In this age, the performance of yajña has been forbidden because no one can properly chant the Vedic mantras. If Vedic mantras are chanted properly, the desire for which a sacrifice is performed must be successful. Therefore the Hare Kṛṣṇa chant is called the mahā-mantra, the great, exalted mantra above all other Vedic mantras, because simply chanting the Hare Kṛṣṇa mahā-mantra brings so many beneficial effects. As explained by Śrī Caitanya Mahāprabhu (Śikṣāṣṭaka 1):

ceto-darpaṇa-mārjanaṁ bhava-mahā-dāvāgni-nirvāpaṇaṁ
 śreyaḥ-kairava-candrikā-vitaraṇaṁ vidyā-vadhū-jīvanam
ānandāmbudhi-vardhanaṁ prati-padaṁ pūrṇāmṛtāsvādanaṁ
 sarvātma-snapanaṁ paraṁ vijayate śrī-kṛṣṇa-saṅkīrtanam

“Glory to the Śrī Kṛṣṇa saṅkīrtana, which cleanses the heart of all the dust accumulated for years and extinguishes the fire of conditional life, of repeated birth and death. This saṅkīrtana movement is the prime benediction for humanity at large because it spreads the rays of the benediction moon. It is the life of all transcendental knowledge. It increases the ocean of transcendental bliss, and it enables us to fully taste the nectar for which we are always anxious.”

Therefore, the best performance of yajña given to us is the saṅkīrtana-yajña. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ (Bhāg. 11.5.32). Those who are intelligent take advantage of the greatest yajña in this age by chanting the Hare Kṛṣṇa mahā-mantra in congregation. When the Hare Kṛṣṇa mantra is chanted by many men together, the chanting is called saṅkīrtana, and as a result of such a yajña there will be clouds in the sky (yajñād bhavati parjanyaḥ). In these days of drought, people can gain relief from scarcity of rain and food by the simple method of the Hare Kṛṣṇa yajña. Indeed, this can relieve all of human society. At present there are droughts throughout Europe and America, and people are suffering, but if people take this Kṛṣṇa consciousness movement seriously, if they stop their sinful activities and chant the Hare Kṛṣṇa mahā-mantra, all their problems will be solved without difficulty. In other processes of yajña there are difficulties because there are no learned scholars who can chant the mantras perfectly well, nor is it possible to secure the ingredients to perform the yajña. Because human society is poverty-stricken and men are devoid of Vedic knowledge and the power to chant the Vedic mantras, the Hare Kṛṣṇa mahā-mantra is the only shelter. People should be intelligent enough to chant it. Yajñaiḥ saṅkīrtana-prāyair yajanti hi sumedhasaḥ. Those whose brains are dull cannot understand this chanting, nor can they take to it.