Cow in the Veda is pre-eminently symbolical of speech

That cow in the Veda is pre-eminently symbolical of speech is evident from a mantra seen by seer Nema Bhārgava. He starts his vision of Vāk with the query as to where has gone the highest form of it while out of the Vāk used by inconscient beings one has settled down as the power of gods and four ones are milking energy in the form of milk (Rigveda, VIII.100.10).

In the immediately following mantra, it is stated that the goddess Vāk has been generated by gods and is being used as such by all kinds of living beings and that she as cow with all the profundity of her voice should yield for us the required energy and come to us whenever earnestly prayed to (Rigveda, VIII.100.11).

It is evident from these mantras that it had become quite usual for the Vedic seer to talk of speech through the symbolism of cow. In this symbolic usage, several qualities of the cow were instrumental. At least two of them are quite evident from these mantras themselves. One of them is the melodious and profundity of the cow’s voice. It is quite close to the dilated pronunciation of the sacred Om. Due to this closeness, perhaps, just as the word Om came to be regarded as the source of all words whatever, even so the cow’s voice was taken to include within itself all possible varieties of voices capable of being produced by living beings.

By virtue of this quality of her voice, she came to be used frequently as a significant symbol of Vāk. Secondly, cow’s voice is closely associated with her milk yielded particularly for her calf. In fact, the cow produces her voice most frequently in the evening when after day’s grazing in the pasture she becomes eager to feed her calf with the milk. Due to this association of her voice with the milk intended to be delivered to the calf, cow became symbolical particularly of that Vāk, which is supposed to be most fruitful.

No doubt Vāk is Vedic in origin. This point is brought out beautifully in one of the mantras of the hymn to jñāna. In this mantra it has been stated that one who serves the cow of the Vedic Vāk well understanding it precisely and profoundly in the same sense as was intended to be communicated by the seer, he is comparable to the wrestler who does not get defeated anywhere, as against one who does not understand the meaning, is comparable to him who although keeping a cow for all practical purposes is really shorn of her since on account of not yielding any milk she is a cow just in name and appearance. (Rigveda, X.71.5) Symbolism of Cow - III from Vedic Vision of Consciousness & Yoga by MukeshVeda