The heavyweight girls are only 24 weeks of age, but I feel the time has come to make a humane decision.
Yesterday one of the girls sustained a bad leg injury, possibly triggered by the rooster's added weight during treading. She was clearly in serious pain, and when I had a close look I could see that her hock looked abnormal, with a hard knot above the joint, and the joint itself was too loose. After I'd put her down I saw that the tendon was completely severed from the leg bone. There was no way she could have healed or had a good quality of life.
However I also noted that her liver was enlarged and friable, as well as being an unhealthy orange colour. She was extremely fat, with a huge amount adhering to the underside of her skin as well as filling the cavity. These obviously point to fatty liver syndrome, which is fairly common in commercial birds (both layers and meat hybrids) due to a combination of high food intake and the kinds of diets we rely on to meet production goals.
A few weeks ago I had to dispatch a leg-stricken bird, and also one a couple of weeks before that; both birds had slight liver discolouration but no signs of fatty liver or enlargement. In today's bird the liver was in appalling shape; I would say close to rupture. The difference in colour to earlier livers was astonishing.
In order to bring the birds into lay and help with shell quality I began 4 weeks ago both increasing feed amounts and adding commercial layer crumble and meat bird finisher (with extra shell grit) as well as soy meal to the diet. This resulted in a net decrease in the amount of whole grain (albeit sprouted) being fed. Although laying improved, the health decline in these 4 weeks has been incredible.
I'm not sure if this decline is merely due to a protein increase in percentage terms. Yes, meat bird finisher is higher in protein than layer feed and probably a fair bit higher than my sprout mix. However protein in general doesn't produce fatty liver syndrome; and I haven't found any literature describing soybean meal as a cause. In general fatty liver is put down to a diet high in quick-burning carbohydrates. Fatty liver syndrome is also, in my reading, more strongly associated with the use of artificial methionine against natural methionine; notably, meat bird finisher is high in the artificial form.
What else can I say? This has been an interesting and at times upsetting project, but I don't feel deterred from continuing with half-leghorn offspring (presuming some hatch). Earlier today I felt rather discouraged and indeed a little sickened by the poor birds' plight. I felt it would be cruel to produce chicks from these birds. However having thought it through (and writing it out here helps me do that -- apologies for any errors or omissions, but this is a work-in-progress) I can see that the excess fat and liver degeneration are dietary. These birds are programmed to overeat, and hence dietary issues appear more strongly in them than in other birds. I'll simply need to be more careful with what I feed in future.
But of course the whole project might prove too hard if the offspring of these meat hybrids suffer similar issues in terms of reproductive health. If I have to use commercial meat bird finisher to increase protein so the birds can lay at all, I'm walking a burning tightrope. But I've gone this far and have eggs set, so for now I might as well keep walking.