Vargas Jurado N, Tanner A E, Blevins S R, Rich J, Mayes R W, Fiske D, Swecker W S, Lewis R M
J Anim Sci. 2015 Apr;93(4):1565-72. doi: 10.2527/jas.2014-8453.
Assessing feed efficiency in pasture-based systems can be challenging due to difficulties in measuring feed intake and diet preference. Furthermore, heifers may modify their intake of a particular forage species depending on its nutritive and physical attributes and on their own physiological status. For instance, heifers growing to larger mature sizes have higher maintenance requirements, which may affect their dietary preferences. The present study was designed to determine the influence of frame size (FS) on feed intake and diet selection at 2 age subclasses: postweaning, at, on average, 313 d of age (317 kg BW for larger FS and 285 kg BW for smaller FS), and as yearlings, at, on average, 403 d of age (391 kg BW for larger FS and 343 kg BW for the smaller FS). Twenty-four Angus-cross heifers were evaluated at these ages. They were housed in a drylot equipped with a Broadbent Feeding System and had ad libitum access to cubed red clover and cubed fescue hay in separate buckets. Following 3-wk acclimation periods, DMI of each forage species was assessed daily for 10 d. Body weights were also recorded daily over those 10 d. Larger and smaller FS cattle did not differ in ADG, feed conversion ratio, and Kleiber ratio at either age (P > 0.05). However, heifers from larger FS category had higher total and red clover DMI at both ages (P < 0.05) and higher proportions of red clover in their diet as yearlings (P < 0.001). Cumulative differences between clover and fescue DMI were similar at postweaning in both FS (P > 0.05); however, at yearling age, the larger FS cattle ate consistently and cumulatively more red clover (P < 0.001). The intake and diet selection of heifers across ages were not consistent for either FS category (P = 0.748), suggesting difficulties in predicting lifetime feed efficiencies based on an early measure. Although gains and feed conversion ratios were similar between FS categories, the larger FS heifers ate more. At yearling age, these heifers of larger metabolic size also selected a greater proportion of red clover in their diet. Because the FS and, thereby, BW of cattle affect intakes and diet selections, they also may impact the composition and sustainability of grasslands generally. Therefore, the relationship between FS and pasture usage likely needs to be integrated into descriptions of cow efficiency in pasture-based operations.
在基于牧场的系统中评估饲料效率可能具有挑战性，因为测量采食量和饮食偏好存在困难。此外，小母牛可能会根据特定牧草品种的营养和物理特性以及自身的生理状态来调整其对该品种牧草的采食量。例如，生长到较大成熟体型的小母牛维持需求更高，这可能会影响它们的饮食偏好。本研究旨在确定体格大小（FS）对两个年龄亚组采食量和饮食选择的影响：断奶后，平均年龄为313天（较大FS的体重为317千克，较小FS的体重为285千克）；一岁时，平均年龄为403天（较大FS的体重为391千克，较小FS的体重为343千克）。在这些年龄段对24头安格斯杂交小母牛进行了评估。它们被饲养在配备了布罗德本特饲喂系统的干栏中，可随意取用分别装在桶里的块状红三叶草和块状羊茅干草。经过3周的适应期后，每天评估每种牧草品种的干物质采食量（DMI），持续10天。在这10天里每天也记录体重。无论在哪个年龄，较大和较小FS的牛在平均日增重、饲料转化率和克莱伯比率方面均无差异（P>0.05）。然而，较大FS类别的小母牛在两个年龄时的总DMI和红三叶草DMI均较高（P<0.05），且一岁时其饮食中红三叶草的比例更高（P<0.001）。在断奶后，两个FS组中三叶草和羊茅DMI的累积差异相似（P>0.05）；然而，在一岁时，较大FS的牛持续且累积地采食更多红三叶草（P<0.001）。对于任何一个FS类别，不同年龄小母牛的采食量和饮食选择都不一致（P = 0.748），这表明基于早期测量来预测终生饲料效率存在困难。尽管FS类别之间的增重和饲料转化率相似，但较大FS的小母牛吃得更多。在一岁时，这些代谢体型较大的小母牛在其饮食中也选择了更大比例的红三叶草。由于牛的FS以及体重会影响采食量和饮食选择，它们也可能总体上影响草原的组成和可持续性。因此，在基于牧场的养殖操作中，FS与牧场利用之间的关系可能需要纳入奶牛效率的描述中。
