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A trial was conducted to determine the total rumen degradable protein intake (RDPI) required to maximise the digestible organic matter intake (DOMI) of beef cows consuming low quality grass hay from the northern variation of Cymbopogon-Themeda pasture type. Thirty-five pregnant (last trimester) Afrikaner × Simmentaler crossbred cows (517.08 ± 53.06 kg) were stratified according to fasted body weight and allocated to five treatments. A rumen degradable protein (RDP) source, calcium caseinate (90% crude protein on dry matter basis and 100% rumen degradable) was used and mixed with a molasses based concentrate. Treatments provided the following supplemental RDP levels/cow/day from casein: (1) control, 0 g, (2) 180 g, (3) 360 g, (4) 540 g, and (5) 720 g. The cows had ad libitum access to low quality grass hay (2.26% CP; 73.94% neutral detergent fibre, NDF). The trial period consisted of 14 d of adaptation, 21 d of the intake study, and 7 d of the digestibility study. No statistically significant influence of RDP on the apparent digestibility of DM, organic matter, or NDF was detected. The grass DM intake, DOMI, and metabolizable energy intake increased in a quadratic manner with increasing supplementary RDP. The single broken-line model predicted DOMI/kg metabolic weight (BW0.75) with higher accuracy than the quadratic regression procedure. According to this model, 4.03 g daily RDPI/kg BW0.75 or 8.07% RDP of digestible organic matter was required to maximise DOMI of pregnant beef cows consuming winter grassveld hay. [ABSTRACT FROM AUTHOR]