Flaxseed

Other Names: Linseed Nutritional Characteristics:

Flax is grown essentially for its oil content, although in Europe there is still some production of special varieties for linen production. Fat-extracted flax, which is commonly called linseed meal, has traditionally been used for ruminant feeds. Over the last few years, there has been interest in feeding full-fat flaxseed to poultry, because of its contribution of linolenic acid. Flax oil contains about 50% linolenic acid (18:3w3) which is the highest concentration of omega-3 fatty acids within vegetable oils. It has recently been shown that 18:3w3, and its desaturation products docosahexaenoic acid and eicos-apentaenoic acid are important in human health, and especially for those individuals at risk from chronic heart disease. Government agencies in many countries now recognize the importance of linolenic acid in human health, suggesting the need to increase average daily intake, and especially intake in relation to that of linoleic acid.

Feeding flaxseeds to poultry results in direct incorporation of linolenic acid into poultry meat and also into eggs. Feeding laying hens 10% flax results in a 10-fold increase in egg yolk linolenic acid content and eating two such modified eggs each day provides adults with most of their daily recommended allowance of linolenic acid. For each 1% of flaxseed added to a layer diet, there will be a +40 mg increase in total omega-3 fatty acids per egg. Likewise, in broilers, each 1% flaxseed addition will increase total omega-3 fats in the carcass by +2% of total fat. Feeding layers 8% flaxseed will result in an egg with about 320 mg total omega-3 fatty acids. For broiler chickens, there is no need to feed flaxseed for the entire grow-out period. Feeding 10% flaxseed to broilers for only the last 14 d of grow-out, results in significant incorporation of omega-3 fatty acids in the meat. With cooked breast + skin there is an increase in omega-3 content from 150 675 mg/100 g cooked product.

Linolenic acid enriched eggs and poultry meat are therefore an attractive alternative to consumption of oily fish. Linolenic acid is essentially responsible for the characteristic smell of 'fish oils' and undoubtedly flax oil does have a 'paint-type' smell. There is some concern about the taste and smell of linolenic acid-enriched poultry meat and this topic needs more careful study with controlled taste panel work. There is often discussion about the need to grind flaxseed. The seeds are very small, and for birds with an 'immature' gizzard it seems likely that some seeds will pass directly through the bird. Flaxseeds are quite difficult to grind, and are usually mixed 50:50 with ground corn before passing through a hammer mill. Perhaps the greatest benefit to grinding is seen with mash diets. Table 2.5 shows digestible amino acid values, determined with adult roosters for whole and ground flaxseed.

These digestibility values were determined using the force-feeding method, and so the bird is fed only the flaxseed, which is a novel situation to the bird. Over time gizzard activity may increase and so digestibility of whole seeds may improve. Using a classical AMEn bioassay, we have shown a consistent increase in AMEn of flaxseed when diets are steam crumbled (Table 2.6).

Table 2.5 Amino acid digestibility of flaxseed (%)

Flaxseed Whole Ground

Table 2.5 Amino acid digestibility of flaxseed (%)

Flaxseed Whole Ground

Methionine

68

85

Cystine

68

87

Lysine

72

88

Threonine

65

82

Tryptophan

85

95

Arginine

71

92

Isoleucine

66

86

Valine

65

84

Leucine

67

87

Courtesy Novus Int.

Table 2.6 Effect of steam crumbling on AMEn of flaxseed (kcal/kg)

Bird Type

Crumble ^

Broiler chicken

3560

4580 +31%

Rooster

3650

4280 +17%

Laying hen

3330

4140 +24%

Adapted from Gonzalez (2000) and Bean (2002)

Adapted from Gonzalez (2000) and Bean (2002)

These assays were conducted at different times and with different samples of flaxseed. In another study there was an 18% improvement in AMEn for layers when flaxseed was extruded. Conventional pelleting seems sufficient to weaken the seed structure so as to allow greater digestibility of amino acids and energy.

With laying hens, there may be transitory problems with suddenly incorporating 8-10% flaxseed in the diet, usually manifested as reduced feed intake and/or wet sticky manure.

These problems can usually be overcome by gradual introduction of flaxseed, using for ex -ample, 4% for one week, followed by 6% for another week and then the final 8-10% inclusion. It usually takes 15-20 d in order for omega-3 content of eggs to plateau at the desired level of 300 mg/egg. With prolonged feeding there is often greater incidence of liver hemorrhage in layers, even though mortality is rarely affected. Such hemorrhaging occurs even in the presence of 100-250 IU vitamin E/kg diet, which is a regular addition to flax-based diets. Disruption to liver function may become problematic if other stressors occur.

Potential Problems:

Flaxseed should be introduced gradually when feeding young layers. Weekly increments using 4-6 and 8-10% over 3 weeks are ideal to prevent feed refusal. Ground flaxseed is prone to oxidative rancidity, and so should be used within 2-3 weeks of processing. There seem to be advantages to steam pelleting diets contain-ing flaxseed. Flaxseed contains a number of antinutrients including mucilage, trypsin inhibitor, cyanogenic glycosides and considerable quantities of phytic acid. The mucilage is mainly pectin, found in the seed coat and can be 5-7% by weight. The mucilage undoubtedly contributes to more viscous excreta, and there is some evidence that ft-glucanase enzymes may be of some benefit, especially with young birds. Flaxseed may contain up to 50% of the level of trypsin inhibitors found in soybeans, and this is possibly the basis for response to heat treatment and steam pelleting of flaxseed. The main glucosides yield hydrocyanic acid upon hydrolysis, and this has an adverse effect on many enzyme systems involved in energy metabolism.

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