Updated: Jun 7, 2019
Tom Jones sings for it as death row jail bird longing for home and family, and at this time of year some of our horses don't want to be torn away from it, and even go to the lengths of giving us the slip as we try to fetch them in. The green grass does create us a few issues!
When grass is rapidly growing, it's higher in sugary carbs, lower in gut filling fibre, and it's minaral balance shifts, making it lower in magnesium and higher in phosphorus. This has led to the hypothesis that spring grass makes for more excitable and spooky horses, as well as ones that dive for sweet grass, and/or who won't be caught from it.
In treating these behavioural issues it pays to look at your horse's dietary managment as well as your training and handling skills. Spring grass sure tastes good, but doesn't help the horse feel that full - really driving them to stuff themselves. Bringing horses off the grass and providing them with access to higher fibre forage, particularly during the later morning and afternoon as sugars build in the grass can help the horse feel fuller. Feeding a fibre based meal before turnout will also help your horse's desire to binge on that sweet grass. It may also help blunt that potentially low magnesium if your horse can also get some supplementation (seek the advice of a properly qualified and registered nutritionist).
The routine of the herd coming in every day, or using an equicentral system to facilitate less grass and more hay, can also reduce the problem of trying to catch a horse from the field. As does addressing their sudden lack or motivation to come in with you.
From your horse's point of view, they have eaten, but they are not sated and must munch more low fibre, sweet grass to try and feel full (basically how do you feel after excess sweet food?). Nothing must interrupt this eating, especially not a human who can't offer decent compensation. If you and/or your food is failing to be a more attractive option than the grass, then you need to be sure you aren't just creating a "trap", using it to "trick" your horse. Some horses really hate tricks and they will bust you if you try.
The most rewarding thing you can do might be to back off when your horse allows your approach - so they learn that not moving away themselves will serve them well in making you leave. Then you can chip down that distance, and may be leave some freshly sliced carrot when you back off - so it now becomes worth approaching safe knowing that you are leaving! At which point you can pair head collar with magical carrot appearance, then take your head collar and leave while they are still munching. You know, the old leave while they still want more routine. This is the easiest way to turn you and your head collar into carrot announcement, not jail time.
When you have this you can start to have your horse earn the carrot and your ultimate departure by allowing you to:
Stroke their neck = carrot and leave
Stroke neck plus lead rope on neck = carrot and leave
Stroke neck plus lead rope on neck plus head collar on = carrot, head collar off and leave
Stroke neck plus lead rope plus head collar on = lead to other side of gate give bonus bucket of carrot slices
Then we come to that other early summer bug bear of many with dieting horses: grass diving with impunity. I'm sure many of these horses have heard the mantra, "no pain, no gain" and apply it to plunging through our grip and through our electric fences too. They also need to have something more motivating than grass in order to walk beside us, or ride on a reasonable contact rather than a death vice grip on the reins to get from a to b without snacking. You may have tried using a food reward to try and keep your horse from diving and are now laughing through tears of despair.
Let's revisit motivation, aka hunger and look at feeding fibre before going anywhere: yes even the racehorses in our post gastric ulcer awareness age get a dose of fibre to mat their stomach before getting on the gallops, so even your dieting horse can have an appropriate high fibre fill before going anywhere with you to at least curb their desires a little. We can also give our food rewards for "walking with head up" at a reward rate that will stop them thinking about what they are missing - and you might well be machine gunning them with rewards until they fall into the habit, don't worry about this, as the habit forms, you can become meaner and make the reward rate leaner. And don't forget your barrier: short (but light hold) on your lead rope with immediate resistance for head movements to grass (watch for sideways movement too!) and if you need to, a lunge caveson is harder to lean on than a head collar and not as severe (read painful) as a pressure halter (or at least this is the case with most ordinary tack shop cavesons).
Then it is patience and persistence, and to borrow from Jean Donaldson, dog trainer, there are three criteria for stretching out and strengthening the head up while walking and stopping behaviour: duration, distance and distraction. The three DS of getting from a to b without diving for grass: how long, how far, and how much grass we just walked past?
My last word will be this, work hard on your relationship and training outside of grass distraction season too. Your reward will be a smoother ride through it compared to neglecting this aspect of horse keeping. A horse who wants to be with you will still be interested when there is grass.