Chief handicapper Phil Smith has defended his decision to give Grand National topweight Tidal Bay a more lenient handicap rating than would usually be the case.

Paul Nicholls' 13-year-old will still have to saddle 11st 10lb (74.5kg) for the first STG1 million ($A1.83 million) National on April 5, but will be running from 161 rather than his official mark of 168.

Smith, in his role as the British Horseracing Authority head of handicapping, is given dispensation in the National to make adjustments to the usual ratings of horses.

He decided to give Long Run just 1lb (.5kg) less to carry than Tidal Bay, with Nicky Henderson's star currently valued at 163 ahead of a likely outing at Kelso on Thursday.

"We're very, very happy with 114 entries," Smith said.

"In Tidal Bay we've got a creditable topweight and I think we've got the most creditable second topweight we've ever had.

"I don't think we've ever had a horse that has won a Gold Cup and two King Georges that hasn't been topweight.

"Overall it looks absolutely fantastic and, for a handicapper, what you look for most is depth.

"Most of these horses have handicap marks based on three miles (4800m) but over four and a half miles and the Grand National fences, this is a completely different test.

"To carry top weight as Tidal Bay did in the Welsh National is wholly different to what he's being asked to do at Aintree.

"We're trying to attract the really nice horses and the important thing is for them to run and have a chance of winning, but you don't want those just behind to feel at a disadvantage.

"But many of those horses just behind Tidal Bay are entered in the Gold Cup, so they would be happy to take him on at levels, but in the National they get weight from him."