Chemical Composition of Canola Oil
The chemical composition of canola oil contains a high proportion of unsaturated fatty acids, oleic (51%), linoleic (25%) and linolenic (14%) (Bell 1988 {1091}; Kennelly 1983 {1253}) (table 12). Similar FA composition was reported by Kempen and Jansman (1994 {881}). The FA compositions of a number of different fat sources are found in table 12.
Supplementation of 4.0-4.5
g fat/kg DM or 600-700 g/d/h included in the ration increases
milk yield and fat content (Kwiatkowski and
Luczak 1993 {912})
and is necessary to obtain maximum milk yield and to minimize
body weight loss in early lactation (Christensen 1998; Chilliard
1993). Further addition of oil decreases fat and protein content
due to a reduction in digestibility of organic matter (specifically
fiber) in the rumen, that reduces
the bacteria protein synthesis and results in a reduction in protein
and fat content of milk (Kwiatkowski and Luczak 1993
{912}; Christensen
1998). Canola oil can be added at 1% of the concentrate either
alone or in combination with tallow. Unsaturated FA reduces milk
fat and milk protein % (Christensen 1998). Work by Christensen
et al. (1994
{866})
reported that milk yields and milk components,
DMI, and digestible energy intake decreased linearly as the chain
length and degree of unsaturation of abomsal infused long chain
fatty acids increased. The saturated FA had a less detrimental
effect in comparison to the unsaturated.
The negative impact of lipid supplements on DM intake appears to become more marked as lactation advances (Gagliostro and Chilliard 1991 {976}). Early lactating (Chilliard et al. 1990 {1017}; Gagliostro and Chilliard 1991 {967}) and midlactating cows continuously infused with 1.0 to1.1 kg rapeseed oil/d had reduced oil-free DM intake (early-12.7 vs. 15.2 kg/d; Chilliard et al. 1990 {1017}, midlactation-16.5 vs. 13.9 kg oil free DM intake/d, P < 0.05; Gagliostro and Chilliard 1991 {967}) compared to control cows. Chilliard et al. (1990 {1017}) reported similar absorbed metabolizable energy between the control and treatment groups, 39.4 and 41.8 Mcal/d, respectively. Infusion of 640 g rapeseed oil tended to decrease (1 kg/d) oil-free DM intake (Ottou et al. 1995 {812}).
Effect on Milk Yield and Composition
Supplementation of rapeseed oil or canola oil has been reported to increase milk production, result in no change (Gagliostro and Chilliard 1991 {967}; Ottou et al. 1995 {812}; Chilliard et al. 1990 {1017}), or slight to great decrease (Christensen et al. 1994 {866}) in milk production. Short-chain fatty acid with chain lengths less than C16 and about 50% of the C16 are synthesized in the mammary gland (Khorasani et al. 1991 {988}). Precursors for these fatty acids are volatile fatty acids (primarily, acetate and butyrate) arising from microbial fermentation in the rumen. Long-chain fatty acids are incorporated directly into milk fat from dietary sources or adipose tissue reserves (Khorasani et al. 1991 {988}). With the supplementation of fat additional long-chain fatty acids by the mammary gland may inhibit intramammary synthesis of short- and medium chain fatty acids (Storry et al. 1973 as cited by Khorasani et al. 1991 {988}). Inclusion of 200g/d of rapeseed oil in lactating cows on pasture resulted in a more persistent lactation curve with the largest difference occurring in the third month of the experiment (Kwiatkowski and Luczak 1993 {912}). Milk yield for the supplemented group was 1.6 liters/d more in comparison to the non-supplemented group at the end of the trial (Kwiatkowski and Luczak 1993 {912}). Supplementation of 400g/d of rapeseed oil to confined lactating cows tended to decrease fat content of milk (4.05% vs. 3.67%) (Kwiatkowski and Luczak 1993 {912}). The effect of supplemental dietary fat on milk fat percentage will depend on the balance between decreased intramammary synthesis of short and medium chain fatty acids and on the extent of incorporation of additional dietary long-chain fatty acids into milk fat (Khorasani et al. 1991 {988}). Added fats in midlactation regardless of quantities and types resulted in a higher positive response of milk fat percentage when milk fat content from control cows was low (Doureau and Chilliard 1992 as cited by Ottou et al. 1995 {812}). Therefore, a lack of response (Cadden et al. 1984 {1222}) in trials can be due to the high milk fat content of control cows (Ottou et al. 1995 {812}). However a decrease in yield must be interrupted as being due to a decrease in DM intake or a reduction in rumen motility.
A consistent finding with fat supplementation
of dairy cow diets is a decrease in milk protein content (Ottou
et al. 1995 {812}; Gagliostro and Chilliard 1991 {967}), however
others have reported no difference (Chilliard et al. 1990 {1017}).
Duodenal rapeseed oil infusions of 640-1100 g/d decreased milk
protein content (Ottou et al. 1995 {812}; Gagliostro and Chilliard
1991 {967}) from 3.11 to 2.93%. The reason for this effect is
not well known, but is speculated that it may be due to a dilution
effect as milk production increases (Gagliostro and Chilliard
1991 {976}) or as a shortage of precursors for milk protein synthesis
due to a reduction in DM intake (Ottou et al. 1995 {812}). This
is further supported by the fact that when DM intake was not affected,
milk protein content generally was unchanged by feeding protected
unsaturated lipids. Also, late lactation cows having a fixed
oil-free DM intake did not decrease milk protein content. Correlatively,
milk protein content was reduced when DM intake fell. Early lactating
cows infused into the duodenum with 1.03 kg rapeseed oil/d had
decreased milk protein (-1.0 g/l) and increased milk fat (1.3
g/l), but not significantly (Chilliard et al. 1990 {1017}). Milk
lactose content in some studies was unaffected (Gagliostro and
Chilliard 1991 {976}) and in other studies was elevated (Ottou
et al. 1995 {812}).
Secretion of milk fat relies on de novo synthesis
and uptake of preformed FA by the mammary gland. The lack of
a production response for milk fat may result from an increased
uptake of rapeseed oil fatty acids (mainly C18:1 to 18:3) by the
mammary gland which is associated with a inhibitory effect of
these FA on de novo synthesis of short- and medium chain FA (C4:
to C16:0) (Ottou et al. 1995 {812}). Duodenal infusion of 640
g/d of rapeseed oil to midlactating cows increased mammary secretion
of C18, but decreased the secretion of medium-chain FA (Ottou
et al. 1995 {812}). Infusion of 330 g /d of canola oil in the
abomasum or rumen increased the proportion of C18:1 fatty acids
and decreased the proportion of C16:0 fatty acids in the total
milk fat at the sn-2 position (DePeters et al. 1992 {933}). Supplementation
of 400 g/d of rapeseed oil to lactating cows resulted in a decrease
in C12:0, C14:0 and C16:0 fatty acids and a significant increase
in C18:1 oleic acid (Kwiatkowski and Luczak 1993 {912}).
The major changes of dietary lipids in the rumen are hydrolysis and the biohydrogenation of released FA by the microbial population, leading to a 70-90% reduction of the polyunsaturated FA and their transformation to saturated (mainly stearic acid) or trans isomers of monosaturated FA (Chilliard 1993; Bauchart et al. 1990 {1024}). Lipid and fatty acid concentrations are reported to be 1.7-2.2 times higher in the solid-adherent bacteria in comparison to the liquid-associated bacteria and may result from preferential incorporation of dietary fatty acid adsorbed onto food particles (Bauchart et al. 1990 {1024}). Supplementation of dietary lipid reduces fiber digestibility, methane production, and acetate to propionate ratio (Chilliard 1993). These effects can be due to reductions in protozoa and in bacterial growth and metabolism, especially for cellulolytic strains and with polyunsaturated FA (Chilliard 1993).
When dietary fats, particularly those
high in unsaturated long-chain fatty acids, are fed to ruminants,
there is defaunating effect. Friesian bulls fed 0.5 kg/d crude
rapeseed oil (00) had lower rumen protozoal mass (P = 0.01), xylanase
activity was reduced (P = 0.01) and the microbial carboxylmethycelluslase
activity was lower (P = 0.07) in the rumen of animals receiving
rapeseed oil (Tesfa 1992 {943}). As a result of defaunation, the
rumen bacterial population becomes dominated by non-cellulolytic
gram-negative organisms and protozoa.
Lipid supplementation of diets for ruminants often
implies a decrease in OM digestibility, mainly due to a decrease
in fiber digestibility in the rumen (Ben Salem et al. 1993 {894}).
This impaired digestion is related to the amount of lipids added
and to their composition: polyunsaturated fatty acids have a more
negative effect on ruminal digestion than saturated fatty acids
(Ben Salem et al. 1993 {894}). However the nature of the diet
may effect the negative impact on fiber digestion. The negative
impact of lipids on digestion was less important when fiber intake
was high; therefore the decrease in fiber digestion was higher
with corn silage-based diets than with hay-based diets with the
same amount of rapeseed oil and the same level of calcium (Ben
Salem et al. 1993 {894}). Supplementing with oil (7% DM) resulted
in a decrease (P < 0.05) in OM digestibility for the corn silage
based diet only (Ben Salem et al. 1993 {894}). There was a significant
reduction in NDF digestibility for both hay and corn based diets,
4.6 and 10.0%, respectively (Ben Salem et al. 1993 {894}). This
decrease was due to both lignocellulose and hemicelluloses being
impaired and was significant for NDF and hemicelluloses, but not
for ADF (Ben Salem et al. 1993 {894}). High rates of duodenal
rapeseed oil infusion (1000 to 1460 g/d) in midlactating cows
decreased apparent digestibility of DM (DM) and OM (OM) due to
a lower apparent digestibility of added lipids (Chilliard et al.
1991 {975}). Under the same conditions with a moderate amount
of rapeseed oil infusion (640 g/d) digestibilities of DM, OM and
ether extract were not different from the control (Ottou et al.
1995 {812}). Early lactating cows infused (1.03 kg rapeseed oil/d)
into the duodenum had total OM digestibility that was not changed
with added oil (Chilliard et al. 1990 {1017}).