​Bamford NJ, Potter SJ, Baskerville CL, Harris PA, Bailey SR
Influence of dietary restriction and low-intensity exercise on weight loss and insulin sensitivity in obese equids
Journal of Veterinary Internal Medicine December 2018 33(1) DOI: 10.1111/jvim.153​
​

See also:
Why Obese Horses Need Both Diet and Exercise - Christa Lesté-Lasserre, Apr 2019, www.thehorse.com

Pollard D, Wylie CE, Verheyen KLP, Newton JR
Identification of modifiable factors associated with owner-reported equine laminitis in Britain using a web-based cohort study approach
BMC Veterinary Research Feb 2019 15:59 https://doi.org/10.1186/s12917-019-1798-8

Eleanor Jones wrote in Horse & Hound April 2019 that "limiting grazing may increase laminitis risk", saying that horses that wore grazing muzzles for only part of the time they were at grass were more likely to develop laminitis than horses that wore a muzzle all of the time or not at all.

Also that horses that grazed for a brief period in the mornings were at greater risk of laminitis than those that went out all day, overnight or not at all.

Is this true?  Given that restricting grass has been one of the main management strategies to reduce the risk of laminitis, does the study really suggest what Jones reports?

The short answer is no - whilst the study has been written up to suggest some correlation between these factors, no causality is proven.

Danica Pollard's research asked owners to complete an online questionnaire to enter the study, and add updates during the study period.  The study lasted from August 2014 to December 2016, 2 years and 5 months including 3 seasonal rises.

Study figures:
1070 horses, but full data for only 978 horses - these 978 horses were equivalent to 974 Horse Years At Risk (HYAR).
of which 35.9% had a history of laminitis prior to the study - 384 out of 1070 horses.  Or up to 384 horses out of 978 with a history of laminitis prior to the study.
During the study period, 97 horses had laminitis, with 123 laminitis episodes reported, but there was full data for only 100 episodes.
75.3% of these 97 horses had a history of laminitis prior to the study - 73 out of 97 horses.
19 out of the 97 horses had more than 1 episode of laminitis during the study period.

​Muzzle use.
Owners were asked:
2.13 Are you currently using any methods to restrict grass intake? Please indicate all that apply.
 No
 Use a grazing muzzle
 Strip grazing
 Restricted turnout
 Other (please specify)

2.13.1 If your horse grazes with a grazing muzzle, please indicate if this is worn:
 All the time while gazing
​ Part of the time while grazing

Grazing muzzle use was recorded as 1 of 16 variables associated with active episodes of owner-reported laminitis.  There were 100 active episodes of owner-reported laminitis in a cohort of 978 horses and ponies, being 974 horse years at risk.
Laminitis episodes in a combined entry of horses wearing a grazing muzzle the whole time while grazing or not using a grazing muzzle at all was 89/100 out of 950 HYAR (~horses) that either wore a grazing muzzle the whole time while grazing or didn't use a grazing muzzle at all - so 9.37%.  The paper uses this as the hazard ratio reference.
Laminitis episodes in horses wearing a grazing muzzle part of the time while grazing was 11/100, out of 30 HYAR (~horses) that wore a grazing muzzle part of the time while grazing - so 36.7%.  The paper gives a hazard ratio of 3.6 to using a grazing muzzle part of the time while grazing compared to using a grazing muzzle the whole time while grazing combined with not using a grazing muzzle at all.

Ignoring the statistics (remember the quote: "There are three kinds of lies: lies, damned lies and statistics"!), 
the figures reported show that in 978 horses, there were 100 active episodes of laminitis, of which 89% had either been wearing a muzzle the whole time while grazing, or hadn't been wearing a muzzle at all, or presumably hadn't been grazing at all, and of which 11% had been wearing a muzzle part of the time while grazing.  

Grouping together horses that wore a muzzle the whole time while grazing or didn't use a muzzle at all  or didn't have any access to grass surely makes this comparison with horses that wore a muzzle some of the time pointless.  Presumably not all 978 horses had access to grass.  Why are horses with no access to grass at all having their incidence of laminitis compared with horses that grazed, with or without  a muzzle?

Putting aside the strange comparison groups, it is clear that incidence of laminitis was higher as a % of horses that wore a muzzle some of the time when grazing (36.7%) than incidence of laminitis as a % of horses that wore a muzzle all the time/didn't wear a muzzle/didn't graze (9.37%).  So why might this be?

1.  The risk of laminitis regardless of muzzle use was likely to be very different between the two groups (whole/no muzzle v part muzzle).  Although entry into the study with its weight tracker and body condition scoring help was biased towards owners of horses that tended to be overweight or suffer from laminitis (and the study was well publicized on laminitis-related support groups and social media pages), we are aware of several owners who entered horses that had never had any issues with laminitis or being overweight.  So some % of the 978 horses had no reason to be considered at greater than normal risk of laminitis, and therefore are unlikely to have ever worn a muzzle.  Owners generally don't muzzle their horses without good reason - or fear - so it's likely - and the authors will have this information - that the horses that wore a muzzle, either all the time or part of the time while grazing, had a history of laminitis, or were at least good doers and/or overweight, i.e. had a suspected if not actual increased risk of laminitis.  And therefore the 30 odd horses in the part muzzle group possibly all had a history of laminitis, or being overweight, i.e. had a higher risk of laminitis regardless of muzzle use, than some of the horses in the whole/no muzzle group.  

2.  Continuing the likelihood that horses in the part muzzled group had a history of laminitis, and knowing that  x-rays weren't submitted to prove that rotation from previous laminitis had been corrected and that they had adequate sole depth and concavity for comfort, perhaps increased movement associated with turnout  (compared to some horses with no turnout in the whole/no muzzle group) increased foot pain, but it wasn't new hormone-related laminitis that was seen. 

3.  Perhaps the owners of the part muzzled horses weren't so strict about low sugar/starch diets - after all, they were by implication allowing their horses to graze without a muzzle some of the time, compared to some of the horses in the other group being muzzled all the time. 

4.  However, perhaps the reason these horses were only muzzled part of the time was because, when they weren't muzzled, they were on very sparse grazing, or a track system, or grazing under trees where grass might be expected to contain less sugar, and perhaps this grazing was supplemented with hay, so owners felt muzzles should be removed to ensure sufficient intake of preserved forage.

5.  Or perhaps the reason these horses were only muzzled part of the time was because they spent some time in a herd situation where social behaviour was encouraged, e.g. mutual grooming, again perhaps on sparse grazing.  

Remember there were only 2 options and no box for supplementary information for this question - there could be a huge range of options between wearing a grazing muzzle all the time when grazing and wearing a grazing muzzle part of the time when grazing.