Alimentation à base de concentrés pour les bovins en croissance et en finition

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Quelle complémentation alimentaire en concentrés chez les bovins en croissance et en finition pour maximiser le revenu des éleveurs ?
Le niveau optimum d’une complémentation alimentaire en concentrés pour les bovins à viande pendant les périodes hivernales à l’intérieur dépend de la réponse animale (kg de gain / kg de concentré), du taux de substitution des fourrages et du prix relatif des produits animaux et des aliments pour les bovins. La valeur alimentaire (et économique) des ingrédients alimentaires dérivés des sous-produits dépend des pratiques alimentaires, notamment des apports de concentrés.
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In beef production systems, feed provision is the single largest direct cost incurred, accounting for approximately 75% of total costs of production; therefore, small improvements in feed efficiency can have a relatively large influence on farm profitability. Additionally, feed efficient cattle excrete fewer nutrients, and produce less gaseous emissions, to the environment (Kenny et al., 2018).
Due to the considerably lower comparative cost of grazed grass as a feedstuff, beef production systems should aim to increase animal output from grazed pasture. However, the seasonality of grass growth and inclement grazing conditions means that an indoor ‘winter’ period, of varying duration, is inevitable on all Irish farms and the main feed costs on beef farms relate to this period, and especially when feeding finishing cattle. For example, even in grass-based, suckler calf-to-steer beef systems on research farms, grazed grass, grass silage and concentrate account for 65%, 27% and 8%, respectively, of feed Dry Matter (DM) intake annually.

Yet when this feed consumption is expressed in terms of cost (land charge included), the outcome is very different: grazed grass, silage and concentrate account for 44%, 39% and 17% of the total annual feed costs, respectively (see Figure 1).
Clearly, in other production systems, such as weanling-to-finish and ‘winter-finishing’, the proportional cost associated with concentrate feeding will be even higher. Consequently, enhanced feed-cost efficiency during the more expensive indoor feeding periods has a comparatively greater financial impact than during the grazing season. Economic and environmental sustainability of beef production systems therefore depends on optimising the contribution of grazed grass to the lifetime intake of feed, and on providing silage and concentrate as efficiently and at as low a cost as feasible. 
Fig1 annual feed budget 
Figure 1. Annual feed budget for a grass-based suckler calf-to-beef steer system expressed on a DM intake and cost-basis.


The role of concentrates is to make up the deficit in nutrient supply from forages in order for cattle to reach performance targets (McGee, 2005). Indeed, in situations where there is a shortage in winter supplies of forage, it may be better to buy concentrates and feed less forage than to purchase expensive low-quality forage.
• Energy is the most important nutrient required by growing-finishing cattle. Comparisons of feedstuffs should always be based on their ‘net’ energy (and protein) values on a DM basis. It is important to ensure that an adequate level of an appropriate mineral/vitamin mix is included in the ration.
• Dry matter digestibility (DMD) is the primary measure influencing the nutritive value of forage and consequently, the performance of forage-cattle. Low DMD forage means that higher levels of concentrate supplementation have to be used to achieve the same growth rate or performance (Table 1).
• Increasing the level of concentrates in the diet reduces forage intake (“substitution”) and increases live and carcass weight gains, although at a diminishing rate.
• Animal production response to concentrate supplementation is higher with forages of lower DMD.
• Growth response to concentrate feeding is higher in high-growth potential animals. For example,  a recent Teagasc Grange study comparing suckler-bred Charolais with Holstein-Friesian steers offered a high-concentrate diet, found that the older, lighter, slower-growing Holstein-Friesian steers consumed 10% more feed DM resulting in a 20% inferior feed conversion efficiency. Similarly, bulls are inherently more feed efficient - 10 to 20% better - than comparable steers.
• Animal response to concentrate supplementation at pasture primarily depends on the availability and quality of pasture and level of supplementation.
• Increasing the level of concentrate supplementation reduces the importance of forage nutritional value, especially when feeding concentrates ad libitum (to appetite).
• The optimum level of concentrate supplementation primarily depends on animal production response (kg gain/kg concentrate), forage substitution rate and the relative prices of animal product and feedstuffs (McGee, 2015).
Weanling Cattle:
To minimise feed costs and exploit subsequent compensatory (“catch-up”) growth at pasture during the following grazing season, a live weight gain of 0.5-0.6 kg/day through the first winter is acceptable for steers, heifers (and suckler bulls) (McGee et al., 2014). Due to compensatory growth, there is little point in over-feeding weanlings during the first winter. However, cattle growing too slowly (<0.5kg/day) during winter will not reach target weights. This target animal performance level can be achieved on grass silage supplemented with concentrates as outlined in Table 1.
Finishing Cattle:
Efficiency of feed utilisation by finishing cattle primarily depends on weight of animal (decreases as live weight increases), potential for carcass growth (e.g. breed type, gender, compensatory growth potential) and duration (decreases as length increases) of finishing period (McGee, 2015). Even high-quality grass silage is incapable of sustaining adequate growth rates to exploit the growth potential of most cattle so concentrate supplementation is required.  Each 1 unit decline in DMD of grass silage requires an additional ~0.33 kg concentrate daily to sustain performance in finishing cattle. Concentrate supplementation rates for finishing steers to achieve ~1.0 kg live weight/day with grass silage varying in DMD are shown in Table 1. Correspondingly, for finishing heifers (lower growth potential) daily supplementation is reduced by about 1.5 to 2.0 kg and for finishing bulls (higher growth potential) rates should be increased by 1.5 to 2.0 kg to achieve 1 kg live weight.
Where silage DMD is poor (e.g. 60%) and/or in short supply, and animal growth potential is high, feeding concentrates ad libitum should be considered. However, when feeding concentrates ad libitum, particularly cereals, there is a risk of acidosis. Therefore, it is critical to ensure; (i) gradual adaptation to concentrates (over ~3 weeks), (ii) minimum roughage inclusion (~10% of total DM intake) for rumen function, (iii) meal supply never runs out and, (iv) a constant supply of fresh water is provided. 
Grass silage DMD (%)       ~60  ~65  ~70  ~75
Wealings  2.0-3.0       1.5-2.0  1.0-1.5  0-1.0
Finishing steers  -  7.0-8.0       5.5-6.5       4.0-5.0     
Table 1.  Concentrate supplementation (kg/day) necessary for weanlings to grow at ~0.5 kg and for finishing steers (600 kg) to grow at ~1.0 kg live weight/day, when offered grass silage of varying Dry Matter Digestibility (DMD) to appetite.
Although cereals (barley, maize, wheat and to a much less extent, oats) usually predominate, by-product feeds, also known as co-products, are widely available and used extensively in beef rations.  They are secondary products mainly from the food processing industry and the biofuel/ethanol industry. Key by-products include soya hulls, corn gluten feed, maize dried distillers grains, wheat dried distillers grains, palm kernel expeller meal and citrus pulp. By-products generally have little value as a foodstuff for humans, but many are suitable as a feed for cattle due to the ability of cattle to digest fibrous, plant cell-wall material. Indoor feed costs could be reduced through utilisation of alternative, ‘cheaper’ concentrate feed ingredients in beef rations.  However, a potential limitation of feeding by-products to cattle is that significant variation can exist in their chemical composition and nutrient content, and this is liable to change over time as the primary manufacturing processes evolve and become more efficient. This means that periodic re-evaluation of the nutritive value of by-products is required for accurate formulation of rations for beef cattle.
In this context, a series of recent DAFM-funded experiments carried out at Teagasc Grange, has evaluated a number of key cereal and by-product feed ingredients in beef cattle diets. The ‘control’ concentrate offered in all these studies was a barley/soyabean meal-based ration (ca. 862g rolled barley, 60g soya bean meal, 50g molasses, 28g minerals and vitamins/kg); all other rations were compared against this. The optimum inclusion level of a number of by-product feeds was evaluated by replacing rolled barley (and some, or all, of the soyabean meal depending on the protein content of the test feed ingredient) in the ration. All concentrates were prepared as coarse mixtures.
Key findings are as follows:
• Carcass weight was heavier and feed efficiency was better in bulls offered a high-concentrate diet where half of the rolled barley in the control ration was replaced with maize meal, but not flaked-toasted maize; maize inclusion in the ration did not enhance carcass fat deposition (Lenehan et al., 2015a).
• Rolled oats can replace rolled barley in a concentrate supplement (ca. 5.0 kg/day) to high-digestibility grass silage without negatively affecting performance of finishing beef cattle; feeding oats had no effect on carcass fat score (McGee et al., 2018).
• For growing ‘weanling’ cattle, soya hulls and citrus pulp can replace rolled barley in concentrate rations offered at relatively low levels (ca. 2 kg/day), as a supplement to high digestibility grass silage, without negatively affecting performance (Lenehan et al., 2015b; 2017b).
• For finishing cattle diets, citrus pulp can replace rolled barley in the ration at inclusion rates up to 400g/kg without negatively affecting performance when offered ca. 5.0 kg concentrate/day as a supplement to high-digestibility (ca. 75% DMD) grass silage (Kelly et al., 2017).
• For growing cattle offered ca. 3.5 kg/day of concentrate as a supplement to moderate digestibility grass (ca. 65% DMD) silage, and finishing cattle offered ad libitum concentrates, the optimum inclusion level of soya hulls in a barley-based concentrate was ca. 200g/kg (Magee et al., 2015c).
• Dried corn gluten feed had a feeding value comparable to that of rolled barley/soya bean meal when offered as a supplement (ca. 5.0 kg/day) to high-digestibility grass silage (Kelly et al., 2018).  
• Maize dried distillers grains had a superior feeding value (based on dietary feed conversion ratio) to wheat dried distillers grains when the ration was offered as a supplement (3.5 kg/day) to grass silage or ad libitum. The optimal inclusion level of maize and wheat dried distillers grains in the concentrate was about 800g/kg when the concentrate ration was offered as a supplement to moderate-digestibility grass silage and, about 400g/kg for maize, and 200g/kg for wheat, dried distillers when the ration was offered ad libitum (Magee et al., 2015a; 2015b).
• Palm kernel expeller meal can be included in a barley-based concentrate at up to 400 g/kg when offered as a supplement to moderate digestibility grass silage and up to 100 g/kg when offered ad libitum (Magee et al., 2016).
Overall it is concluded that, due to ‘associative effects’, the relative nutritive (and economic) value of by-product feed ingredients depends on concentrate feeding practices; i.e. inclusion level in the ration; whether the ration is offered as a supplement to grass silage or to appetite with restricted grass silage.
Weanling and finishing, steers and heifers, generally do not require protein supplementation when fed barley-based concentrates and high DMD grass silage (McGee, 2005), but for suckler bull weanlings, recent research at Grange showed a significant, but small, response to protein supplementation (Lenehan et al., 2015b). However, all cattle are likely to respond to supplementary protein in barley-based concentrates when grass silage has moderate to low DMD and/or low protein content, especially weanling cattle.
Studies at Teagasc, Grange have shown that at adequate (ca. 20 g DM/kg live weight) grass allowances in autumn, feeding ca. 0.50 to 0.75 kg of concentrate ration per 100 kg live weight resulted in carcass growth responses in steers between 30 and 110 g carcass per kg concentrate. The low growth response to supplementation was associated with grazing very high nutritive value grass herbage. Recent Grange research has shown that concentrate supplementation is a strategy for ‘pasture-finishing’ (achieving adequate carcass fat score) of cattle in autumn, especially late-maturing breed types (Lenehan et al., 2017a; Regan et al., 2018).

Références :
Kelly M.J., Moloney A.P., Kelly A., McGee M. (2017). Replacement of rolled barley with citrus pulp in a concentrate supplement for finishing beef cattle offered grass silage. Journal of Animal Science 95, Suppl. 4, 295.
Kelly M., Moloney A.P., Kelly A., McGee M. (2018). Intake, growth, carcass and selected meat quality traits of steers offered grass silage and supplementary concentrates with increasing levels of dried corn gluten feed. Advances in Animal Biosciences, 9, (1) 150.
Kenny D.A., Fitzsimons C., Waters S.M., McGee M. (2018). Invited Review: Improving feed efficiency of beef cattle – the current state of the art and future challenges. Animal, 12, 1815–1826.
Lenehan C., Moloney A.P., O’Riordan E.G., Kelly A., McGee M. (2015a). Effect of substituting barley with maize on the performance of suckler-bred bulls offered a high concentrate diet. Agricultural Research Forum, Tullamore, Ireland, p82.
Lenehan C., Moloney A.P., O’Riordan E.G., Kelly, A., McGee M. (2015b). Effects of supplementary concentrate type and protein level on growth of suckler-bred weanling bulls offered grass silage. Agricultural Research Forum, Tullamore, Ireland, p74.
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Magee D., Moloney A.P., Kelly A., O’Riordan, E.G., McGee M. (2015a). Replacement of barley with increasing levels of maize dried distillers grains: intake, growth and carcass characteristics of beef cattle. Agricultural Research Forum, Tullamore, Ireland, p80.
Magee D., Moloney A.P., Kelly A., O’Riordan E.G., McGee M. (2015b). Effect of replacing barley with increasing levels of wheat dried distillers grains on intake, growth and carcass traits of beef cattle. Agricultural Research Forum, Tullamore, Ireland, p75.
Magee D., Moloney A.P., Kelly A., O’Riordan E.G., McGee M. (2015c). Intake and performance of beef cattle offered barley-based concentrates with increasing inclusion levels of soya hulls. Agricultural Research Forum, Tullamore, Ireland, p81.
Magee D., Moloney A.P., Kelly A., O’Riordan E.G., McGee M. (2016). Inclusion of palm kernel expeller meal in barley-based concentrates for beef cattle. Book of Abstracts, 67th Annual Meeting of European Federation for Animal Science (EAAP), Belfast UK, p669.
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Regan M., Lenehan C., Moloney A.P., O’Riordan E.G., Kelly A.K., McGee M. (2018). Finishing late-maturing suckler steers and bulls from pasture: Effect of concentrate supplementation. Grassland Science in Europe, 23, 485-487.

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