Dean Revell

Dietary exposure of pregnant ewes to salt dictates how their offspring respond to salt.

We investigated changes in salt preference, food and water intake, renin activity and salt excretion in adult offspring from ewes that were fed a high-salt diet (14% NaCl, high-salt offsrping) or grazed saltbush (saltbush offspring) from day 60 of pregnancy until day 21 of lactation. High-salt offspring were compared to offspring born to ewes consuming a control diet (2% NaCl) and saltbush offspring were compared to offspring from ewes which grazed a control diet of dry pasture. All offspring were weaned at 3 months of age and grazed the same clover-based pasture until testing started at 8 months of age. The preference for a low-salt diet (0.5% NaCl) when offered with an alternative (7% NaCl) did not differ between the offspring groups. High-salt offspring and saltbush offspring had a lower food intake (14% and 27% respectively) and lower water intake (35% and 20% respectively) than their control offspring. Both high-salt offspring and saltbush offspring had lower basal renin activity than their respective controls. After consuming salt, the renin activity of the saltbush offspring continued to be lower than controls whereas the renin activity of the high-salt offspring became similar to controls. In general, the saltbush offspring excreted an oral salt load more rapidly, though this depended on the extent of the salt load. This important adaptation of offspring born to ewes that consumed saltbush during pregnancy may improve their ability to cope with high-salt diets such as saltbush when they consume it themselves. However, the high-salt offspring did not possess such beneficial adaptations.

Programming sheep production on saltbush : adaptations of offspring from ewes that consumed high amounts of salt during pregnancy and early lactation

We investigated if feeding a high salt diet (pellet containing 14% NaCl) or saltbush (Atriplex nummularia) to ewes between day 60 of gestation and day 21 of lactation would allow their offspring to gain more weight, and produce more wool, when grazing saltbush as adults compared to offspring from ewes that were fed a control diet (2% NaCl) or grazed pasture. At 10 months of age, offspring grazed saltbush for 8 weeks then dry pasture for 2 weeks. Throughout this time, liveweights, plasma renin activity and wool growth (g/day) were measured. Greasy and clean fleece weights, and fleece characteristics were measured at 14 months of age, and greasy fleece weight was measured again at 22 months after grazing pasture. Offspring from ewes that consumed the high salt pellet had an 8 and 10% increased fleece weight at 14 and 22 months of age, respectively (P ≤ 0.01). Offspring of ewes that consumed saltbush also showed an 8% increase in greasy fleece weight at 22 months of age (P ≤ 0.05). Offspring from ewes that consumed saltbush had lower plasma renin activity and gained tissue weight when grazing saltbush (P ≤ 0.05), whereas the other three treatment groups all lost weight (P > 0.05). Grazing pregnant ewes on saltbush induces important adaptations in plasma renin activity of their offspring, which allows them to gain weight when grazing saltbush as adults and may also increase the density of their wool follicles. Grazing pregnant ewes on saltbush can profit farmers in three main ways: (i) ability to utilise salt-affected land; (ii) increase weight gain of sheep when grazing saltbush; and (iii) increase fleece weight.

Feeding pregnant ewes a high-salt diet or saltbush suppresses their offspring's postnatal renin activity

If ewes consumed a high-salt diet or saltbush during the last 3 months of pregnancy and for 3 weeks after birth, we expected the renin activity of their lamb to be suppressed at birth and at 3 weeks of age. We also expected an increase in the concentration of cations other than sodium in the ewes milk and an increase in the plasma Na concentration of the lamb at birth. To test these hypotheses, Merino ewes were fed a high-salt diet (14% NaCl) in an animal house and compared to control ewes eating a control diet (2% NaCl). In addition, we compared ewes grazing saltbush (about 13% salt in diet) to ewes grazing pasture from day 60 of pregnancy to 3 weeks after birth. Lambs born to ewes consuming saltbush had 85% lower (P < 0.001) renin activity than offspring from ewes consuming pasture at 3 weeks of age. Similarly, lambs born to ewes consuming a high-salt diet had 20% lower renin activity at birth and 3 weeks (P = 0.07). Feeding ewes a high-salt diet or saltbush altered the mineral composition of the milk; the largest change was a 10% increase in K levels (P 0.05). Consuming a high-salt diet or saltbush lowered the plasma Na of ewes at 130 days of gestation (by 3-5 mmol/l; P < 0.001), but only lambs from ewes fed the high-salt diet had a lower plasma Na at birth (P < 0.05). Suppression of the renin activity of lambs could lead to permanent physiological changes in salt balance in later life.

Australian perennial shrub species add value to the feed base of grazing livestock in low- to medium-rainfall zones

Australian native perennial shrubs that are adapted to drought and infertile soils contribute nutrients to grazing systems that would otherwise support limited ruminant productivity. In this study, we report the nutritive value of 39 Australian shrub species of the genera Atriplex, Rhagodia, Maireana, Chenopodium, Enchylaena, Acacia, Eremophila, and Kennedia. Edible foliage was sampled in winter and summer, and there was little difference in nutritive value between seasons. The in vitro organic matter digestibility of most shrub species was 40–70%. Most species contained medium to high levels of crude protein (12–22% of dry matter, DM) and high concentrations of sulfur (2–8 g/kg DM). In an 8-week grazing experiment in which Merino wethers grazed a ‘shrub system’ containing four shrub species and a sown inter-row of annual pasture, the sheep gained weight during autumn without supplementary feeding. By comparison, sheep fed senesced volunteer pasture and supplementary cereal grain only maintained weight. The forage shrubs provided up to 50% of the total DM intake of sheep grazing the ‘shrub system’ and made a modest contribution to the digestible energy intake of the animals and a large contribution to their crude protein and mineral intake. Considering the timely and predictable provision of limiting nutrients and benefits such as gut health and the provision of shade and shelter, we suggest that Australian shrub species can make a valuable addition to the feed base of low- to medium-rainfall zones in southern Australia.

Feeding behaviour in ruminants: a consequence of interactions between a reward system and the regulation of metabolic homeostasis

Feeding behaviour, through both diet selection and food intake, is the predominant way that an animal attempts to fulfil its metabolic requirements and achieve homeostasis. In domestic herbivores across the wide range of production practices, voluntary feed intake is arguably the most important factor in animal production, and a better understanding of systems involved in intake regulation can have important practical implications in terms of performance, health and welfare. In this review, we provide a conceptual framework that highlights the critical involvement and interconnections of two major regulatory systems of feeding behaviour: the reward and the homeostatic systems. A review of the literature on ruminants and rodents provides evidence that feeding behaviour is not only shaped by homeostatic needs but also by hedonic and motivational incentives associated with foods through experiences and expectations of rewards. The different brain structures and neuronal/hormonal pathways involved in these two regulatory systems is evidence of their different influences on feeding behaviours that help explain deviation from behaviour based solely on satisfying nutritional needs, and offers opportunities to influence feeding motivation to meet applied goals in livestock production. This review further highlights the key contribution of experience in the short (behavioural learning) and long term (metabolic learning), including the critical role of fetal environment in shaping feeding behaviour both directly by food cue–consequence pairings and indirectly via modifications of metabolic functioning, with cascading effects on energy balance and body reserves and, consequently, on feeding motivation.

Feed intake of sheep when allowed ad libitum access to feed in methane respiration chambers

The patterns of feed intake when animals are allowed ad libitum access to feed in a respiration chamber is not known, nor are the potential effects of the artificial environment of chambers on voluntary feed intake. The objectives of the study were to describe the pattern of hourly feed intake of sheep when fed for ad libitum intake in respiration chambers and determine the repeatability of this pattern and the correlation between feed intake and methane production calculated at hourly intervals. Daily and hourly measurements of methane production and feed intake of 47 Merino wethers were measured in respiration chambers twice, 4 wk apart. We found that hourly feed intake of sheep with ad libitum access to feed in respiration chambers showed a repeatable pattern over the 2 measurement periods (r = 0.89, P < 0.001). During both measurements, sheep ate continuously throughout the 23 h period, but most of the eating occurred during the first 8 h in the respiration chambers. There was a significant linear correlation (r = 0.22) between hourly feed intake and hourly methane production (P < 0.001). An unexpected result from this study was that despite using an accepted and published acclimatization procedure to habituate the animals to the respiration chambers, sheep had 15 to 25% lower feed intake in the respiration chambers compared with their feed intake during the previous week in the animal house pens. In addition, daily feed intake in the respiration chamber was not correlated with feed intake in any of the 7 d before entering the chamber (P > 0.05). Future methane research may consider using feed intake and changes in intake levels as a quantitative indicator of habituation to the methane measurement procedure and environment, which, given the tight association between feed intake and methane production, will be crucial in providing accurate values for methane production.

Reproductive capacity of Merino ewes fed a high-salt diet.

An option to increase the productivity of saline land is to graze sheep on salt-tolerant plants, which, during the summer/autumn period, can contain 20% to 25% of their dry matter as salt. This study assessed the impact of coping with high dietary salt loads on the reproductive performance of grazing ewes. From the time of artificial insemination until parturition, 2-year-old maiden Merino ewes were fed either a high-salt diet (NaCl 13% of dry matter) or control diet (NaCl 0.5% of dry matter). Pregnancy rates, lamb birth weights, milk composition and the plasma concentrations of hormones related to salt and water balance, and energy metabolism were measured. Leptin and insulin concentrations were lower (1.4 ± 0.09 v. 1.5 ± 0.12 ng/ml; (P < 0.05) and 7.2 ± 0.55 v. 8.2 ± 0.83 ng/ml; P < 0.02) in response to high-salt ingestion as was aldosterone concentration (27 ± 2.7 v. 49 ± 5.4 pg/ml; P < 0.05), presumably to achieve salt and water homeostasis. Arginine vasopressin concentration was not significantly affected by the diets, but plasma concentration of T3 differed during gestation (P < 0.02), resulting in lower concentrations in the high-salt group in the first third of gestation (1.2 ± 0.18 v. 1.3 ± 0.14 pmol/ml) and higher concentrations in the final third of gestation (0.8 ± 0.16 v. 0.6 ± 0.06 pmol/ml). T4 concentration was lower in ewes ingesting high salt for the first two-thirds of pregnancy (162 ± 8.6 v. 212 ± 13 ng/ml; P < 0.001). No substantial effects of high salt ingestion on pregnancy rates, lamb birth weights or milk composition were detected.

Consumption of a high-salt diet by ewes during pregnancy alters nephrogenesis in 5-month-old offspring

Maternal nutrition during pregnancy can affect kidney development in the foetus, which may lead to adverse consequences in the mature kidney. It was expected that high-salt intake by pregnant ewes would lead to a reduction in foetal glomerular number but that the ovine kidney would adapt to maintain homoeostasis, in part by increasing the size of each glomerulus. Merino ewes that were fed either a control (1.5% NaCl) or high-salt (10.5% NaCl) diet during pregnancy, as well as their 5-month-old offspring, were subjected to a dietary salt challenge, and glomerular number and size and sodium excretion were measured. The high-salt offspring had 20% fewer glomeruli compared with the control offspring (P < 0.001), but they also had larger glomerular radii compared with the control offspring (P < 0.001). Consequently, the cross-sectional area of glomeruli was 18% larger in the high-salt offspring than in the control offspring (P < 0.05). There was no difference in the daily urinary sodium excretion between the two offspring groups (P > 0.05), although the high-salt offspring produced urine with a higher concentration of sodium. Our results demonstrated that maternal high-salt intake during pregnancy affected foetal nephrogenesis, altering glomerular number at birth. However, the ability to concentrate and excrete salt was not compromised, which indicates that the kidney was able to adapt to the reduction in the number of glomeruli.

Animal grazing selectivity and plant chemistry issues impact on the potential of Rhagodia preissii as an anthelmintic shrub

Rhagodia preissii had shown significant in vitro anthelmintic activity in a previous study, we examined the effect of including this shrub in the diet of sheep infected with Trichostrongylus colubriformis. Worm-infected merino wethers were grazed for 7 weeks on either R. preissii or annual pasture, and faecal egg counts (FECs) were conducted weekly. Plant material was collected weekly from eaten and uneaten plants, and analysed for levels of plant secondary metabolites (tannins, oxalates, saponins) and in vitro anthelmintic activity. While mean FECs were consistently lower in sheep grazing R. preissii compared to pasture (reductions of 20-74%), the differences were not significant. There was no relationship between grazing preference (eaten or uneaten) and in vitro anthelmintic activity of plant extracts. The levels of saponins and oxalates did not correlate with grazing preference or in vitro anthelmintic activity, while tannins were not responsible for the anthelmintic activity. While the identity of the grazing deterrent and in vitro anthelmintic compounds remain unknown, the presence of plants which were both highly preferred by the sheep and showed in vitro anthelmintic activity indicates a potential to develop the species as an anthelmintic shrub through selection of shrub populations dominated by such plants.

Integrated and innovative livestock production in drylands

By around 2050, it is estimated that the demand for meat and milk will be 60–70 % greater than it is today. Production from ruminants in dryland areas is expected to play an important role in the responses to this challenge. However, livestock production in dryland areas is risky because of a number of uncertainties, the majority of which can be attributed to variation in key climatic factors that can limit productivity and also fuel societal opinions about the ethics of animal production. In this chapter, we have described and analysed the risks and problems for livestock production in difficult environments, and have proposed management options. Based on the work done with Australian native shrubs and pasture species, we have shown how silvopastoral systems can offer innovative and versatile options for livestock production in dryland regions if animal behaviour is managed to influence diet and habitat selection. Versatile livestock systems that include compatible livestock genotypes, forage species and management strategies, can maximise productivity and improve ecosystem health while providing food for humanity.