R. Baumont

Associative effects between forages on feed intake and digestion in ruminants

The feeding value of forage mixtures from permanent and temporary multi-species grasslands cannot always be precisely defined. Indeed, the digestibility and feed intake of a combination of forages can differ from the balanced median values calculated from forages considered separately. In order to present an overview of the associative effects between forages on digestion and intake, a literature study was carried out. The associative effects can be studied in a complementary way in vitro to test digestive interactions of a large number of mixtures and to carry out explanatory experiments, and in vivo to investigate intake and digestion at the whole animal scale. We identified three main situations in which interactions between forages can lead to associative effects on intake and digestion: (i) increased intake that can be observed with grass and legume association can be explained by fast digestion of the soluble fraction of legumes, and a higher rate of particle breakdown and passage through the rumen, (ii) increased digestion when a poor forage is supplemented by a high nitrogen content plant can be explained by stimulation of the microbial activity and (iii) modification of digestive processes in the rumen, including proteolysis and methane production when certain bioactive secondary metabolites such as tannins, saponins or polyphenol oxidase are present. According to the type and concentration of these compounds in the diet, the effects can be favourable or unfavourable on intake and digestive parameters. Reported associative effects between forages show a large variability among studies. This reflects the complexity and multiplicity of nutritional situations affecting intake and the rumen function in a given animal. In order to provide more reliable information, further accumulation of data combining in vitro and in vivo studies is required. A better understanding of the associative effects between forages could help to optimise feed use efficiency, resulting in greater productivity, a reduction of the environmental impact of animal emissions and more sustainable animal production.

The ability of sheep at different stocking rates to maintain the quality and quantity of their diet during the grazing season

The degree to which grazers maintain the amount (organic matter intake) or the quality (organic matter digestibility) of their diet without adversely affecting the other component was addressed by investigating how sheep managed trade-offs between quantity and quality throughout the grazing season in an upland area of central France. Two groups of five dry ewes, grazing two plots of contrasting areas from April to the end of September 2000. were studied. On the smaller plot (1500 m 2 ), the application of a high stocking rate (HSR) produced a resource of good quality but in low quantity; and on the larger plot (3000 m 2 ),a low stocking rate (LSR) created a sward of low quality but in good quantity. In spring, in both conditions, the sheep maintained their intake of digestible organic matter (OM) at between 1000 and 1250 g/day. Both organic matter intake and digestibility remained high at both stocking rates. In summer, the intake of digestible OM decreased to between 750 and 1000 g/day. On HSR, this was mainly due to a decline of intake in relation to the decrease of intake rate and bite weight. On LSR a decrease in digestibility and to a lesser extent in intake was involved. At both stocking rates, the sheep maximized the digestibility of their diet by selecting the green laminae throughout the grazing season. In spring, the sheep modulated their daily grazing time to compensate the decrease in intake rate and maintained a high daily intake. In summer and autumn, the sheep failed to maintain their daily intake at the same level as previously observed. On both plots they modulated their daily grazing time to cover their needs (730 g/day), but they did not increase it further to maximize daily intake. This may be attributable to the costs involved in selecting (LSR) or taking (HSR) the best components from the sward.

Effect of feeding on Fos protein expression in sheep hypothalamus with special reference to the supraoptic and paraventricular nuclei: an immunohistochemical study

The hypothalamus plays an important role in the control of food intake in different species, but there is little relevant information for ruminants like sheep. In order to study the putative role of several hypothalamic nuclei in food intake in sheep, Fos expression, a marker of cellular activity, was compared by immunohistochemistry between fed and unfed ewes. The expression of Fos protein was stimulated in the supraoptic nucleus of fed ewes, whereas it was increased in the paraventricular nucleus of unfed animals. In the latter nucleus, Fos immunoreactivity was mainly localized close to the third ventricle, an area corresponding to the parvocellular system of the nucleus, but never in the magnocellular system. In the paraventricular nucleus, the number of corticotrophin releasing factor‐immunoreactive neurons and the number of Fos/corticotrophin releasing factor double‐labelled neurons were not affected by feeding or lack of feeding. The number of Fos‐immunoreactive neurons was higher in the lateral septum, the infundibular, the ventromedial and in the dorsomedial nuclei of unfed ewes than in those of fed ewes. Our results show for the first time that the dorsomedial and ventromedial nuclei are involved in the control of feeding in sheep as in rodents. The supraoptic nucleus of sheep is activated by the same conditions as in rodents but, conversely, the paraventricular nucleus is activated in unfed sheep, whereas in rodents and primates, this nucleus is activated by satiety as well as by fasting. In sheep, unlike in rodents, corticotrophin releasing factor did not appear to be involved in short‐term regulation of food intake.

Occurrence of associative effects between grasses and legumes in binary mixtures on in vitro rumen fermentation characteristics

When animals are fed a grass and legume mixture, digestive interactions can occur in the rumen between the substrates contained in the different plants, and the response of the animal to the combination of forages can differ from the balanced median values of their components considered individually. Our objective was to assess the associative effects between temperate forages in 8 grass-legume binary combinations on in vitro rumen fermentation characteristics to highlight synergies or antagonisms in terms of nutritional and environmental impacts. Two grasses (cocksfoot, CF; ryegrass, RG) and 4 legume species (alfalfa, A; white clover, WC; red clover, RC; sainfoin, SAN) were incubated alone and in grass-legume mixture (1:1, wt/wt) in batches containing buffered rumen fluid during 3.5 and 24 h. For each substrate and each incubation time, parameters describing the degradation of the energetic and nitrogenous compounds and their partition into fermentation end products were measured. Data were subjected to ANOVA using a mixed procedure to test quadratic contrasts. At 3.5 h of incubation, many quadratic effects were observed. The presence of A, WC, or RC in mixtures quadratically increased the NH(3)-N production (up to +28% when compared with the calculated value, P < 0.001), whereas the presence of condensed tannin (CT) activity in SAN considerably decreased it (up to -67%, quadratic effect, P < 0.001) and the N disappearance. To a lesser extent, the presence of SAN reduced the proportion of methane in the gas produced (up to 7%, quadratic effect, P = 0.018). Generally, the degradation of OM and NDF was not stimulated by mixing grass and legume, except for SAN. However, the presence of SAN severely impaired the NDF digestion at the early phase of fermentation. At 24 h of incubation, few associative effects were observed in comparison with those observed at 3.5 h of incubation, but the effect of CT of SAN on N metabolism was still clearly present. This study shows that, among the grass-legume mixtures tested in vitro, only the presence of SAN can interact with the grasses to reduce the degradation of proteins and the production of methane but with a transitory negative effect on fiber digestion.

Several subpopulations of neuropeptide Y– containing neurons exist in the infundibular nucleus of sheep: An immunohistochemical study of animals on different diets

Conversely to rodents, the involvement of hypothalamic neuropeptide Y (NPY) neurons in the control of nutrition is poorly understood in ruminants such as sheep. Therefore, the aim of this work was to describe the NPY neurons of the diencephalon in ewes submitted to different diets. In colchicine‐treated animals, large populations of NPY‐immunoreactive (‐ir) neurons were observed in a ventral and a lateral subpopulation of the infundibular nucleus (IN), in the median eminence, the pituitary stalk, and the dorsomedian and dorsocaudal nuclei. No labeled perikaryon was observed in the magnocellular neurons of the hypothalamus, although numerous labeled fibers were noted in the neural part of the pituitary. The pattern of distribution of NPY‐ir neurons in the sheep hypothalamus is similar in many ways to those of rodents, but it presents also many specific characteristics that have not been previously described. In ewes that were fasted for 24 hours, or fed ad libitum, the number of NPY‐ir neurons was the same whatever the hypothalamic structures. In underfed ewes (40% of maintenance for 24 weeks), the lateral subpopulation of the IN presented a higher number of NPY‐ir neurons than observed in the 100% fed ewes. Conversely, in the ventral subpopulation, the animals refed ad libitum (at least 150% of maintenance for 4 days) presented a lower number of NPY‐ir neurons than the other groups. The other NPY neuronal populations of the hypothalamus were not significantly modified by the dietary treatments. For the first time, we demonstrated the presence of two functionally distinct subpopulations of NPY neurons in the sheep IN. The variations of labeled neurons were correlated with plasma nonesterified fatty acid levels but not with leptinemia. J. Comp. Neurol. 444:129–143, 2002.

Models based on variable fractional digestion rates to describe ruminal in situ digestion

Using a first-order model to describe ruminal in situ digestion implies that the rate of digestion is affected only by the quantity of potentially digestible substrate remaining. Other factors, like the microbial efficacy for digesting substrate, are assumed to be constant. However, microbes are not only the cause but also the result of digestion, being one of the digestion end-products. Two sigmoidal models (a logistic and a Gompertz-like model) were derived from a general digestion function in which the rate of digestion equals the product of the quantity of potentially digestible substrate remaining and a non-constant fractional rate of digestion (microbial efficacy function). The models were compared with a first-order model with a discrete lag time. The logistic model specifically accounted for the conversion of substrate mass to microbial mass, but did not describe microbial migration between the substrate and the ruminal fluid. In contrast, the Gompertz-like model assumed that the change in microbial efficacy was only time-dependent. There was little difference between models in estimates of scale parameters, but the asymptotic microbial efficacy was consistently higher for the logistic model than for the other models. Estimates of discrete lag time in the first-order model were biased towards obtaining values identical to the independent variable. Scale estimators appeared to be more robust than kinetic estimators. Lack-of-fit was present for most model-data set combinations. The similar patterns of residuals between models suggested that a four-parameter model may be insufficient to describe the data. It was concluded that if a four-parameter model is to be used, the model with a discrete lag time would be the least biologically appropriate.

Perception and hedonic value of basic tastes in domestic ruminants.

Taste is one of the five senses that give ruminants and other animals an awareness of their environment, especially for food selection. The sense of taste, which recognizes sweet, bitter, salty, sour and umami basic tastes, is often considered of paramount importance as it is the last sense in use before foods are swallowed. It thus plays a fundamental biological role in aiding animals to regulate intake of suitable food and reject unsuitable food. However, despite potentially relevant production and welfare issues, only a few studies have investigated how ruminants perceive and evaluate the basic tastes. Here we review current knowledge on tasting abilities and hedonic value of basic tastes in domestic ruminants via the analysis of both their anatomical and neurological structures and their behavioral preferences. Studies of the organization and functioning of the anatomical and neurological structures responsible for the perception of taste in ruminants have shown that sheep, cattle and goats all have lingual receptors for all five basic tastes. However, these studies have mainly focused on the sweet and bitter tastes. They have shown in particular that cows have fewer genes coding for the bitter receptors than other mammals, making them more tolerant to this taste. This pattern has been linked to the differences in the range of toxins and so potentially in the occurrence of bitterness encountered by different species in their environment, depending on the nature of their diet. Studies of ruminant feeding behavior have shown that the taste inducing the greatest consensus in preferences is the umami taste, with a high positive hedonic value. The bitter taste seems to have a rather negative hedonic value, the salty taste either a positive or a negative one depending on body needs, while the sweet taste seems to have a positive value in cattle and goats but not in sheep. Finally, the hedonic value of the sour taste is uncertain. Besides the hedonic value, the animal may assign a signal value to the tastes. In ruminants, the unanimous preference for the umami taste, the need dependent preference for the salty taste, and the reluctance of sheep to associate a positive reward with the bitter taste suggest that these tastes would signal the presence of proteins, minerals and toxins, respectively.

Sheep use preingestive cues as indicators of postingestive consequences to improve food learning.

Previous work has shown that herbivores successfully learn to associate food sensory characteristics with postingestive consequences when the conditioning procedure is simple, whereas this ability breaks down when the learning task is made more complex. We hypothesized that sensory characteristics could act as indicators of postingestive consequences and that the presence of preingestive cues would improve the food learning of sheep in situations varying in complexity. Sixteen sheep were subjected to a first conditioning phase to associate 2 flavors added to alfalfa hay with either a positive or a negative consequence, induced by intraruminal administration of starch (330 mg/g of DMI) or LiCl (5 mg/g of DMI). Sheep progressively decreased their choice of the flavored hay associated with the negative consequence (P < 0.05). This procedure provided sheep with experience with postingestive consequences associated with the different flavors. In a second conditioning phase, the experienced sheep and 16 naïve sheep were divided into groups of 8 and subjected to either a simple or a complex conditioning procedure [i.e., the 2 flavors were offered on separate days (simple conditioning) or simultaneously within a day (complex conditioning)]. The 2 flavors applied to grass hay were associated with either positive (starch, 330 mg/g of DMI) or negative (LiCl, 10 mg/g of DMI) consequences. As hypothesized, sheep in the simple conditioning group expressed a greater aversion to the flavored hay associated with the negative consequence than did those in the complex conditioning group (0.303 +/- 0.035 vs. 0.474 +/- 0.035 respectively; P < 0.01). Experienced sheep rejected the flavor associated with the negative consequence more strongly than did naïve sheep, regardless of the conditioning procedure (0.304 +/- 0.029 vs. 0.470 +/- 0.041 respectively; P < 0.05). The initial increased preference for aniseed (0.80 +/- 0.04), however, greatly influenced food learning because sheep negatively conditioned on this flavor expressed less avoidance than those negatively conditioned on orange (0.53 +/- 0.04 vs. 0.25 +/- 0.03 respectively; P < 0.01). In conclusion, the simultaneous scenario was actually perceived as complex by all sheep, and experienced sheep were more efficient in food learning than naïve sheep in both the simple and complex learning contexts. The sheep were thus able to generalize the association between sensory cues and postingestive consequences, especially in a complex environment, and then to use these sensory cues as indicators of postingestive consequences.

NIRS prediction of the feed value of temperate forages: efficacy of four calibration strategies.

Near infrared reflectance spectroscopy (NIRS) of 924 fresh temperate forages were used to develop calibration models for chemical composition - crude ash (CA) and crude protein (CP) - organic matter digestibility (OMD) and voluntary intake (VI). We used 110 samples to assess the models. Four calibration strategies for determining forage quality were compared: (i) species-specific calibration, (ii) family-specific calibration, (iii) a global procedure and (iv) a local approach. Forage calibration data sets displayed CA values ranging from 52 to 205 g/kg of dry matter (DM), CP values from 50 to 280 g/kg DM, OMD values from 0.48 to 0.85 g/g and VI values from 22.5 to 115.2 g DM/kg metabolic body weight (BW0.75). The calibration models performed well for all the variables except for VI. For CA, local procedure showed lower standard error of prediction (SEP) than species-specific, family-specific or global models. For CP, the calibration models all showed similar SEP values (11.13, 11.08, 11.38 and 11.34 g/kg DM for species-specific, family-specific, global and local approaches). For OMD, the local procedure gave a similar SEP (0.024 g/g) to specific species and global procedures (0.027 g/g) and a lower SEP than the family-specific approach (0.028 g/g). For VI, the local approach and species-specific calibration showed lower SEP (7.08 and 7.16 g/kg BW0.75) than the broad-based calibrations (8.09 and 8.34 g/kg BW0.75 for family-specific model and global procedure, respectively). Local calibration may thus offer a practical way to develop robust universal equations for animal response determinations.

Body fat content and feeding level interact strongly in the short- and medium-term regulation of plasma leptin during underfeeding and re-feeding in adult sheep

Circulating leptin is regulated by food intake in the long, medium and short term; however, little is known about putative remnant effects of these successive regulations at any given time. To clarify this, two experiments were conducted in adult sheep, during which body condition parameters and plasma leptin were measured. During experiment 1, twenty ewes with normal body condition were either well fed (101 % of maintenance energy requirements (MER)) or underfed (41 % MER) for 166 d, then rapidly re-fed (at a mean of 208 % MER) for 3 d. Leptinaemia decreased after 14 d of underfeeding, remained depressed until day 166 and did not increase after 3 d re-feeding, whereas it was increased (+153 %; P < 0.05) by re-feeding the previously well-fed ewes. During experiment 2, twenty-four fat or lean ewes were either well fed (114 % MER) or underfed (52 % MER) for 94 d, and gradually re-fed for 2 d and maintained at a high feeding level (235 % MER) for 9 d. Underfeeding decreased leptinaemia in fat (from 4.19 to 2.63 ng/ml) but not lean ewes, and re-feeding increased leptinaemia after 5 d in lean previously well-fed (+123 %; P < 0.05) but not underfed ewes. In fat ewes, the impact of re-feeding was rapid (+144 %; P < 0.001 at 5 d) in previously well-fed ewes, whereas it was more gradual with a maximum at 11 d (+162 %; P < 0.01) in previously underfed ewes. In conclusion, leptinaemia is modulated by short-term energy intake level in interaction with long-term regulations involving nutritional history and body fatness, suggesting that a biological threshold of adiposity (about 20 %) is necessary to allow short- and medium-term leptin regulation.