Longissimus Dorsi Muscle Diameter, Backfat Thickness, Body Condition Scores and Skinfold Values Related to Metabolic and Endocrine Traits in Lactating Dairy Cows Fed Crystalline Fat or Free Fatty Acids

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Abstract
Summary: An experiment was performed using 18 dairy cows with ≥ 2 lactations from parturition until week 20 of lactation to investigate the effects of feeding rumen‐protected crystalline fat (group A) or free fatty acids (group B) compared with a control group not fed supplemental fat or fatty acids (group C). The feeding effects studied were those on milk yield and composition, body weight (BW), body conditions scores (BCS), skinfold values (a measure of subcutaneous fat tissue thickness), backfat thickness and longissimus dorsi muscle diameter (measured by ultrasound), as well as on blood plasma metabolic and endocrine traits. Fat and fatty‐acid feeding reduced roughage intake. Net energy and protein balances during the first week of lactation were negative. Milk yield was similar in all groups, but concentrations of milk fat were reduced in group B and of milk protein in groups A and B. BW, BCS, skinfold value, backfat thickness and longissimus dorsi muscle diameter similarly decreased in all groups until weeks 8–12. Concentrations of glucose and fructosamine decreased transiently during early lactation, whereas levels of β‐hydroxybutyrate, urea, total protein and albumin increased reversibly and similarly in all groups. Concentrations of creatinine decreased similarly in all groups until week 4 and those of nonesterified fatty acids until week 12. Triglyceride concentrations increased continuously until week 18, whereas concentrations of phospholipid and cholesterol increased until week 10 and then remained elevated. Triglyceride, phospholipid and cholesterol concentrations were hïgher in group B than in groups A and C throughout the experiment. Concentrations of insulin, insulin‐like growth factor‐1, thyroxine and 3,5,3′‐triiodothyronine started to increase after the first 6–8 weeks of lactation, while those of growth hormone were transiently low in week 16. None of the hormones exhibited group differences. In this study rumenprotected fat or fatty‐acid supplementation failed to improve the energy and protein balances during early lactation and did not affect milk yield, likely due to decreased roughage intake. BCS, skinfold values and backfat ultrasound measurements of subcutaneous adipose tissue mass and of longissimus dorsi muscle diameter indicated mobilization of fat tissue and protein mobilization in early lactation and were mirrored by typical changes of metabolic and endocrine traits.

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