Here is what the GIRM says about the Responsorial Psalm:
- After the first reading comes the responsorial Psalm, which is an integral part of the Liturgy of the Word and holds great liturgical and pastoral importance, because it fosters meditation on the word of God.
The responsorial Psalm should correspond to each reading and should, as a rule, be taken from the Lectionary.
It is preferable that the responsorial Psalm be sung, at least as far as the people’s response is concerned. Hence, the psalmist, or the cantor of the Psalm, sings the verses of the Psalm from the ambo or another suitable place. The entire congregation remains seated and listens but, as a rule, takes part by singing the response, except when the Psalm is sung straight through without a response. In order, however, that the people may be able to sing the Psalm response more readily, texts of some responses and Psalms have been chosen for the various seasons of the year or for the various categories of Saints. These may be used in place of the text corresponding to the reading whenever the Psalm is sung. If the Psalm cannot be sung, then it should be recited in such a way that it is particularly suited to fostering meditation on the word of God.
In the dioceses of the United States of America, the following may also be sung in place of the Psalm assigned in the Lectionary for Mass: either the proper or seasonal antiphon and Psalm from the Lectionary, as found either in the Roman Gradual or Simple Gradual or in another musical setting; or an antiphon and Psalm from another collection of the psalms and antiphons, including psalms arranged in metrical form, providing that they have been approved by the United States Conference of Catholic Bishops or the Diocesan Bishop. Songs or hymns may not be used in place of the responsorial Psalm.
Note, that it does not say that you can chop off the psalm. Here what the General Introduction to the Lectionary says:
b) The Responsorial Psalm
- The responsorial psalm, also called the gradual, has great liturgical and pastoral significance because it is an "integral part of the liturgy of the word."36 Accordingly, the faithful must be continually instructed on the way to perceive the word of God speaking in the psalms and to turn these psalms into the prayer of the Church. This, of course, "will be achieved more readily if a deeper understanding of the psalms, according to the meaning with which they are sung in the sacred Liturgy, is more diligently promoted among the clergy and communicated to all the faithful by means of appropriate catechesis."37
Brief remarks about the choice of the psalm and response as well as their correspondence to the readings may be helpful.
- As a rule the responsorial psalm should be sung. There are two established ways of singing the psalm after the first reading: responsorially and directly. In responsorial singing, which, as far as possible, is to be given preference, the psalmist, or cantor of the psalm, sings the psalm verse and the whole congregation joins in by singing the response. In direct singing of the psalm there is no intervening response by the community; either the psalmist, or cantor of the psalm, sings the psalm alone as the community listens or else all sing it together.
- The singing of the psalm, or even of the response alone, is a great help toward understanding and meditating on the psalm’s spiritual meaning.
To foster the congregation’s singing, every means available in each individual culture is to be employed. In particular, use is to be made of all the relevant options provided in the Order of Readings for Mass38 regarding responses corresponding to the different liturgical seasons.
- When not sung, the psalm after the reading is to be recited in a manner conducive to meditation on the word of God.39 The responsorial psalm is sung or recited by the psalmist or cantor at the ambo.40
It does not say that the psalm can be edited at will.