The relationship of lactating beef cow metabolizable energy intake to energy partitioning, milk composition, and calf performance

A 2-yr experiment was conducted to determine the impact of maternal metabolizable energy intake (MEI) on energy partitioning and performance of beef cows and calves. Forty mature crossbred Angus beef cows (6 ± 2.0 yr, 534 ± 60 kg BW) were used each year along with their suckling steer calves (84 ± 8.7 d, 130 ± 15 kg BW). Cows were stratified by early lactation milk yield and randomly assigned to 1 of 5 levels of MEI. Each treatment group was housed in a dry lot pen. During year 1, maternal MEI ranged from 225 to 320 kcal·BW0.75·d-1, while in year 2, MEI ranged from 215 to 288 kcal·BW0.75·d-1. Calves were provided ad libitum access to the same TMR diet as the cows in a creep area. Parameters evaluated included apparent total tract nutrient digestibility, milk production and nutritional composition, cow body weight (BW), body condition score (BCS), ultrasound carcass characteristics, calf BW, and calf creep feed intake. Data were analyzed using the GLIMMIX procedure of SAS (SAS Inst. Inc., Cary, NC) as a general linear mixed model. The final model included the linear effect of maternal MEI as a fixed effect and the year of the experiment as a random effect. Significance of the fixed effects was declared when P ≤ 0.05, while tendencies were declared when 0.05 < P ≤ 0.10. Cow 100-d BW, BCS, and ADG linearly increased (all, P < 0.01) with increasing levels of maternal MEI. Similarly, calf 100-d BW and ADG linearly increased (P = 0.03, P < 0.01, respectively) with increasing maternal MEI. Milk yield, milk energy production and all milk nutrients linearly increased (P ≤ 0.04) with increasing maternal MEI, except for milk urea nitrogen which significantly decreased (P < 0.01). Cow energy partitioned to tissue accretion and milk production linearly increased (both, P < 0.001). As maternal MEI increased, the proportion of net retained energy partitioned to maternal tissue accretion initially increased. However, at approximately 275 kcal·BW0.75·d-1 of maternal MEI, the proportion plateaued. Increased maternal MEI reduced the efficiency of calf BW gain (BW gain per unit of calf MEI from both creep feed and milk) in a linear fashion (P = 0.03). These findings suggest that maternal energy intake, rather than genetic capacity for milk yield, limited milk energy production in these cows. Furthermore, the maximum proportion of retained energy as maternal tissue was achieved at about 275 kcal·BW0.75·d-1.