The Giants second team's hopes of completing a double over Saints were dashed by missed tackles, wrong options and a poor completion rate.
Holroyd, fit again after an ankle injury, darted over for the first try after just 41 seconds, and Paul White converted.
But Saints took a 12-6 lead with tries by Ian Hardman and Peter Cook and two goals from Steve Rowlands, who had a 100% kicking record.
Stand-off Holroyd and kicker White twice got the Giants back on level terms, first after 12 minutes, when Jon Grayshon put in the spadework.
Then, after James Graham had struck for Saints, Holroyd grabbed his third try after good work by Eorl Crabtree in the 31st minute.
The Giants were competing strongly at that stage, but found themselves 36-18 behind after tries by Liam Williams (36 minutes), John Wilkin (42) and Alan Kilshaw (58).
Ricky Sheriffe, whose 61st- minute try was goaled by White, and Eorl Crabtree, who crashed over for an unconverted score six minutes later, gave the Giants hope, but the fightback failed to materialise.
Huddersfield's next game is at Wigan tomorrow week.
The Giants Junior Academy side face Keighley at Cougar Park tomorrow (2.30).
Team: Hull, Sutherland, Knight, George, Fagborun, Hemingway, Simpson (L), Brown, Hawkyard, Wilkinson, Robinson, Joseph, Clare. Subs (from): Holdsworth, Samuel, Tsang, Simpson (M), Watson, Crabtree, Mennell.