that drinking water contain less than 5 g/L TDS, while water at 7 g/L TDS should be avoided. In summary, sheep and goats exhibit greater tolerance to saline water than beef and dairy cattle, pigs, and poultry, as reported in previous studies. Therefore, goats and sheep may be more suitable for coastal areas during the dry season when freshwater is scarce.

Figure 1. Salt tolerance in meat goats (A) and dairy goats (B): The percentage of fluid intake from two buckets of preference test from phase 1 (FW/FW) and phase 2 (FW/DSW). * Significant difference in water intake between FW and DSW from each preference test day p<0.05). FW = fresh water, DSW = diluted seawater. Source (Nguyen Thiet et al 2023

Effects of drought and saline water on growth and yield of forage species under tropical conditions

Nguyen Thiet (2020a, b) reported that under adequate irrigation, Mombasa guinea grass exhibited superior growth and yield compared to Paspalum atratum . However, when subjected to a 5-day water deficit, the yields of the two species were comparable. This suggested the need for further trials with extended drought periods to determine the drought tolerance threshold and to assess the effects of drought duration on forage yield. In a subsequent study, Nguyen Thiet (2020c) evaluated the impact of prolonged drought on Mombasa guinea grass and Paspalum atratum. Results showed that withholding water for 10 to 15 days significantly reduced growth and yield, while increasing fiber and nitrate concentrations. These findings indicate that both Mombasa guinea grass and Paspalum atratum can withstand up to 5 consecutive days without irrigation without yield loss (Figure 2). Additionally, Hoang Van Tao (2015) found that Mulato II and Mombasa guinea grass were more drought-tolerant than Napier grass, TD8 guinea grass, and Paspalum atratum. Based on these studies in Vietnam, it can be concluded that Mombasa guinea grass and Mulato II have superior growth and yield performance compared to several other commonly grown forage species under limited water conditions.

Saltwater intrusion deep into inland areas has reduced the land available for rice cultivation, prompting many farmers to switch to forage production for livestock. In the Mekong Delta, using salt-tolerant forage species is among the most practical and cost-effective strategies for maintaining agricultural productivity, especially when compared to more expensive measures such as soil reclamation or the construction of saltwater barriers (Le Hong Giang and Nguyen Bao Toan, 2014).

Cattle raising has a long history and remains well-developed in the Mekong Delta, particularly in provinces such as Ben Tre, Tien Giang, and Tra Vinh. Forage for cattle mainly comes from common cultivated grasses such as TD58 guinea grass (Panicum maximum), Mombasa guinea grass, Napier grass (Pennisetum purpureum), and ruzi grass (Brachiaria ruziziensis), along with crop by-products. However, under hot weather conditions and with irrigation water affected by salinity, both forage cultivation area and yields decline significantly.

Figure 2. Effect of drought duration on dry matter yield percentage change of Mombasa grass (unpublished data)

Studies worldwide have documented the salt tolerance of certain forage and cereal grasses, such as para grass (Brachiaria mutica), seashore paspalum (Paspalum vaginatum), and foxtail millet (Setaria italica), which can grow well in saline soils and brackish water (Kafi et al 2009; Roy and Chakraborty, 2014; Mukhtar et al 2016). Introducing salt-tolerant forage species in coastal Mekong Delta areas could not only ensure feed supply for ruminants but also make productive use of high-salinity lands. In Vietnam, several studies have examined the effects of saline irrigation on the yield of common forage grasses under greenhouse conditions. Nguyen Thi Hong Nhan (2018) found that Setaria sphacelata demonstrated greater salt tolerance, whereas Paspalum atratum produced higher yields at salinity levels ≤ 6‰. Key chemical composition indicators, including dry matter, crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), and ash, were not significantly affected in her salinity tolerance trials. Similarly, Vo Hoang Viet (2019) investigated three forage species—para grass (Brachiaria mutica), setaria grass (Setaria sphacelata), and paspalum (Paspalum atratum)—under saline irrigation levels of 0, 5, 10, 15, and 20‰. Results indicated that Paspalum grass was the least salt-tolerant, with complete mortality at 15‰ salinity. Setaria grass exhibited salt injury symptoms such as leaf burn and stem wilting at 15 and 20‰. In contrast, para grass maintained growth performance, biomass production (shoot and root), and chlorophyll and proline content even at salinity levels up to 20‰.

Conclusions

Under conditions of high temperatures and saline water, goats and sheep are considered more suitable livestock species due to their greater tolerance compared with cattle and buffalo. Among forages, Mombasa guinea grass (Panicum maximum cv. Mombasa) and Mulato II (Brachiaria hybrid cv. Mulato II) exhibit good growth and productivity under water-deficit conditions, while para grass (Brachiaria mutica) maintains its yield even when salinity levels reach 20‰ under experimental conditions.

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