I have found no-one who addresses the question of tzaar baalei chayim with regard to this. Shechita, the seh la-a-zal-zel etc. were instant deaths.How about the famous story about Rabbi BM 84b-85a??? ( Psalms 145:9 )
Rabbi Simcha Freedman, boca raton, florida 33433
Rav Joseph Pearlman replies:
Animals exist for the service of mankind and are for food or sacrifice or for other positive function. This includes the performance of Mitzvos and also reasonable experimentation for medical purposes for the benefit of mankind (Rema, Even ha'Ezer 5:14).
Thus, the goat to Azazel of the Avodah on Yom Kipur (Vayikra 1610) was necessary for the sake of Hash-m's commandment, however cruel it might seem to us. Another good example is in the Shulchan Orech, Yoreh Deah 24:8, Rema and Shach (#8), quoting the Mordechai and others, and see also the Pri Megadim in Sifsei Da'as there, regarding plucking the wool or feathers from an animal that interfere with the Shechitah.
It should be noted also that the Noda b'Yehudah (Mahadura Tinyana, YD #10) holds that Tza'ar Ba'alei Chayim, the prohibition against cruelty to animals, has no application where the animal is being put to death. Others, though, disagree.
Hence, Chato'os ha'Meisos does not pose a problem of Tza'ar Ba'alei Chayim, as there is no alternative possibility with sound Halachic reasons as to what to do with the animal. Since it is being left to starve for the sake of serving a Halachic purpose as mandated by G-d, it is not considered Tza'ar Ba'alei Chayim.
However, where these considerations do not apply, one is certainly obligated to treat animals with kindness, as a number of sources in the Gemara and Halachah clearly indicate. Moreover, apart from all of the specific Mitzvos in the Torah -- such as lifting the heavy burden off of an animal that has fallen down, the prohibition against muzzling an animal to prevent it from eating while working, letting one's animal rest from work on Shabbos, the prohibition against working two different types of animals together, the requirement to feed one's animals first before eating, and Tza'ar Ba'alei Chayim -- there is the general overriding principle of emulating G-d's trait of "v'Rachamav Al Kol Ma'asav," Hash-m "is merciful upon all of His creations."