Simple SummarySelection for an increased meatiness in beef cattle has resulted in double-muscled (DM) animals, owing to the inactivation of the myostatin gene. These animals are characterized by an excellent conformation and an extremely high carcass yield, coinciding with a reduced organ mass. As a consequence, voluntary feed intake is reduced, but feed efficiency is considerably improved, although maintenance requirements are not clearly reduced. DM animals are more susceptible to respiratory disease, stress and dystocia, requiring extra attention for accommodation and welfare. Carcasses of DM animals are very lean, and intramuscular fat content is low. The fatty acid profile is different when compared with non-DM animals, containing less saturated fatty acids. Collagen content of the meat is lower, so that meat from double-muscled animals is mostly more tender. However, meat tenderness, color and juiciness are not always improved. A different metabolism as a consequence of faster glycolytic myofibers can be partly responsible for this phenomenon. DM animals are interesting for the producer and butcher, and beneficial for the consumer, if an appropriate nutrition and accommodation, and adequate slaughter conditions are taken into account.AbstractMolecular biology has enabled the identification of the mechanisms whereby inactive myostatin increases skeletal muscle growth in double-muscled (DM) animals. Myostatin is a secreted growth differentiation factor belonging to the transforming growth factor-β superfamily. Mutations make the myostatin gene inactive, resulting in muscle hypertrophy. The relationship between the different characteristics of DM cattle are defined with possible consequences for livestock husbandry. The extremely high carcass yield of DM animals coincides with a reduction in the size of most vital organs. As a consequence, DM animals may be more susceptible to respiratory disease, urolithiasis, lameness, nutritional stress, heat stress and dystocia, resulting in a lower robustness. Their feed intake capacity is reduced, necessitating a diet with a greater nutrient density. The modified myofiber type is responsible for a lower capillary density, and it induces a more glycolytic metabolism. There are associated changes for the living animal and post-mortem metabolism alterations, requiring appropriate slaughter conditions to maintain a high meat quality. Intramuscular fat content is low, and it is characterized by more unsaturated fatty acids, providing healthier meat for the consumer. It may not always be easy to find a balance between the different disciplines underlying the livestock husbandry of DM animals to realize a good performance and health and meat quality.

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Prenatal development is known to be extremely sensitive to maternal and environmental challenges. In this study, we hypothesize that body growth and lactation during gestation in cattle reduce nutrient availability for the pregnant uterus, with consequences for placental development. Fetal membranes of 16 growing heifers and 27 fully grown cows of the Belgian Blue (BB) breed were compared to determine the effect of body growth on placental development. Furthermore, the fetal membranes of 49 lactating Holstein Friesian (HF) cows and 27 HF heifers were compared to study the impact of dam lactation compared to dam body growth. After parturition, calf birth weight and body measurements of dam and calf were recorded, as well as weight of total fetal membranes, cotyledons and intercotyledonary membranes. All cotyledons were individually measured to calculate both the surface of each individual cotyledon and the total cotyledonary surface per placenta. Total cotyledonary surface was unaffected by breed or the breed × parity interaction. Besides a 0.3 kg lower cotyledonary weight ( P = 0.007), heifer placentas had a smaller total cotyledonary surface compared with placentas of cows (0.48 ± 0.017 v. 0.54 ± 0.014 m 2 , respectively, P < 0.001). Within the BB breed, fetal membranes of heifers had a 1.5 kg lower total weight and 1.0 kg lower intercotyledonary membrane weight ( P < 0.005) compared with cows. A cotyledon number of only 91 ± 5.4 was found in multiparous BB dams, while growing BB heifers had a higher cotyledon number (126 ± 6.7, P < 0.001), but a greater proportion of smaller cotyledons (<40 cm 2 ). Within the HF breed, no parity effect on intercotyledonary membrane weight, cotyledon number and individual cotyledonary surface was found. Placental efficiency (calf weight/total cotyledonary surface) was similar in HF and BB heifers but significantly higher in multiparous BB compared with multiparous HF dams (106.0 ± 20.45 v. 74.3 ± 12.27 kg/m 2 , respectively, P < 0.001). Furthermore, a seasonal effect on placental development was found, with winter and spring placentas having smaller cotyledons than summer and fall placentas ( P < 0.001). Main findings of the present study are that lactation and maternal growth during gestation entail a comparable nutrient diverting constraint, which might alter placental development. However, results suggest that the placenta is able to manage this situation through two potential compensation mechanisms. In early pregnancy the placenta might cope by establishing a higher number of cotyledons, while in late gestation a compensatory expansion of the cotyledonary surface is suggested to meet the nutrient demand of the fetus.

Summary Six normal‐conformation and six double‐muscled Belgian White‐blue bulls were involved in a 2 × 2 × 2 factorial experiment to investigate the effect of genotype (normal versus double‐muscled), body weight (400 versus 600 kg) and dietary crude protein (135 versus 165 g per kg dry matter) on voluntary feed intake, digestion and its consequences on the nutritive value, metabolites in blood and urine and nitrogen retention. The diet consisted of concentrate and maize silage (50:50, dry matter basis) and was fed ad libitum. Double‐muscled bulls showed a significantly lower feed consumption (67.1 g DM per kg W0.75) compared to normal bulls (81.5 g). When adjusted for dry matter intake, digestibility was not affected by beef type. Some feed components tended to be better digested when body weight (crude fibre and nitrogen‐free extractives) or dietary crude protein content (dry matter, organic matter, crude protein, nitrogen‐free extractives and energy) were higher. Blood urea nitrogen was not dependent on genotype, but increased with body weight and dietary protein. Creatinine concentration in the blood and daily creatinine excretion in the urine were highest in double‐muscled bulls and heavier animals. Urinary 3‐methylhistidine excretion was not different between double‐muscled and normal bulls, but increased with body weight. This was confirmed by the fact that nitrogen retention relative to intake was similar for both beef types and decreased with a higher body weight. A negative effect of stress on feed intake, muscle protein degradation and nitrogen retention in double‐muscled bulls was not excluded. Zusammenfassung Einfluß von Genotyp, Lebendmasse und Proteingehalt der Ration auf Futteraufnahme, Verdaulichkeit, Metaboliten im Blut und Harn und Stickstoffbilanz. Der Einfluß von Genotyp (konventionell oder Doppellender), Lebendmasse (400 und 600 kg) und Proteingehalt der Ration (135 oder 165 g Rohprotein pro kg Trockensubstanz) auf Verdaulichkeit, Metaboliten im Blut und Harn und Stickstoffbilanz wurde mit 6 konventionellen Bullen und 6 Doppellendern der Belgischen weißblauen Rasse untersucht. Die Ration bestand aus Kraftfutter und Maissilage (50:50 auf Basis von Trockensubstanz) und wurde ad libitum gefüttert. Die Trockensubstanzaufnahme der Doppellender lag niedriger im Vergleich mit den konventionellen Bullen (67, 1 bzw. 81, 5 g pro kg LM0.75). Nach mathematischer Anpassung an die verschiedene Trockensubstanzaufnahme war die Verdaulichkeit nicht durch den Genotyp beeinflußt. Die Verdaulichkeit einiger Futterkomponenten lag höher, wenn die Lebendmasse (Rohfaser, N‐freie Extraktstoffe) und der Proteingehalt (Trockensubstanz; Organische Substanz, Protein, N‐freie Extraktstoffe und Energie) höher waren. Die Blutharnstoffkonzentration war nicht abhängig vom Genotyp, aber sie erhöhte sich mit der Lebendmasse und dem Proteingehalt. Die Blutkreatininkonzentration und die tägliche Exkretion im Harn lagen höher bei den Doppellendern und den schweren Tieren. Die 3‐Methylhistidinexkretion im Harn war nicht verschieden...