LITERATURE REVIEW

Fat addition in the diet of dairy ruminants and its effects on productive parameters^¤

Adición de grasa en la dieta de rumiantes lecheros y sus efectos sobre parámetros productivos

Efeito da adição de gordura à dieta de ruminantes leiteiros sobre os resultados produtivos

Andrés L Martínez Marín^*, MVZ, PhD; Manuel Pérez Hernández, MVZ, PhD; Luis M Pérez Alba, MVZ, PhD; Domingo Carrión Pardo, MVZ, PhD; Ana I Garzón Sigler, Biol, PhD; Gustavo Gómez Castro, MVZ, PhD.

* Corresponding author: Andrés L. Martínez Marín. Departamento de Producción Animal, Campus Universitario de Rabanales, Carretera Madrid-Cádiz, km. 396, 14071 Córdoba, España. e-mail: pa1martm@uco.es

Departamento de Producción Animal, Facultad de Veterinaria, Universidad de Córdoba, España.

(Received: September 19, 2011; accepted: October 3, 2012)

Summary

This review analyzes the current knowledge on the effects of fat supplementation in dairy ruminant diets. Research conducted on dairy cows and goats shows that dry matter intake decreases when diets contain extra fat due to a negative effect on digestibility. In dairy cows dry matter intake can be also decreased by gut peptides released in response to extra fat. This effect has not been observed in ewes and goats. Milk yield increases in dairy cows, ewes, and goats in a curvilinear manner with increasing amounts of dietary fat. When fat supply is low, the increase in milk yield is probably caused by a higher energy intake. However, milk yield decreases when fat supply is high, which may be related to diminished energy availability for milk production due to negative effects on rumen digestion and/or dry matter intake. Unprotected lipids negatively impact milk fat content in dairy cows, but not in dairy ewes and goats; negative effects of supplemental lipids on milk protein content observed in dairy cows and ewes has not been observed in dairy goats.

Key words: digestibility, lipids, milk, voluntary intake.

Resumen

Esta revisión examina la información actual sobre el efecto de añadir fuentes de grasa a la dieta de hembras rumiantes lecheras. Estudios con vacas y cabras han demostrado que el consumo de materia seca se reduce cuando la grasa extra incluida en la dieta tiene un efecto negativo sobre la digestibilidad. En vacas, el consumo también puede reducirse por efecto de la grasa extra sobre la secreción de mediadores hormonales en el intestino. Este efecto no ha sido demostrado en ovejas y cabras. En vacas, ovejas y cabras, la respuesta de la producción de leche a la inclusión de cantidades crecientes de lípidos suplementarios en la dieta es curvilínea: cuando el aporte de grasa es bajo, el incremento de la producción puede explicarse por un mayor consumo de energía. Por el contrario, la disminución de la producción cuando el consumo de grasa es elevado puede relacionarse con la reducción de la energía disponible debido al efecto negativo sobre la digestión ruminal y/o el consumo de materia seca. La adición a la dieta de lípidos no protegidos no tiene efectos negativos sobre el contenido de grasa láctea en ovejas y cabras, pero sí en vacas. El efecto negativo de los lípidos suplementarios sobre el contenido de proteína láctea en vacas y ovejas no ha sido descrito en cabras.

Palabras clave: consumo voluntario, digestibilidad, leche, lípidos.

Resumo

Nesta revisão se examina as informações atuais sobre o efeito da adição de fontes de gordura nas dietas de ruminantes (fêmeas leiteiras). Estudos com vacas e cabras têm demonstrado que o consumo de matéria seca é reduzido quando o excesso de gordura na dieta promove efeito negativo sobre a digestibilidade. Em vacas, o consumo também pode ser reduzido pelo efeito da gordura suplementar sobre a secreção de mediadores hormonais no intestino. Este efeito não foi demonstrado em ovelhas e cabras. Em vacas, ovelhas e cabras a resposta da produção de leite pela inclusão de quantidades crescentes de gordura suplementar na dieta é curvilínea: quando a suplementação de gordura é baixa, o aumento da produção pode ser explicado pelo aumento do consumo de energia. Por outro lado, a diminuição da produção de leite quando a ingestão de gordura é alta pode ser relacionada com a redução da energia disponível, devido ao efeito negativo sobre a digestão ruminal e/ou o consumo de matéria seca. A adição de lipídios não protegidos na dieta não tem efeitos negativos sobre o conteúdo de gordura do leite em ovinos e caprinos, tendo efeito apenas no leite da vaca. O efeito negativo de lipídios suplementares sobre o teor de proteínas lácteas em vacas e ovelhas não tem sido descrito em caprinos.

Palavras chave: consumo voluntário, digestibilidade, leite, lipídios.

Introduction

Interest has recently increased on improving fatty acids profile of milk fat by including appropriate fat sources in dairy ruminant diets. The aim is to reduce the content of medium chain saturated fatty acids (FA), which have a hypercholesterolemic effect, and increase the fatty acids considered beneficial, such as polyunsaturated FA of the n-3 series and rumenic acid (cis- 9,trans-11 18:2) in milk fat (Lock and Bauman, 2004). Oilseeds and their oils, whether protected or unprotected from rumen digestion, seem to be suitable fat sources (Ashes et al., 1997; Chilliard et al., 2001).

On the other hand, fat sources can have negative effects on dry matter intake (DMI) (Chilliard et al., 1993), fibrous carbohydrate digestibility (Sauvant and Bas, 2001), or milk composition (Doreau and Chilliard, 1992; Wu and Huber, 1994). These effects should be taken into account when attempting to improve milk FA profile.

The aim of the present paper was to review the effects of the addition of fat sources to the diet of dairy cows, ewes, and goats on DMI, milk production, and milk composition.

Effect of dietary lipids on dry matter intake

Data in tables 1 and 2 show that inclusion of fat in the diet negatively affects cows' DMI. The magnitude of this effect depends on the fat source; the effect tends to be greater when the dietary forage is corn silage. On the other hand, most research on ewes and goats has not shown that fat source negatively affects DMI (Tables 3 and 4). The reduction of DMI after dietary fat inclusion may be a result of 1) an increase in ruminating time due to negative effects on rumen digestion; 2) a slowdown of rumen emptying due to a metabolic effect of long chain FA (Chilliard et al., 1993). In both situations a satiety effect due to rumen replenishment could occur.

The relationship between digestion and DMI is difficult to establish because there is a scarcity of publications in this area. Murphy et al. (1987) did not observe differences in DMI although rumen dry matter digestibility was lower in diets with 2.4% and 4.7% extra fat compared with a control diet (46.3%, 44.4%, and 52.1%, respectively).

On the contrary, Pantoja et al. (1994) observed a linear reduction of organic matter intake (19.4 kg/d to 16.6 kg/d) combined with a linear decrease of neutral detergent fiber (NDF) rumen digestibility (from 51.4% to 43.8%), presumably due to the degree of unsaturation of the added fat. Regarding research work on cows' total tract digestibility, Martin et al. (2008) observed that a decrease of 10.1% in acid detergent fiber (ADF) digestibility was parallel to a drop of 3.1 kg/d in DMI. However, organic matter total tract digestibility dropped by 18% without affecting DMI, according to Beauchemin et al. (2009). The relationship between decreased total tract NDF digestibility and reduced DMI is also not evident in goats. Silva et al. (2007) observed that including soybean oil in the diet decreased NDF digestibility by 10% and reduced DMI by 0.3 kg/d compared with results from a control diet, although including ground soybeans decreased DMI by the same amount without affecting NDF digestibility.

The metabolic effect of fat source on DMI has been studied in cows through abomasal infusion experiments using oils and free FA. Free FA addition with increasing unsaturation showed a greater impact on DMI. Litherland et al. (2005) observed a linear reduction of DMI with infusions of unsaturated FA triacylglycerols and the corresponding free FA into the abomasum; the reduction was greater with the latter. This effect increased with the amount infused: 200 g/d, 400 g/d, and 600 g/d of free FA or triacylglycerols infused lowered the DMI 3.8 kg/d, 5.5 kg/d, and 10.6 kg/d or 1.1 kg/d, 2.6 kg/d and, 4.4 kg/d, respectively, compared with the intake observed for the control treatments (19.8 kg/d and 22.4 kg/d, respectively).

The effect of the degree of unsaturation was investigated by Bremmer et al. (1998), who infused 450 g/d of five mixtures of differently unsaturated fat sources into the abomasum (18:1 + 18:2 + 18:3 = 70 g/d, 187 g/d, 200 g/d, 257 g/d, and 384 g/d) and observed that DMI dropped from 1.2 kg/d with the more highly saturated fat mixture to 3.7 kg/d with the less saturated one, compared with intake observed for the control treatment (22.8 kg/d). It has been suggested that the observed negative effect may be due to secretion of unidentified chemical mediators as a response to the flow of fat into the duodenum. Litherland et al. (2005) did not observe changes in plasma cholecystokinin concentration as a result of fat abomasal infusion, but DMI reduction was significantly related to the increase of plasmatic concentration of glucagon-like peptide-1. Harvatine and Allen (2005) pointed out that the reduction of DMI observed when including unsaturated FA-rich fat into the diet may be related to a satiety effect of decreasing digestive motility as a result of intestinal peptide secretion.

Experiments similar to those reported in cows have not been published with ewes or goats. In the paper by Pérez Alba et al. (1997), feeding extra fat (120 g/d of calcium salts of olive oil FA or 1.3 g extra fat/kg live weight) did not lower DMI. In studies by Teh et al. (1994) and Brown-Crowder et al. (2001) increasing amounts of extra fat included in the diet resulted in a trend towards reduced DMI with the highest level used in each experiment (9% calcium salts of palm oil FA and 6% tallow, respectively). The daily fat intake with those levels (3.6 g/kg and 2.4 g/kg live weight, respectively) was higher than the infused amount of triacylglycerols and unsaturated FA (0.3 g/kg live weight), which reduced DMI in the work of Litherland et al. (2005).

Effects of dietary lipids on milk production

Diets with added fat generally increase milk production compared with a control diet without added fat in cows, ewes, and goats (Tables 1, 3, and 4). In cows, the increase is greater when given encapsulated animal fats or calcium salts of palm oil FA and when the saturation degree is higher (Table 1). From the information presented in tables 3 and 4 it is not possible to establish which type of fat source elicits the highest milk production response in ewes and goats. As a rule, the relationship between milk production and amount of extra fat included into the diet is curvilinear in the three species: production rises with increasing amount of fat to a maximum and decreases from a certain level of inclusion (Chilliard et al., 1993; Brown-Crowder et al., 2001; Gargouri et al., 2006).

Increase in milk production when the dietary inclusion of extra fat is low could be explained by a higher energy concentration of the feed consumed. A straight substitution of 3% plant oil for the same amount of corn in a diet with 1.55 Mcal NEl/kg DM amounts to a 7% increase in energy concentration and a higher energy consumption if DMI remains unchanged. In fact, cow data (Table 1) show that the increase in energy consumption offset the observed reduction in DMI, with few exceptions.

The reduction of milk production when high dietary fat supplement is provided could be related to a combined negative effect on digestion and DMI. In studies by Martin et al. (2008) using cows, and Silva et al. (2007) with goats, milk yield reduction of animals fed diets with added fat (5.7% in the diets of the first study, and 4.5% in those of the second) compared with the animals fed the control diet without added fat (20.8 kg/d and 18.9 kg/d vs. 23.0 kg/d, and 1.8 kg/d and 1.5 kg/d vs. 2.2 kg/d, respectively) was parallel to a simultaneous decrease of fibrous carbohydrate digestibility and DMI. However, Maia et al. (2006a, 2006b), working with goats, observed a 24% reduction in NDF digestibility when the diet included 5.1% of either rice, rapeseed, or soybean oils, but DMI and milk production were no lower than those of the control diet without added fat. Also, Beauchemin et al. (2009) observed that including 4.2% or 3.7% extra fat of sunflower or flax seeds in diets for cows reduced organic matter digestibility by 19% and 9%, respectively, compared with a control diet which included 3.1% fat from calcium salts of palm oil FA, but the DMI and milk production were not different between the three diets. These reports support that increasing energy concentration of the diet by fat inclusion can offset the reduced digestibility if DMI is unaffected.

Effect of dietary lipids on milk fat

In cows, the effect of fat source on milk-fat content depends on rumen digestion, subsequently related to the degree of protection of fat sources used by rumen microorganisms, and their processing and unsaturation degree (Doreau and Chilliard, 1992). Regarding protected fats (Table 1), calcium salts of palm oil FA increase milk fat content less than encapsulated plant oils because the former also increase milk production causing a dilution effect. In the non-protected fat sources group (Table 1) plant oils and animal fats reduce milk fat content; conversely, as a rule the effect of the oilseeds is negative with exceptions depending on their processing and unsaturation degree. Extruded oilseeds reduce milk fat content more frequently than when oilseeds are untreated or treated in other ways. Gonthier et al. (2005) observed a 0.26% reduction in milk fat content when they included 12.7% extruded linseed in a control diet, but fat content increased 0.20% and 0.14% when the same linseed was raw or micronized, respectively. Regarding saturation level, Ortiz et al. (1998) observed that high-oleic acid sunflower seeds (80% oleic acid in true fat) did not affect milk fat content as opposed to regular sunflower seeds (65% linoleic acid in true fat), which saw a reduction of 0.31%. Casper et al. (1988) obtained similar results and pointed out that negative results could be due to the effect of polyunsaturated FA in regular sunflower seeds on rumen fiber digestion, which would reduce acetate production in the rumen.

The negative effect of non-protected fat sources on milk fat content of cows is modulated by their inclusion rate and the main source of forage. Dhiman et al. (2000) observed a quadratic increase in milk fat content (3.44%, 3.60%, 3.56%, 2.86%, and 2.93%) as a response to growing levels of soybean oil included in the diet (control without oil, 0.5%, 1%, 2%, and 4% oil), which suggests there is a limit in the capacity of rumen cellulolytic bacteria to hydrogenate the oil consumed and to lessen the toxic effect of unsaturated FA on their growth. On the other hand, literature points out that a milk fat reduction is more likely when non-protected fat sources are included in diets based on corn silage but the reduction decreases with other forages, specially when alfalfa hay totally or partially replaces corn silage (Table 2).

The addition of fat sources of different type and origin (animal, plant, processed or whole oilseeds, calcium salts) to the diet of ewes and goats generally increases milk fat content (Tables 3 and 4) as opposed to dairy cows. In those species, milk fat percentage increases linearly when growing levels of the fat source are included in the diet (Teh et al., 1994; Rotunno et al., 1998; Brown-Crowder et al., 2001; Casals et al., 2006). In ewes, the increase of milk fat content is greater at the beginning than at the end of lactation. From the work by Pérez-Alba et al. (1997) and Casals et al. (2006) it can be calculated that the increase during the suckling period (up to 35 days of lactation) is 6.2 ± 4.1 g of fat/kg of milk higher than in the milking period. On the contrary, the response in goats is bigger from 150 days in milk than in the first 3 months of lactation: 1.0 ± 0.2 g of fat/kg of milk (Mir et al., 1999; Rapetti et al., 2002; Bernard et al., 2005) versus 0.6 ± 0.1 g of fat/kg of milk (Teh et al., 1994; Brown-Crowder et al., 2001; Bouattour et al., 2008).

Regarding the relationship between extra fat and forage in the diet, Mele et al. (2006) did not observe interaction between extra fat and percentage of forage (75% or 60%) on milk fat content of ewes. However, Reynolds et al. (2006) observed that milk fat content tended to increase when 3% of a 2:1 combination of soybean-algae oils was included in a diet based on alfalfa haylage, but tended to decrease when the forage was corn silage (+13.8 g/ kg vs. -11.7 g/kg, respectively). Chilliard and Ferlay (2004) showed that the increase in goat milk fat is lower when linseed or high oleic sunflower oils are included in corn silage based diets compared to alfalfa hay (1.5 g/kg vs. 6.3 g/kg, respectively).

Different digestive and metabolic causes have been advanced to explain the negative effect of non-protected fat sources on cow's milk fat content. Glasser et al. (2007) suggested that milk secretion of 4 to 16-carbon FA could be limited by the mammary availability of 18-carbon FA for the initial acylation of glycerol in lipid-poor diets (<3%), while milk secretion of 18-carbon FA could in turn be limited by a lack of 4 to 16-carbon FA for complete glycerol esterification when high fat diets (3% to 6%) are fed. The reduction of short and medium chain FA is related to a lower de novo synthesis in the mammary gland, which may be due to two reasons: 1) decreased rumen production of volatile FA, which are their substrate in diets with added fat; 2) negative metabolic effect of long chain FA, or conjugated linoleic acid isomers on mammary enzymes (Chilliard and Ferlay, 2004). Sanz Sampelayo et al. (2007) pointed out that the higher rate of rumen transit in ewes and goats could turn down the negative effect of non-protected fat sources on rumen digestion of fibrous carbohydrates and the consequent decrease of precursors for the de novo synthesis in the udder. According to Chilliard et al. (2003) the high rumen transit rate could reduce the effect of dietary unsaturated FA on rumen production of conjugated linoleic acid isomers, which negatively affect mammary lipogenesis.

Effect of dietary lipids on milk protein

Tables 1, 2, and 3 show that dietary fat supplementation in cows and ewes decreases milk protein content compared to diets without added fat. Results of reports presented in table 4 show that this effect does not happen in goats consuming diets with added fat from different sources.

In cows, the negative effect is greater after the peak of lactation; its size is not related to a particular fat source, and increases with the degree of unsaturation (Wu and Huber, 1994; Schroeder et al., 2004). The decrease of protein content can be explained by a reduction of milk casein content when added-fat diets are given. DePeters et al. (1987) observed that the addition of 3.5% and 7% extra fat to the diet reduced protein and casein content in a similar magnitude in both treatments when compared to the control diet (1.2 g protein and 0.9 g casein/kg of milk) while the percentage of casein in total nitrogen was only significantly reduced with the 7% fat added diet (74.53% vs 75.47% in control diet). Doreau and Chilliard (1992) calculated that the addition of fat to dairy cow diets reduces milk protein, casein, and casein/ protein ratio by 1.6 g/kg, 1.5 g/kg, and 0.8%, respectively, while whey proteins and non-protein nitrogen are barely affected (±0.8 g/kg and ±0.2 g/ kg, respectively).

In ewes, Osuna et al. (1998) observed that the addition of 4.5% extra fat to the diet as calcium salts of palm oil FA or its combination with oilseeds had little effect on milk protein content but reduced the proportion of casein in total protein. Zhang et al. (2006) obtained milk with less total protein and casein (46.5 g/kg and 35.4 g/kg) when ewes were fed sunflower seeds compared to a control diet (47.6 g/kg and 36.7 g/kg) or a diet supplemented with linseed (47.1 g/kg and 36.1 g/kg). In this paper, the percentage of casein in milk for the sunflower seed diet was 1.3% lower than for the control diet. On the other hand, Gargouri et al. (2006) found a negative linear regression between milk protein content and consumption of calcium salts of palm oil FA: protein content (g/L) = -0.0210 x calcium salts intake (g/d).

In goats, works by Teh et al. (1994), Mir et al. (1999), Brown-Crowder et al. (2001), Bernard et al. (2005), Schmidely et al. (2005), Maia et al. (2006a), and Fernandes et al. (2008) show that the content and production of milk protein are not negatively affected by the type of fat, unsaturation degree or level of inclusion into the diet. Regarding milk protein components, Sanz Sampelayo et al. (2002) observed that feeding a concentrate with 12% of calcium salts rich in polyunsaturated FA increased the percentage of serum albumin in whey proteins by 76% and reduced α-casein in total caseins by 22% without affecting the contents of total whey protein and total casein. This is most likely because there were non significant changes in the rest of the fractions for both groups of milk proteins.

Several hypotheses have been advanced to explain the observed reduction in milk protein content when fat sources are included in the diet of cows and ewes (Doreau and Chilliard, 1992). After reviewing the available literature, Wu and Huber (1994) pointed out that the decrease in protein content could be partially due to a dilution effect of the increased amount of milk produced because of the extra fat, as well as a shortage of available amino acids for the synthesis of protein to maintain pace with the increased milk production. In this sense, replacing dietary rumen digestible carbohydrates by lipids can bring a reduction of rumen microbial protein production (Coppock and Wilks, 1991), or it can increase the utilization of amino acids for gluconeogenesis due to a lower production and availability of propionate (Lobley, 1992). Other authors have suggested that the cause may be a low secretion of somatostatin (Casper and Schingoethe, 1989) or insulin (Mackle et al., 2000), which would result in a low mammary amino acid uptake. The causes of the differences between cows and ewes and goats have not been documented, and could be due to unknown physiological and metabolic factors (Chilliard et al., 2003).

In conclusion, the addition of extra fat to diets for cows may have counterproductive effects on milk production and composition when nonprotected fat sources rich in unsaturated FA are used. Milk production and milk fat content are not negatively affected in goats and ewes by the consumption of diets with added fat, but milk protein content decreases in ewes. More research is clearly needed to identify the possible reasons for the different responses observed in cows, ewes, and goats.

Notes

^¤ To cite this article: Martínez AL, Pérez M, Pérez LM, Carrión D, Garzón AI, Gómez G. Effect of dietary fat on the productive results of dairy ruminants. Rev Colomb Cienc Pecu 2013; 26:69-78.

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