In the Rehit Marayada (http://www.sgpc.net/rehat_maryada/section_six.html), Section Six, it states:

The undermentioned four transgressions (tabooed practices) must be avoided
1. Dishonouring the hair;
2. Eating the meat of an animal slaughtered the Muslim way(Kutha);
3. Cohabiting with a person other than one`s spouse
4. Using tobacco.

Sikh Rehit Maryada

Throughout Sikh history, there have been movements or subsects of Sikhism which have espoused vegetarianism. I think there is no basis for such dogma or practice in Sikhism. Certainly Sikhs do not think that a vegetarian`s achievements in spirituality are easier or higher. It is surprising to see that vegetarianism is such an important facet of Hindu practice in light of the fact that animal sacrifice was a significant and much valued Hindu Vedic ritual for ages. Guru Nanak in his writings clearly rejected both sides of the arguments - on the virtues of vegetarianism or meat eating - as banal and so much nonsense, nor did he accept the idea that a cow was somehow more sacred than a horse or a chicken. He also refused to be drawn into a contention on the differences between flesh and greens, for instance. History tells us that to impart this message, Nanak cooked meat at an important Hindu festival in Kurukshetra. Having cooked it he certainly did not waste it, but probably served it to his followers and ate himself. History is quite clear that Guru Hargobind and Guru Gobind Singh were accomplished and avid hunters. The game was cooked and put to good use, to throw it away would have been an awful waste.

Sikhs and Sikhism by I.J. Singh, Manohar, Delhi

The ideas of devotion and service in Vaishnavism have been accepted by Adi Granth, but the insistence of Vaishnavas on vegetarian diet has been rejected.

Guru Granth Sahib, An Analytical Study by Surindar Singh Kohli, Singh Bros. Amritsar

Commenting on meat being served in the langar during the time of Guru Angad: However, it is strange that now-a-days in the Community-Kitchen attached to the Sikh temples, and called the Guru`s Kitchen (or, Guru-ka-langar) meat-dishes are not served at all. May be, it is on account of its being, perhaps, expensive, or not easy to keep for long. Or, perhaps the Vaishnava tradition is too strong to be shaken off.

A History of the Sikh People by Dr. Gopal Singh, World Sikh University Press, Delhi

As a true Vaisnavite Kabir remained a strict vegetarian. Kabir far from defying Brahmanical tradition as to the eating of meat, would not permit so much, as the plucking of a flower (G.G.S. pg 479), whereas Nanak deemed all such scruples to be superstitions, Kabir held the doctrine of Ahinsa or the non-destruction of life, which extended even to that of flowers. The Sikh Gurus, on the contrary, allowed and even encouraged, the use of animal flesh as food. Nanak has exposed this Ahinsa superstition in Asa Ki War (G.G.S. pg 472) and Malar Ke War (G.G.S. pg. 1288).

Philosophy of Sikhism by Gyani Sher Singh (Ph.D), Shiromani Gurdwara Parbandhak Committee. Amritsar

The Gurus were loath to pronounce upon such matters as the eating of meat or ways of disposing of the dead because undue emphasis on them could detract from the main thrust of their message which had to do with spiritual liberation. However, Guru Nanak did reject by implication the practice of vegetarianism related to ideas of pollution when he said, `All food is pure; for God has provided it for our sustenance` (AG 472). Many Sikhs are vegetarian and meat should never be served at langar. Those who do eat meat are unlikely to include beef in their diet, at least in India, because of their cultural proximity to Hindus.

A Popular Dictionary of Sikhism, W.Owen Cole and Piara Singh Sambhi, England

In general Sikhism has adopted an ambivalent attitude towards meat eating as against vegetarianism. But if meat is to be taken at all, Guru Gobind Singh enjoined on the Khalsa Panth not to take kosher meat ie. Halal meat slaughtered and prepared for eating according to the Islamic practice. In fact it is one of the kurahits for every amritdhari Sikh. One who infringes it becomes patit (apostate).

Sikhism, A Complete Introduction by Dr. H.S. Singha and Satwant Kaur, Hemkunt Press, Delhi

A close study of the above-mentioned hymns of Guru Nanak Dev clarifies the Sikh standpoint regarding meat-eating. The Guru has not fallen into the controversy of eating or not eating animal food. He has ridiculed the religious priests for raising their voice in favour of vegetarianism. He called them hypocrites and totally blind to the realities of life. They are unwise and thoughtless persons, who do not go into the root of the matter. According to him, the water is the source of all life whether vegetable or animal. Guru Nanak Dev said. "None of the grain of corn is without life. In the first place, there is life in water, by which all are made green" (Var Asa M.1, p. 472). Thus there is life in vegetation and life in all types of creatures.

Real Sikhism by Surinder Singh Kohli, Harman Publishing, New Delhi

The Gurus neither advocate meat nor banned its use. They left it to the choice of the individual. There are passages against meat, in the Adi Granth. Guru Gobind Singh however prohibited for the Khalsa the use of Halal or Kutha meat prepared in the Muslim ritualistic way.

Introduction to Sikhism by Dr. Gobind Singh Mansukhani, Hemkunt Press, Delhi

There are no restrictions for the Sikhs regarding food, except that the Sikhs are forbidden to eat meat prepared as a ritual slaughter. The Sikhs are asked to abstain from intoxicants.

Introduction to Sikhism by G.S. Sidhu, Shromini Sikh Sangat, Toronto

According to the Maryada booklet `Kutha`, the meat prepared by the Muslim ritual, is prohibited for a Sikh. Regarding eating other meat, it is silent. From the prohibition of the Kutha meat, it is rightly presumed that non-Kutha meat is not prohibited for the Sikhs. Beef is prohibited to the Hindus and pork to the Muslims. Jews and Christians have their own taboos. They do not eat certain kinds of meat on certain days. Sikhs have no such instructions. If one thinks he needs to eat meat, it does not matter which meat it is, beef, poultry, fish, etc., or which day it is. One should, however, be careful not to eat any meat harmful for his health. Gurbani`s instructions on this topic are very clear. "Only fools argue whether to eat meat or not. Who can define what is meat and what is not meat? Who knows where the sin lies, being a vegetarian or a non-vegetarian?" (1289) The Brahmanical thought that a religious person should be a vegetarian is of recent origin. Earlier, Brahmans had been eating beef and horse meat. In conclusion, it is wrong to say that any person who eats meat (of course Kutha, because of the Muslim rituals is prohibited) loses his membership of the Khalsa and becomes an apostate.

The Sikh Faith by Gurbakhsh Singh, Canadian Sikh Study and Teaching Society, Vancouver

The above discussion leads us to the conclusion that the Sikh Gurus made people aware of the fact that it is very difficult to distinguish between a plant and an animal, therefore, it is difficult to distinguish between a vegetarian and a non-vegetarian diets and there is no sin of eating food originating from plants or animals.

Scientific Interpretation of Gurbani, Paper by Dr. Devinder Singh Chahal

The practice of the Gurus is uncertain. Guru Nanak seems to have eaten venison or goat, depending upon different janamsakhi versions of a meal which he cooked at Kurukshetra which evoked the criticism of Brahmins. Guru Amardas ate only rice and lentils but this abstention cannot be regarded as evidence of vegetarianism, only of simple living. Guru Gobind Singh also permitted the eating of meat but he prescribed that it should be Jhatka meat and not Halal meat that is jagged in the Muslim fashion.

Mini Encyclopaedia of Sikhism by H.S. Singha, Hemkunt Press, Delhi.