--- a/taskcluster/docs/parameters.rst
+++ b/taskcluster/docs/parameters.rst
@@ -17,20 +17,16 @@ file if necessary, rather than starting 
 complete set of parameters.
 
 The properties of the parameters object are described here, divided rougly by
 topic.
 
 Push Information
 ----------------
 
-``triggered_by``
-   The event that precipitated this decision task; one of ``"nightly"`` or
-   ``"push"``.
-
 ``base_repository``
    The repository from which to do an initial clone, utilizing any available
    caching.
 
 ``head_repository``
    The repository containing the changeset to be built.  This may differ from
    ``base_repository`` in cases where ``base_repository`` is likely to be cached
    and only a few additional commits are needed from ``head_repository``.

--- a/taskcluster/mach_commands.py
+++ b/taskcluster/mach_commands.py
@@ -144,20 +144,16 @@ class MachCommands(MachCommandBase):
                      type=int,
                      default=0)
     @CommandArgument('--owner',
                      required=True,
                      help='email address of who owns this graph')
     @CommandArgument('--level',
                      required=True,
                      help='SCM level of this repository')
-    @CommandArgument('--triggered-by',
-                     choices=['nightly', 'push'],
-                     default='push',
-                     help='Source of execution of the decision graph')
     @CommandArgument('--target-tasks-method',
                      help='method for selecting the target tasks to generate')
     def taskgraph_decision(self, **options):
         """Run the decision task: generate a task graph and submit to
         TaskCluster.  This is only meant to be called within decision tasks,
         and requires a great many arguments.  Commands like `mach taskgraph
         optimized` are better suited to use on the command line, and can take
         the parameters file generated by a decision task.  """

--- a/taskcluster/taskgraph/cron/decision.py
+++ b/taskcluster/taskgraph/cron/decision.py
@@ -11,18 +11,16 @@ import pipes
 import yaml
 import re
 import os
 import slugid
 
 
 def run_decision_task(job, params):
     arguments = []
-    if 'triggered-by' in job:
-        arguments.append('--triggered-by={}'.format(job['triggered-by']))
     if 'target-tasks-method' in job:
         arguments.append('--target-tasks-method={}'.format(job['target-tasks-method']))
     return [
         make_decision_task(
             params,
             symbol=job['treeherder-symbol'],
             arguments=arguments),
     ]

--- a/taskcluster/taskgraph/cron/schema.py
+++ b/taskcluster/taskgraph/cron/schema.py
@@ -28,19 +28,16 @@ cron_yml_schema = Schema({
 
         # Description of the job to run, keyed by 'type'
         Required('job'): Any({
             Required('type'): 'decision-task',
 
             # Treeherder symbol for the cron task
             Required('treeherder-symbol'): basestring,
 
-            # --triggered-by './mach taskgraph decision' argument
-            'triggered-by': basestring,
-
             # --target-tasks-method './mach taskgraph decision' argument
             'target-tasks-method': basestring,
         }),
 
         # when to run it
 
         # Optional set of projects on which this job should run; if omitted, this will
         # run on all projects for which cron tasks are set up.  This works just like the

--- a/taskcluster/taskgraph/decision.py
+++ b/taskcluster/taskgraph/decision.py
@@ -152,17 +152,16 @@ def get_decision_parameters(options):
         'head_rev',
         'head_ref',
         'message',
         'project',
         'pushlog_id',
         'pushdate',
         'owner',
         'level',
-        'triggered_by',
         'target_tasks_method',
     ] if n in options}
 
     # Define default filter list, as most configurations shouldn't need
     # custom filters.
     parameters['filters'] = [
         'target_tasks_method',
     ]

--- a/taskcluster/taskgraph/parameters.py
+++ b/taskcluster/taskgraph/parameters.py
@@ -23,17 +23,16 @@ PARAMETER_NAMES = set([
     'message',
     'moz_build_date',
     'optimize_target_tasks',
     'owner',
     'project',
     'pushdate',
     'pushlog_id',
     'target_tasks_method',
-    'triggered_by',
 ])
 
 
 class Parameters(ReadOnlyDict):
     """An immutable dictionary with nicer KeyError messages on failure"""
     def check(self):
         names = set(self)
         msg = []