Associative effects between forages and grains: consequences for feed utilisation

Abstract

Intake of metabolisable energy (ME) when forages and grains are fed together to ruminants may, due to digestive and metabolic interactions, be lower or higher than expected from feeding these components separately. These interactions, or associative effects, are due primarily to changes in the intake and/or the digestibility of the fibrous components of forage. Effects on voluntary forage intake (substitution effects) are usually much larger than on the digestibility of fibrous components, although the changes in forage intake may be a consequence of changes in the rate of digestion of the fibrous components. Positive associative effects, where grains increase voluntary intake and/or digestion of forage, are usually due to the provision of a limiting nutrient (eg. nitrogen, phosphorus) in the grain which is deficient in the forage. Negative associative effects, where grains decrease voluntary intake and/or digestion of forage, occur frequently and can cause low efficiency of utilisation of grain. Rate of substitution of grain for forage is related to forage intake, forage digestibility, the proportion of grain in the diet, and the maturity of the animal. Substitution rates are usually high in ruminants consuming high intakes of forage of high digestibility, probably due to the metabolic mechanisms which control voluntary intake reducing forage intake. Substitution rates are often low when animals are consuming forage of low to medium digestibility. Since voluntary intake of such forages is most likely determined by the capacity of the rumen to accommodate and pass to the lower gastrointestinal tract undigested forage residues, and of the rate of forage fibre digestion in the rumen, substitution is likely to be determined by changes in these processes. Reduced rate of fibre digestion in the rumen is often due to low rumen pH and/or an insufficiency of essential substrates for rumen microorganisms. Use of grains for lactating dairy cows involves an additional constraint since dietary grain may severely reduce milk fat content. Negative associative effects can be alleviated by ensuring supply of essential microbial substrates, feeding management, and modification of grain to minimise their adverse effects on fibre digestion, while ensuring satisfactory digestion of the grain and efficient microbial protein production.

The effects of concentrate added to pineapple (Ananas Comosus linn. Mer.) waste silage in differing ratios to form complete diets, on digestion, excretion of urinary purine derivatives and blood metabolites in growing, male, Thai swamp buffaloes

“…The increasing level of intake indicated that nitrogen sources and other essential ingredients might be required to sustain optimum rumen fermentation. This has been earlier reported by several studies in sheep and cattle (Kaur et al 2008;Kozloski et al 2006;Dixon and Stockdale 1999). However, the lack of any effect of concentrate proportion on the feed intake which was recorded here (P40:C60 vs P60:C40) is similar to those described by Phipps et al (1987) and Sloan et al (1988).…”

Efficiency of utilisation of different diets with contrasting forages and concentrate when fed to sheep in a discontinuous feeding pattern

“…This has been previously reported by numerous workers in both sheep and cattle (Mould et al, 1983;Kolver and Muller, 1998;Dixon and Stockdale., 1999). In the present study, the higher intake by sheep fed the C25, C35 and C45 diets may have been due to higher proportion of concentrate in the diets in comparison to C15.…”

“…However, when fed twicea-day at milking, the response to feeding high amounts of concentrates is low (Fulkerson et al, 2000;Walker et al, 2001) likely due to the adverse influence on rumen environment. In this regard, feeding high levels of concentrates, over short periods during milking, causes decrease in rumen pH leading to a decrease in fibre digestion (Dixon and Stockdale, 1999). However, the effect of a drop in rumen pH is likely to be dependent on the type of fibre being fed; thus, more rapidly digested fibre is less likely to be affected than slowly degraded fibre.…”

Milk Yield and Composition and Pasture Ruminal Digestion in Grazing Dairy Cows Receiving Three Levels of Energy Concentrate Supplementation

“…It was postulated that substitution rate often increases with increasing concentrate intake [48] but this general relationship is inconsistent for high yielding grazing cows [11] in agreement with results obtained in the present study (0.59 and 0.58 kg DM pasture kg DM concentrate −1 for T7.0 and T10.5, respectively). A higher substitution rate could be explained by negative associative effects at the ruminal level [53] coupled to reductions in ruminal pH, activity or number of cellulolytic bacteria leading to a low rate of fiber digestion and pasture intake reduction [53]. In the present study, the increase in concentrate intake affected neither the degradability nor the rate of digestion of pasture NDF [54]), who suggested that the lack of response in conversion efficiency would be explained by the substitution rate.…”