Former Rosh Kollel in Johannesburg (1998-2002)
Currently Rav of Beit Knesset Hechadash, Netanya

There are two types of “agricultural gifts” – matanot aniyim”: 
1) those, one is obligated to give at the preliminary stages of agricultural production, for example the mitzvah of bikkurim.
2) Those one ‘gives’ at the end of growing one’s crops, for example the mitzvah of peah.
[There are ‘gifts’ given in the intermediary stages such as leket and shichecha– stalks of grain or fruit that were dropped or forgotten during the harvest must be left to the poor.]
The mitzvah of bikkurim entails taking the first fruits grown on ones’ land to the Beit hamikdash and given to the Kohanim accompanied with a declaration.
The mitzvah of peah requires not harvesting one’s field completely but rather leaving a corner (of the last fruits) to the poor.
Both these ‘gifts’ have no set amount to be given, as mentioned in mishna Peah :
”אלו דברים שאין להם שיעור, הפאה והביכורים…..”                        

Similarly there are two types of Berachot:
1) those we recite before we partake of foods.
2)Those we recite after having eaten ,for example birkat hamazon that we recite after eating bread, as mentioned in our parasha.
Which beracha, (or agricultural gift), is more difficult to fulfill – the “Before-type” or the “After-type”?
It seems that the “After type” – peah and birkat hamazon, firstly because they are mentioned in the Torah in contrast to Berachot before eating, which are obligatory only on a Rabbinic level.
 There is also a psychological reason: It is more difficult to desist from an activity one has already commenced, than to restrain oneself a little longer. It requires a greater effort to change from activity to a different activity than to continue a passive status quo.
 To stop eating – an activity so ‘self-centered’ (who else but one’s own stomach benefits from that food?) – and then bless a Being outside one’s self, namely Hashem, is more difficult than extending a situation of not eating until one recites a beracha beforehand.
The same is true of peah and bikkurim – to restrain oneself having finally commenced the activity of harvesting, accompanied by the joy of bringing ones toil to realization, is extremely difficult.
[Similarly, Yosef is lauded not only for avoiding the temptations of Photiphar’s wife but more so, for overcoming once the temptation had already begun – ”She grabbed him by his cloak. He ran away from her…and fled outside.” (Bereishit 39:12)
The Meshech Chochma elaborates on the psychological reason/dimension of birkat hamazon. He explains: it is the nature of man, experiencing satisfaction, that he develops a sense of smugness, insensitivity and forgetfulness of Hashem. To counter this, Hashem commanded that we remember our source of our contentment and express gratitude to Him through Blessings after the meal.
The Meshech Chochma applies this principle to explain a difficulty in gemarah Erechin 4a. The Talmud states that Kohanim eating korbanot are also required to recite birkat hamazon,
despite the fact that they are eating only as part of atonement afforded to the owner.

In other words, one who brings a sacrifice achieves atonement by
1) Having his animal and bread sacrificed by the Kohanim.
2) Having the Kohanim eat of the sacrifice.
The Meshech Chochma explains that the novelty of the Talmud is thus: Since the Kohanim eating in this context is for a holy purpose I would have thought that there is no concern of an “inner spiritual rebellion” and hence no need to counter the “rebellion” with requirement of birkat hamazon. Despite all this, birkat hamazon must still be recited by them.